System Diagnostic Tools

Adjacent VAV boxes or terminal units are in cooling mode and heating mode
  • Explanation: One zone is at the perimeter and one is interior.

  • Explanation: Zone loads may vary significantly over short time spans.

  • Problem: Thermostats are improperly located.

AHU preheat/heating coil and cooling coil are both on simultaneously

Introduction

Note: This symptom is limited to single-duct systems that have both heating and cooling coils located in the air handling unit (AHU). Dual-duct and multizone systems are specifically designed for simultaneous operation of both coils.

With no cooling or heating (valve position for both is closed, or % = 0), supply-air temperature (SAT) should equal the mixed-air temperature (MAT) plus a few degrees for fan heat (usually two degrees in most commercial systems, and up to four degrees in high-pressure systems). If the temperature difference is greater than what can be attributed to fan heat, then there may be a problem with the heating coil.

Preheat and heating coils are typically located between the outside-air (OSA) intake and the cooling coil to protect the cooling coil from freezing conditions. This applies to both draw-through and blow-through configurations. If humidity control is available, then the primary (or a supplemental) heating coil may be downstream of the cooling coil. The heating coil can be electric, hydronic (heating water), or steam, and different causes of this symptom will apply to each type.

How This Wastes Energy

With both coils operating, the downstream cooling coil must remove some, if not all, of the heat added by the heating coil to maintain the SAT setpoint.

Possible Causes of Symptom

The list below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: The system is controlling humidity.
  2. Explanation: Chilled-water pump could be running, possibly due to freezing conditions.
  3. Problem: Temperature setpoints are overridden.
  4. Problem: Valve actuators can't overcome the differential water pressure to fully shut off the system.
  5. Problem: Valves are manually adjusted.
  6. Problem: Coil control valve has failed, is stuck in the open position, or is allowing fluid to leak by.

How to Find the Problem(s) by Inspection

As noted above, for this condition to occur, both the cooling and heating systems must be active.

Inspection Step 1

Review the design criteria for the system to see if humidity control is part of the design. Under some conditions, the cooling coil will be active to reduce moisture in the air stream. It typically cools the air below the dry-bulb-temperature setpoint, so the air stream must be reheated to reach the SAT setpoint. In such cases, the heating coil is located downstream of the cooling coil. If this applies in your case and conditions call for dehumidification, then having both coils active is permissible. However, if multiple AHUs supply the same area, make sure they are operating from a single set of sensors and not independently. All AHUs should be operating in the same mode at the same time. Otherwise, they can work against each other- one is heating while the other is cooling.

Inspection Step 2

Review the design criteria and the DDC-system programming to see if the chilled-water-system pump is online for freeze protection. It is relatively common to circulate chilled water through the coil to prevent freezing, especially in climates where freezing is rare (in which case, protecting the equipment is considered more important than conserving energy). Alternatives to this practice are:

  • Draining the cooling coils in the winter. This is standard practice in two-pipe distribution systems.
  • Having solenoid dump valves that automatically drain the cooling coil if a potential freezing condition exists. (Make sure your water additives meet your local waste codes.)

If this is condition applies, the total impact on energy usage is the sum of the chilled-water pump energy and the energy that the cooling coil removes from the air stream.

Inspection Step 3

Inspect the DDC system to verify that the SAT setpoint has a reset schedule and has not been overridden. Verify the original setpoint as commissioned or as specified in the operating plan. If the setpoints have been overridden, find out why before correcting the problem. Sometimes software interlocks are not programmed to prohibit simultaneous heating and cooling, and the coils may be controlling to different setpoints. Heating may be active to try to reach a certain temperature based on a reset schedule using OSA, while cooling is active to try to maintain a terminal-unit damper at a certain position.

Inspection Step 4

Inspect the DDC commands to the heating- and cooling-coil valves. Only one should be active. If there are no end switches providing feedback about valve position to the DDC system, you will need to inspect the valves. Valve actuators for 2-way and 3-way valves have different torque requirements. Actuators on 2-way valves need more torque to overcome the pump static head pressure. Three-way valves just divert the flow, not overcome it, so their actuators need much less torque. A 3-way valve may have been converted to a 2-way but the actuator was not replaced. Inspect the valves to make sure their position is as indicated by the DDC system.

Inspection Step 5

Some motorized and pneumatic control valves can also be adjusted manually. Make sure the valves can freely travel their entire stroke as commanded by the DDC system and that there are no manual adjustments that limit their stroke. If it is a hybrid control system-that is DDC sensors with pneumatic controllers-verify that the electro-pneumatic (EP) controller is working properly and allowing full control air to the pneumatic diaphragm. If the control valve is wide open, make sure the isolation valves to the coil are not being used for flow control.

Inspection Step 6

If a valve can't make the full stroke or doesn't respond to a DDC command, remove and inspect the actuator for proper operation. The valve should also be removed and dismantled for inspection and repair. Look for problems with the seating surfaces, both on the valve disc and body. Inspect the strainer upstream of the control valve and make sure it is clean and that a screen is installed. (A good indicator of a leaking valve is a differential temperature between the inlet and outlet piping that you can identify using a thermometer or your hands. Just make sure you don't get burned while checking for this.)

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Supply-air temperature (SAT)
  • Mixed-air temperature (MAT)
  • Return-air temperature (RAT)
  • Cooling valve (%)
  • Heating valve (%)
  • Heating-coil leaving-air temperature (LAT)

Trending the heating-coil LAT will most likely require portable temperature loggers. Graph the temperatures and see how the coil LATs vary with respect to the valve positions, or percent of capacity.

Example of Normal Operation

In this example, the system is economizing so the MAT is 2F below the SAT setpoint because of fan heat. The AHU is modeled as constant volume to simplify the graphs.

The graph below shows normal cooling operation. The cooling-coil control valve is at 0% when the OSAT is 2 degrees below the SAT. The heating valve is closed throughout the day. If your graph looks like this, the problem should be solved.

Normal operation

Example of Abnormal Operation

The graph below is similar to the one above, except that the baseline for the cooling-coil control valve appears to be 25%. If your graph looks like this, a leaky valve at the heating coil is causing heat gains. The heating-coil LAT was trended, showing a constant pickup of about 8 degrees. Re-inspect the system starting from Inspection Step 4.

Abnormal operation: leaking heating valve

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
AHU supply-air temperature is low during cold weather

Introduction

Low supply-air temperature (SAT) can be a problem when the system is in heating mode. This symptom is closely related to the symptom Mixed-air temperature near outside-air temperature during heating mode. You should review that symptom for economizer and pressurization problems in this situation. If your air-handling unit (AHU) does not have a heating coil, follow the steps for that symptom to find the problem. If your AHU has a heating coil, continue investigating this symptom.

How This Wastes Energy

A low SAT can cause terminal units to expend more energy than necessary reheating air to reach the required zone temperature.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: The system is serving a process load.
  2. Explanation: Two-pipe distribution system is still in cooling mode.
  3. Problem: Temperature reset is not scheduled or is overridden.
  4. Problem: HVAC zone may require a low supply-air temperature to compensate for an abnormal heat load.
  5. Problem: Temperature sensor for supply air is miscalibrated or improperly located.
  6. Problem: The AHU casing leaks or an access door is open, allowing outside air to enter downstream of the mixing box.
  7. Problem: The AHU heating coil is not operating properly.
  8. Problem: An uninsulated supply duct passes through unheated space.

How to Find the Problem(s) by Inspection

Inspection Step 1

Review the design criteria for the system to make sure the AHU SAT is not being controlled by a process-cooling load. Many times a small server room or elevator machinery room is inadvertently added to the air-distribution system, forcing the system to maintain a low SAT regardless of OSAT.

Inspection Step 2

If the piping system is a two-pipe design, verify that the system has been fully shifted to heating mode. Otherwise this may be a temporary condition due to an unexpected cold day.

Inspection Step 3

Inspect the direct-digital-control (DDC) system to verify that the SAT setpoint has a reset schedule and has not been altered. Verify the original setpoint as commissioned or as specified in the operating plan. If the setpoints have been overridden, find out why before correcting the problem.

Inspection Step 4

Investigate the conditions at each terminal unit using the DDC system. Is there a unit requiring 100% cooling? A terminal unit may not be able to meet its zone loads at a higher SAT due to an air balance problem or the addition of appreciable heat loads.

Inspection Step 5

Make sure the SAT and OSAT sensors are correctly installed and calibrated. If a sensor is not located properly in the air stream, it could provide an incorrect temperature reading to the DDC system.

Inspection Step 6

Make sure the AHU casing is airtight, that no access doors are open, and that no air is passing through a damaged gasket. (This condition is primarily associated with rooftop units.)

Inspection Step 7

Make sure the AHU heating coil is operating for properly. (This could also apply to a heat-recovery coil, if present.) Some possible problems might be:

  • If electric, make sure the coil is powered and not disconnected locally. Inspect the flow-switch interlock and make sure there are no frozen or burned out contacts.
  • Make sure the heating-water system is active and that the supply line to the coil is at the expected temperature. If the system is active but not providing adequate heat to the coil, check the following:
  • Make sure there is adequate flow to the coil, and that it is not air-bound. Vent the high point of the coil header to make sure there is no air present. Make sure the pumps are operating as expected.
  • Make sure the boilers are sequencing properly to meet the programmed heating-water-supply temperature (HWST) setpoint.
  • If steam, make sure the service is active and that the traps are operating properly. A flooded condensate system or failed trap can fill the coil with condensate and sharply reduce heat transfer. Freezing is a serious concern, in that case.
  • If face and bypass dampers are installed, make sure the damper linkage and actuator are operating properly. Cycle the actuator using the DDC system to ensure proper operation. Ensure that all damper blades are properly installed and tight.

Inspection Step 8

If this symptom occurs not at the AHU but at a terminal unit, inspect the HVAC plans to see if the ducting passes through an unheated space. The insulation could be damaged in that area.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Return-air temperature (RAT)
  • Mixed-air temperature (MAT)
  • Supply-air temperature (SAT)
  • SAT setpoint
  • Heating-coil control valve, % open

Graph the temperatures and see how the SAT varies with respect to the SAT setpoint, the RAT, and the OSAT.

Example of Normal Operation

The graph below depicts a system with a high OSA component, set at 80%. The heating coil modulates as required to maintain the SAT at its setpoint of 65 degrees. This graph illustrates normal, efficient operation of the system. If your graph looks similar to this, the problem should be solved.

Normal operation:SAT at setpoint

Example of Abnormal Operation

In the graph below, the MAT and SAT track the OSAT, with a small offset due to the fan heat. If your graph looks something like this, you may not be getting any heat from your heating coil; the "Heating %" is a constant 0%, indicating that there is no temperature differential across the heating coil. Re-inspect the system starting from Inspection Item 1.

Abnormal operation: SAT not at setpoint

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
All restrooms have excessive odors
  • Problem: Toilet exhaust system is programmed off during occupied period.
  • Problem: Fan has failed.
  • Problem: Fire damper is closed.
Boiler starts and stops frequently
  • Explanation: Load is below the minimum boiler capacity.
  • Problem: Flow switch is malfunctioning.
  • Problem: Water-temperature high-limit switch is set too low.
  • Problem: Deadband between on/off is too narrow.
  • Problem: Boiler is overfiring or the flue or turbulence-inducing inserts in fire tubes may be clogged.
  • Problem: Heating-water pump is cycling. Boiler cycles via the system flow switch.
  • Problem: NG pressure at the manifold is low.
  • Problem: NOx controls are malfunctioning.
  • Problem: Flow of induced-draft fan is inadequate.
Building does not go into warm-up as expected
  • Problem: Optimum start is not programmed or is improperly scheduled. 
  • Problem: Temperature sensor is miscalibrated or improperly located.
Building goes into warm-up during occupied hours
  • Problem: Optimum start schedule is improperly programmed.
Building takes longer than expected to warm up
  • Problem: Optimum start function is not activated, time clock is not set for cold weather.
  • Problem: Long (4-day) weekend allowed building mass to cool the space below the setpoint.
  • Problem: Setback temperature for "unoccupied" periods is too low.
  • Problem: Heating system is locked out.
  • Problem: Outside-air reset prevents the heating system from controlling temperature properly.
  • Problem: There are problems with the pumps or distribution system of the heating system.
  • Problem: Heating system is undersized.
  • Problem: Outside-air temperature is below the design condition.
  • Problem: Outside-air dampers open during the warm-up cycle.
  • Problem: Building exhaust fans operate during warm-up causing infiltration problems.
  • Problem: HVAC system has parallel VAV boxes at the perimeter and pinch-off VAV boxes in the interior with no reheat. Only the VAV boxes, not the main AHU, are scheduled for warm-up.
  • Problem: Sections of the building are unoccupied and have little or no internal heat gains.
Chilled-water pump operates significantly more hours than chiller
  • Explanation: Load is below the unit's minimum capacity.
  • Explanation: Pump sequence calls for chilled water to circulate during cold weather to prevent freezing coils.
  • Problem: Pumps operate too long after chiller shuts down.
  • Problem: Pumps operate in manual mode at the local controller or are overridden in the DDC system.
Chilled-water supply stays at constant design temperature

Introduction

Some chilled-water systems have a reset logic based on outside-air temperature (OSAT). This conserves energy when the cooling load is less than the design load. The reset logic allows the chilled-water supply temperature (CHWST) to rise above the design temperature and increases the chiller efficiency by reducing the amount of work it has to do. The air-handling-unit (AHU) fans will compensate for the corresponding elevated supply-air temperature (SAT) as needed, but using less energy.

How This Wastes Energy

Keeping the CHWST lower than necessary causes the chiller to operate in a less efficient region of its performance curve. The rule of thumb is that raising the CHWST one degree Fahrenheit reduces chiller energy consumption roughly 2%. Lower CHWST can also cause higher heating bills because of reheating at terminal units.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: A small process load (e.g., server room) is connected to the system and requires a constant chilled-water temperature.
  2. Explanation: Control over chiller setpoints (either local or via the direct digital control system) is absent.
  3. Problem: Temperature-reset schedule is not programmed, or is overridden.
  4. Problem: Temperature sensor is miscalibrated or improperly located.
  5. Problem: HVAC zone may require low supply-air temperature to compensate for a lack of airflow.
  6. Problem: Lack of insulation or long piping runs cause excessive heat gains in the piping distribution system.
  7. Problem: Distribution system is undersized.
  8. Problem: Chilled-water system is improperly balanced.

How to Find the Problem(s) by Inspection

Inspection Step 1

Review the design documents for your chilled-water system to see if a process load is served by the system. Possible loads include a fan coil serving a small data room, an elevator machinery room, or a lighting-control room. Any one of these would force the chilled-water system to maintain a steady CHWST since the process load is constant.

Inspection Step 2

Familiarize yourself with your chiller's capabilities. Can it reset the CHWST through the direct-digital-control (DDC) system? If it has an OSAT sensor installed, can it do it using control logic integral to the chiller controls, Can it reset CHWST based on the AHU that requires the most cooling? Older chillers often have no temperature-reset capability. Your system will operate at a set chilled water temperature if reset logic is not included in the stand-alone software running the unit, or if there is no control from the DDC system.

If you have no automatic-reset capability, you may want to consider adjusting the temperature manually for seasonal conditions. This will require that a detailed record be kept showing occupant complaints as compared to OSAT and CHWST. Always operate the CHWST as high as possible without causing tenant complaints. Be aware, though, that allowing the AHU cooling-coil temperature to rise too high may allow humidity to become unacceptably high.

Inspection Step 3

Inspect the override logs of the DDC system to verify that the reset-schedule setpoints have not been altered. Verify the original setpoints as commissioned or in the operating plan. If the setpoints have been overridden, find out why before correcting the problem. Some reasons for override may be:

  • A process load connected to the system requires constant chilled-water temperatures.
  • An AHU that can't meet its loads with a higher chilled-water temperature due to a water- or air-balance problem.

Inspection Step 4

Check sensor location and calibration:

  • See if the CHWST probe is correctly installed and calibrated. If the probe is not located properly in the water stream, it will provide elevated temperatures to the DDC system, forcing the chiller to drop the supply-water temperature.
  • Inspect the OSAT sensor location and installation. Is it located where it provides an accurate reading? Check to see if it shares a DDC system point from another controller. If communication was lost between controllers, the reset logic may be using the last OSAT value transmitted.

Inspection Step 5

Check the trouble log of the HVAC system. A zone may lack airflow and require supply air at a temperature below its normal setpoint to meet its load. If this is the case, resolve the airflow problem before addressing the reset logic.

Inspection Step 6

Review your chilled-water piping. You may have excessive heat gain in the piping due to very long runs or inadequate insulation. Wet insulation, due to a break in the vapor barrier or a leak can increase temperature rise dramatically.

Inspection Step 7

Review the design drawings for flow and piping size. The piping may be undersized such that the chilled water must be cooled below its design setpoint due to a lack of adequate flow.

Inspection Step 8

Review the latest test-and-balance report.

  • Verify that the system was properly balanced at the time the report was prepared. Compare those readings to the current conditions. Inspect your system for possible bypassing through 3-way valves. Check balancing valves for proper position. Passing too much water through a coil can actually reduce the heat transfer rate by not allowing the water enough time in the coil. (More flow is not always a good thing.)
  • Sometimes water flow at the far end of the system is inadequate due to poor balancing. The pump may be at the design rate but most of the flow is passing through the units closest to the pumps due to the higher pressure differential. The most common response to this is to turn on another pump, which only makes the problem worse.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Chilled-water supply temperature (CHWST)
  • Chilled-water return temperature (CHWRT)
  • Cooling-coil valve positions-feedback if possible, and not output from the DDC system
  • Secondary pump speed-feedback from the variable speed drive (VSD) if possible, and not output from the DDC system

Graph the CHWST with respect to the other four points. If the OSAT line is flat, then the OSAT sensor is the problem. If a cooling-coil valve is always 100% open, then that unit may be undercooling (or over-flowing). If the secondary pump speed is constant at 100%, the system demand may exceed the chiller capacity. (The return-water temperature can also be above design in this case.)

Normal Operation

The graph below illustrates normal, efficient operation of the system. The system starts at 09:00 (based on an OSAT of 60 degrees) with the CHWST at 50 degrees, and the cooling valve 20% open. As the OSAT rises, the CHWST starts to drop to its design point of 45 degrees. The CHWRT starts to rise at the same time, showing that the chilled water is bringing back more heat to the chiller. The pump speed increases as the load increases. The chiller is operating at maximum differential temperature at 14:00. The AHU-cooling-coil valve continues to open as the zone load increases until it is open 100%. If your graph looks like this then the problem should be solved.

Normal system operation

Abnormal Operation - Absence of Reset Sechedule

The graph below illustrates abnormal operation of the system. The system starts at 09:00 (when the OSAT reaches 60 degrees) with the CHWST at 45 degrees, and the cooling valve 20% open. As the OSAT rises, the CHWST stays constant at its design point of 45 degrees. The CHWRT starts to rise, showing that the chilled water is bringing back more heat to the chiller. The pump speed increases as the load increases. The chiller is operating at maximum differential temperature at 14:00. The AHU-cooling-coil valve continues to open as the zone load increases, until it is 100% open. If your graph looks like this, the problem is a lack of reset.

Abnormal operation: reset problem

Abnormal Operation - Water Flow or Balance

The graph below illustrates abnormal operation of the system. The system starts at 09:00 (when the OSAT reaches 60 degrees) with the CHWST at 45 degrees. As the OSAT rises, the CHWST stays constant at its design point of 45 degrees, trying to satisfy the AHU demand. The CHWRT starts to rise, showing that the chilled water is bringing back more heat to the chiller from the entire system. The chiller is operating at maximum differential temperature at 14:00. If your graph looks like this, then the problem is either an airflow or water-flow issue. If you have trended the chilled-water pump speed and it is constant at 100%, you may have a water-flow or water-balance problem. In either case, go back to Inspection Step 5 and re-investigate. Continue trending to verify your new findings.

Abnormal operation: chilled-water valve and VFD at 100%

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
  • Test-and-balance technician
Chiller operates when outside air temperature is low

Introduction

First, familiarize yourself with the capabilities of both your chilled-water and direct-digital-control (DDC) systems. Determine if and how your chiller is intended to be locked out when mechanical cooling is not expected to be needed. Usually it is based on reaching a certain outside-air temperature (OSAT) where economizers can operate more efficiently.

How This Wastes Energy

Energy is wasted by operating the chilled-water system when air-side economizers can be used for free cooling-that is, when OSAT is more than 2 degrees below the supply-air temperature (SAT). This also reduces the life span of all operating equipment. (The 2-degree offset accounts for heat added to the air stream by fans.) Low loads on chillers can also cause refrigerant "slugging" within a chiller, which can damage it, resulting in large repair charges or even replacement.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: A small process load, e.g., server room, is connected to the system and requires constant chilled-water.
  2. Explanation: Lack of air-side economizers.
  3. Explanation: HVAC system is a medium-temperature system with a low (45 degree) supply-air temperature.
  4. Problem: OSAT sensor is miscalibrated or improperly located. OSAT sensor could be a shared point from another DDC controller and communication between controllers was lost.
  5. Problem: Economizer is malfunctioning.
  6. Problem: Lockout setpoint is not provided, is overridden, or has been reset below the design setpoint.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect the as-built drawings of the chilled-water system. Look for process loads such as computer-room cooling units with chilled-water coils, or elevator-machinery rooms with chilled-water fan coils. Sometimes even condensing units on large freezers, refrigerators, or ice machines were designed to use chilled water to reject heat. If you have such a condition, a capital improvement project is most likely the best solution.

Inspection Step 2

Check the HVAC design to see if air-side economizers are part of the design. There are situations where air quality or building configuration overrides energy considerations, as with clean rooms. Some type of water-side economizer is probably the best solution for this situation.

Inspection Step 3

Is the HVAC system designed as a medium-temperature system with SATs in the mid-40s? (Typical HVAC systems use a design setpoint of 53-55 degrees.) This type of system may have been designed with smaller ducting to reduce first costs, but it greatly decreases the number of hours that the economizer cycle can operate. Chilled-water-supply temperatures (CHWST) in a system like this can be in the high 30s. If your system is designed to provide chilled water in the high 30s, this may not be a problem.

Inspection Step 4

Check the location and installation of the OSAT sensor. Is it located where it provides an accurate reading? Check to see if it is a shared point from another controller. If communication was lost between controllers, the reset logic may be using the last OSAT value transmitted.

Inspection Step 5

Make sure the air-handling unit (AHU) economizer dampers are operating properly. A single failed damper system can force a large chilled-water plant to operate year-round.

Inspection Step 6

Inspect the DDC-system programming to see if a lockout setpoint has been provided, and see if it is active or overridden. Check to see if it is programmed but overridden, or possibly set at below an OSAT of 50 degrees. Possible causes for this condition are:

  • An AHU has a depressed SAT, maybe in the high 40s. This is sometimes seen in units that serve surgery suites.
  • An AHU cannot meet its zone loads at design conditions due to a water- or air-balance problem.
  • An AHU has a problem getting access to true OSA. It may be diluted with return air (RA) before the mixing box, falsely elevating the OSAT reading for that unit. This is known to occur where an OSA shaft exists in a building core that serves multiple AHUs. The shaft is under maximum negative pressure during economizing and tends to pull air from the occupied spaces through wall cracks, diluting the OSA as it penetrates deeper into the building.

How to Confirm the Problem(s) by Trend Logging

Trend log the following for the problem AHU identified during the inspection:

  • Outside-air temperature (OSAT) - General location, plus for each suspect AHU, if supplied by an OSA shaft
  • Chilled-water-supply temperature (CHWST)
  • Chilled-water-return temperature (CHWRT)
  • Chiller status
  • Supply-air temperature (SAT)
  • Return-air temperature (RAT)
  • Mixed-air temperature (MAT)

Graph the chiller status with respect to the other points.

Example of Normal Operation

If your graph looks like the figure below, your chiller-lockout sequence is operating properly and is turning off the chiller when the OSAT reaches the programmed setpoint. The economizer dampers modulate to maintain SAT until the OSAT approaches SAT minus 2 degrees. At that point your chiller energizes, even though the economizers are 100% open. The OSAT is still lower than RAT, so it is more efficient to cool 100% outside air (OSA) than use return air (RA). Once OSAT exceeds RAT, the economizer dampers go to minimum position, 20% in this case.

In the afternoon, when it starts to cool, your economizer dampers switch back to 100% OSA when the OSAT drops below your RAT. The chiller continues to operate until your OSAT drops 2 degrees below your SAT, at which point it shuts down.

Normal operation: chiller lockout

Example of Abnormal Operation

If your graph looks like the figure below, your OSA damper is locked at minimum position, and the economizer is inoperative. The MAT is never below the SAT so the chiller must run to drop the air temperature to 2 degrees below the SAT (allowing for fan heat). The SAT-reset logic is still active, allowing some savings during the shoulder hours.

Abnormal operation: chiller not locked out, AHU OSA damper fixed at 20%

If you have an OSA shaft, you may note that OSAT gradually increases on each floor or AHU, the deeper you get into the shaft away from the OSA louver. Even though it is cold enough at the AHU closest to the OSA louver, the chiller plant must remain on until the AHU farthest away from the louver can operate on its economizer cycle for 100% free cooling. If this is the case, you may need to seal the OSA shaft airtight. It is most likely a 2-hour-rated shaft, so refer to your local building codes for sealing requirements.

If the OSAT line is flat throughout the day, then the OSAT sensor is most likely the problem, or a communication link between the OSAT controller and the HVAC controller is down.

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
  • Architect, if envelope issues are involved
Chillers

Introduction

Chillers are a key component of air conditioning systems for large buildings. They produce cold water to remove heat from the air in the building. They also provide cooling for process loads such as file-server rooms and large medical imaging equipment. As with other types of air conditioning systems, most chillers extract heat from water by mechanically compressing a refrigerant.

Chillers are complex machines that are expensive to purchase and operate. A preventive and predictive maintenance program is the best protection for this valuable asset.

Learn more about establishing a Best Practice O&M Program.

Chillers commonly use more energy than any other piece of equipment in large buildings. Maintaining them well and operating them smartly can yield significant energy savings.

graphic figure

Chiller and associated HVAC systems

Types of Chillers

Mechanical Compression

During the compression cycle, the refrigerant passes through four major components within the chiller: the evaporator, the compressor, the condenser, and a flow-metering device such as an expansion valve. The evaporator is the low-temperature (cooling) side of the system and the condenser is the high-temperature (heat-rejection) side of the system.

graphic figure

The refrigeration cycle

Mechanical Compressor Chillers

Mechanical compression chillers are classified by compressor type: reciprocating, rotary screw, centrifugal and frictionless centrifugal.

Reciprocating: Similar to a car engine with multiple pistons, a crankshaft is turned by an electric motor, the pistons compress the gas, heating it in the process. The hot gas is discharged to the condenser instead of being exhausted out a tailpipe. The pistons have intake and exhaust valves that can be opened on demand to allow the piston to idle, which reduces the chiller capacity as the demand for chilled water is reduced. This unloading allows a single compressor to provide a range of capacities to better match the system load. This is more efficient than using a hot-gas bypass to provide the same capacity variation with all pistons working. Some units use both methods, unloading pistons to a minimum number, then using hot-gas bypass to further reduce capacity stably. Capacities range from 20 to 125 tons.

Reciprocating compressor

Rotary Screw: The screw or helical compressor has two mating helically grooved rotors in a stationary housing. As the helical rotors rotate, the gas is compressed by direct volume reduction between the two rotors. Capacity is controlled by a sliding inlet valve or variable-speed drive (VSD) on the motor. Capacities range from 20 to 450 tons.

Screw compressor

Centrifugal: The centrifugal compressor operates much like a centrifugal water pump, with an impeller compressing the refrigerant. Centrifugal chillers provide high cooling capacity with a compact design. They can be equipped with both inlet vanes and variable-speed drives to regulate control chilled water capacity control. Capacities are 150 tons and up.

Centrifugal compressor

Frictionless Centrifugal: This highly energy-efficient design employs magnetic bearing technology. The compressor requires no lubricant and has a variable-speed DC motor with direct-drive for the centrifugal compressor. Capacities range from 60 to 300 tons.

Turbocor© frictionless centrifugal compressor

Absorption Chillers

Absorption chillers use a heat source such as natural gas or district steam to create a refrigeration cycle that does not use mechanical compression. Because there are few absorption machines in the Northwest U.S., this document covers only mechanical-compression chillers. You can learn more about absorption chillers at the Energy Solutions Center.

Key Components of Mechanical Compression Chillers

Evaporator

Chillers produce chilled water in the evaporator where cold refrigerant flows over the evaporator tube bundle. The refrigerant evaporates (changes into vapor) as the heat is transferred from the water to the refrigerant. The chilled water is then pumped, via the chilled-water distribution system to the building's air-handling units.

Learn more about Operation and Maintenance of HVAC Water Distribution Systems.

Learn more about Operation and Maintenance of Air Distribution Systems.

The chilled water passes through coils in the air-handler to remove heat from the air used to condition spaces throughout the building. The warm water (warmed by the heat transferred from the building ventilation air) returns to the evaporator and the cycle starts over.

Compressor

Vaporized refrigerant leaves the evaporator and travels to the compressor where it is mechanically compressed, and changed into a high-pressure, high-temperature vapor. Upon leaving the compressor, the refrigerant enters the condenser side of the chiller.

Condenser

Inside the condenser, hot refrigerant flows around the tubes containing the condenser-loop water. The heat transfers to the water, causing the refrigerant to condense into liquid form. The condenser water is pumped from the condenser bundle to the cooling tower where heat is transferred from the water to the atmosphere. The liquid refrigerant then travels to the expansion valve.

Learn more about Operation and Maintenance of Cooling Towers.

Expansion valve

The refrigerant flows into the evaporator through the expansion valve or metering device. This valve controls the rate of cooling. Once through the valve, the refrigerant expands to a lower pressure and a much lower temperature. It flows around the evaporator tubes, absorbing the heat of the chilled water that's been returned from the air handlers, completing the refrigeration cycle.

Controls

Newer chillers are controlled by sophisticated, on-board microprocessors. Chiller control systems include safety and operating controls. If the equipment malfunctions, the safety control shuts the chiller down to prevent serious damage to the machine. Operating controls allow adjustments to some chiller operating parameters. To better monitor chiller performance, the chiller control system should communicate with the facility's direct digital control (DDC).

Safety Issues

Chillers are typically located in a mechanical equipment rooms. Each type of refrigerant used in a chiller compressor has specific safety requirements for leak detection and emergency ventilation. Consult your local mechanical code or the International Mechanical Code for details.

The EPA has enacted regulations regarding the use and handling of refrigerants to comply with the Clean Air Act of 1990. All personnel working with refrigerants covered by this act must be appropriately licensed.

Best Practices for Efficient Operation

The following best practices will improve chiller performance and reduce operating costs:

Operate multiple chillers for peak efficiency: Plants with two or more chillers can save energy by matching the building loads to the most efficient combination of one or more chillers. In general, the most efficient chiller should be first one used.

Raise chilled-water temperature: An increase in the temperature of the chilled water supplied to the building's air handlers will improve its efficiency. Establish a chilled-water reset schedule. A reset schedule can typically adjust the chilled-water temperature as the outside-air temperature changes. On a centrifugal chiller, increasing the temperature of chilled water supply by 2-3°F will reduce chiller energy use 3-5%.

Reduce condenser water temperature: Reducing the temperature of the water returning from the cooling tower to the chiller condenser by 2-3°F will reduce chiller energy use 2-3%. The temperature setpoint for the water leaving the cooling tower should be as low as the chiller manufacturer will allow for water entering the condenser.

Purge air from refrigerant: Air trapped in the refrigerant loop increases pressure at the compressor discharge. This increases the work required from the compressor. Newer chillers have automatic air purgers that have run-time meters. Daily or weekly tracking of run time will show if a leak has developed that permits air to enter the system.

Optimize free cooling: If your system has a chiller bypass and heat exchanger, known as a water-side economizer, it should be used to serve process loads during the winter season. The water-side economizer produces chilled water without running the chiller. Condenser water circulates through the cooling tower to reject heat, and then goes to a heat exchanger (bypassing the chiller) where the water is cooled sufficiently to meet the cooling loads.

Verify Performance of hot-gas bypass and unloader: These are most commonly found on reciprocating compressors to control capacity. Make sure they operate properly.

Maintain refrigerant level: To maintain a chiller's efficiency, check the refrigerant sight-glass and the superheat and subcooling temperature readings, and compare them to the manufacturer's requirements. Both low-level and high-level refrigerant conditions can be detected this way. Either condition reduces a chiller's capacity and efficiency.

Maintain a daily log: Chiller O&M best practices begin with maintaining a daily log of temperatures, fluid levels, pressures, flow rates, and motor amperage. Taken together, these readings serve as a valuable baseline reference for operating the system and troubleshooting problems. Many newer chillers automatically save logs of these measurements in their on-board control system, which may be able to communicate directly with the DDC. Below is an example of a daily log that can be adapted for use with your chiller.

Download this table as a Word Document

Sample Operating Log for Chillers

Best Practices for Maintenance

Compared to a major chiller failure, a sound preventive and predictive maintenance program is a minor cost. Implementing a best-practice maintenance plan will save money over the life of the chiller and ensure longer chiller life. For more information on this topic go to Best Practice O&M Program.

Substandard operating practices frequently go unnoticed and become the accepted norm. Training personnel in both maintenance and operating practices is the best prevention. Many chiller manufacturers offer training for building operating engineers in operating and maintaining their chillers.

To effective maintain chillers, you must 1) bring the chiller to peak efficiency, and 2) maintain that peak efficiency. There are some basic steps that facilities professionals can take to make sure their chillers are being maintained properly. Below are some of the key practices.

Reduce Scale or Fouling

Failure of the heat exchanger tubes is costly and disruptive. The evaporator and condenser tube bundles collect mineral and sludge deposits from the water. Scale buildup promotes corrosion that can lead to the failure of the tube wall. Scale buildup also insulates the tubes in the heat exchanger reducing the efficiency of the chiller. There are two main preventive actions:

Checking water treatment: Checking the water treatment of the condenser-water open loop weekly will reduce the frequency of condenser tube cleaning and the possibility of a tube failure.

Learn more about Operation and Maintenance of Cooling Towers.

Checking the water treatment of the chilled-water closed loop monthly will reduce the frequency of evaporator tube cleaning and the possibility of a tube failure.

Learn more about Operation and Maintenance of HVAC Water Distribution Systems.

Inspecting and cleaning tubes: The tubes in the evaporator and condenser bundles should be inspected once a year, typically when the chiller is taken offline for winterizing. Alternately, for systems that operate all year to meet process loads, tube scaling and fouling can be monitored by logging pressure drop across the condenser and evaporator bundles. An increase in pressure from the inlet to the outlet of 3-4 PSI indicates a probable increase in scale or fouling requiring tube cleaning.

Inspect for Refrigerant Leaks

If possible, monitor the air-purge run timer. Excessive or increased air-purge time may indicate a refrigerant leak. If an air-purge device is not installed, bubbles in the refrigerant sight-glass may also indicate refrigerant leak. Gas analyzers can also be used to identify refrigerant leaks.

The table below provides a checklist for maintenance tasks.

Download this table as a Word document

Maintenance Schedule for Chillers
Description Comments Maintenance Frequency
Fill out daily log Check all setpoints for proper setting and function. Make sure there are no unusual sounds and the space temperature is acceptable. Daily (4x)
Chiller use/sequencing Turn off or sequence unnecessary chillers Daily
Check chilled water reset settings and function Check settings for approved sequence of operation at the beginning of each cooling season Annually
Check chiller lockout setpoint Check settings for approved sequence of operation at the beginning of each cooling season Annually
Clean evaporator and condenser tubes Indicated when pressure drop across the barrel (tube bundle) exceeds manufacturer's recommendations, but at least annually. Annually
Verify motor amperage load limit Motor amperage should not exceed manufacturer's specification Annually
Compressor motor and assembly Conduct vibration analysis: Check all alignments to specifications. Check all seals. Lubricate where necessary. Annually
Compressor oil system Perform analysis on oil and filter. Change if necessary. Check oil pump and seals Check oil heater and thermostat Check all strainers, valves, etc. Annually
Electrical connections Check all electrical connections and terminals for full contact and tightness Annually
Check refrigerant condition Add refrigerant if low. Record amounts and address leakage problems. Annually
Check for condenser and evaporator tube corrosion and clean as needed. Indications include: poor water quality, excessive fouling, and age of chiller. Eddy current testing may be done to assess tube condition. As needed

References

FEMP 2004. O&M Best Practices Guide 2.0.

FEMP 2002. Continuous Commissioning Guidebook for Federal Energy Managers.

Chiller starts and stops frequently
  • Explanation: Load is below the minimum chiller capacity.
  • Problem: The chiller is stopped due to a lack of condenser water flow, which triggers the high-pressure-limit switch for the refrigerant.
  • Problem: The chiller is stopped due to the high temperature of water entering the condenser, which triggers the high-pressure-limit switch for the refrigerant.
  • Problem: Deadband for CHWST (chilled-water supply temperature) is too narrow.
  • Problem: The sensor for leaving-water temperature or differential pressure across the evaporator or condenser has failed.
  • Problem: Chilled/condenser water pump is cycling. Chiller cycles via the system flow switch.
  • Problem: Refrigerant is short.
  • Problem: Compressor draws too much current due to wear and tear.
Condenser-water pump operates significantly more hours than chiller
  • Explanation: Pump sequence calls for condenser water to circulate during cold weather to prevent freezing.
  • Explanation: Loop is being used to provide cooling water for supplemental AC units serving computer rooms, elevator machinery rooms, etc.
  • Problem: Pumps operate too long after chiller shuts down.
  • Problem: Pumps operate in manual mode at the local controller or are overridden in the DDC system.
Condensing pressure or temperature are significantly higher than setpoint for air-cooled unit
  • Explanation: Improperly sized or located enclosure is causing air-side to short-cycle.
  • Problem: Fan motor is not operating.
  • Problem: Condenser coils are dirty.
Cooling-tower fans are always on high, even during cool weather

Introduction

Depending on the number of cells and configuration, cooling-tower systems can be designed with a wide variety of capacity-control schemes. There are control schemes for parallel towers, each having multiple cells, and single towers with a single cell. This analysis deals with a single cell having one of four control schemes:

  • Variable-speed drive (VSD)
  • 2-speed motor
  • 2-motor assembly (primary/pony)
  • Multiple small fans

As you try to determine the cause of this symptom, remember that each facility has its own unique HVAC system-design criteria.

How This Wastes Energy

Operating cooling-tower fans excessively (for example, a VSD running too fast, or a 2-speed fan running on high rather than low speed) can cause the leaving-water temperature to be cooled lower than is called for by the chiller. This wastes fan-motor energy, and can also affect make-up water and chemical treatment usage. Fan-motor energy should be kept as low as possible while keeping the leaving-water temperature (LWT) of the cooling tower within its design parameters.

If the cooling towers are controlled by a system that optimizes overall central-plant energy use, then this condition might not be a problem. In some installations, the energy use of the entire plant is monitored and control logic for all equipment is based on minimizing total energy use. In this case there might be times when running the tower fans at full speed to reduce the LWT reduces energy use by the chiller and pumps more than it increases energy use by the tower fan.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspections Steps:

  1. Explanation: Water-side economizer logic is active.
  2. Problem: VSD control for cooling-tower fan is set to manual or is overridden.
  3. Problem: Temperature sensor for condenser water is miscalibrated or improperly installed.
  4. Problem: Two-speed motor or pony motor has failed.
  5. Problem: Relay contacts on the motor starter are frozen.

How to Find the Problem(s) by Inspection

Inspection Step 1

Review your design documents to see if a water-side economizer is part of the system design. This would include a heat exchanger to transfer heat directly from the chilled-water system to the condenser-water loop without having the chiller online. The presence of a water-side-economizer loop indicates that the chilled-water system has loads that are not dependent on OSAT, but on some other criteria. These are usually process loads such as computer-room air-handling units (AHUs) that require 24-hour cooling. If this is the case, the condenser-water temperature required at the heat exchanger is the basis for controlling the tower fans.

Inspection Step 2

Inspect the fan starter, or VSD if present. Verify that the unit is in automatic mode. If it is in manual or bypass mode, find out who made the change and ask why before resetting any controls.

There are situations where the cooling-tower fan can run at full speed and not reduce the LWT low enough to trip the chiller offline. The LWT can vary with both load and OSA conditions, especially the wet-bulb temperature.

If a 3-way diverter valve is installed in the system piping, the valve could be properly controlling the condenser-water-supply temperature to the chiller, offsetting or masking the problem at the cooling tower.

Inspection Step 3

Make sure the sensor for the cooling-tower LWT is properly located and installed, and provides an accurate value.

Inspection Step 4

Inspect the override logs of the DDC system to verify that the control logic for LWT or condenser-fan speed have not been altered. If the setpoints have been overridden, find out who changed them and ask why before resetting them. Determine the original intended setpoints either as designed or as specified in the operating plan. If the reason these settings were overridden has been resolved, you can reset the system to the proper setpoints.

Inspection Step 5

If a two-speed motor is used, make sure it can operate at low speed and does not default to high speed due to a winding failure.

If a pony motor is used, make sure it operates properly. A failed pony motor can force the primary motor to operate at all times.

Inspection Step 6

Make sure no contacts or relays have failed. Relays for the primary motor or high-speed winding may be frozen closed, preventing the motor from responding to external control input.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Leaving-water temperature (LWT)
  • Fan speed (for VSDs) or motor status (for two-speed motors or dual motors). If the system uses multiple small fans, you might need to trend log the total kW demand or amperage.

While the wet-bulb temperature affects the cooling-tower LWT, most control systems do not have a sensor installed for it, so it is not included in this analysis.

Graph the listed parameters and see how the LWT and fan operation vary with respect to the EWT. Typically, the cooling load follows the OSAT, so the fan curve should have a similar profile to the EWT. The relative percent of the cooling load is denoted by the difference between the EWT and LWT as compared to the design differential.

For a VSD, the fan speed should vary to maintain the LWT at a single setpoint with no variation.

For a two-speed fan, the unit should shift between high and low speed to maintain the LWT within a certain operating range.

For a cell with a main and pony motor, operation should shift between them in a manner similar to a two-speed motor to maintain the LWT within a certain operating range.

Example of Normal Operation - VSD

The graph below shows that the LWT is maintained at a constant setpoint. The fan VSD responds to control logic and modulates as required. This illustrates normal, efficient operation of the system. If your graph looks like this, the problem should be solved.

Normal system operation with a VSD

Example of Normal Operation - 2-Stage Fan Operation

The graph below shows temperatures in a system that uses a 2-speed motor or a primary motor and a pony motor. The LWT is kept between 70 and 80 degrees. The fan controller responds to control logic and shifts between high and low as required. This illustrates normal, efficient operation of the system. If your graph looks like this, the problem should be solved.

Normal system operation with two-speed or dual motors

Example of Abnormal Operation - Fan at 100%

The graph below shows a fan operating at 100%. The time interval for this example is shorter than usual because the problem would occur on a very short cycle and could go undetected with 30-minute readings. The LWT is controlled by cycling the fan on and off instead of using the installed capacity-control scheme. The fan drives the LWT down to the lower allowable setpoint. If there is no lower setpoint and the LWT drops below the chiller low-temperature safety, the chiller will trip offline. If your graph looks like this, the problem has not been solved. Return to the inspection process.

Note: If your cooling tower fan is single-speed, then the graph below indicates normal operation. The fan cycles on and off in response to the water upper and lower temperature limit setpoints.

Example of a staged or modulating fan at constant speed

Labor Skills Required to Find and Resolve the Problem

  • DDC-system operator/programmer
  • Service mechanic
Cooling-tower fans are always on low, even in hot weather
  • Explanation: Cooling load demanded by chiller is low.
  • Problem: VSD control on cooling-tower fan is set to manual or is overridden.
  • Problem: Two-speed motor or pony motor has failed.
  • Problem: Relay contacts on the motor starter are frozen.
Dimmable lights do not dim under daylight conditions
  • Problem: Photocells are miscalibrated, damaged, or improperly installed.
  • Problem: Controls are set to manual or are overridden.
  • Problem: Low-voltage relay is frozen.
Equipment operates during unoccupied hours

Introduction

In most buildings with office-function occupancies (including hospitals with office areas), HVAC systems are scheduled to operate only at certain times. This includes a morning warm-up or cool-down period, plus normal occupancy. The systems are shut down after occupants leave the building at night. The occupancy schedule is usually dictated in the space lease agreement.

As you investigate this symptom, remember that each facility has its own unique HVAC system schedule. Some facilities have occupancy schedules that vary for each floor if they have floor-by-floor air handlers.

Buildings with large central systems tend to have strict occupancy schedules for all tenants to conserve energy. Otherwise, a system with several hundred horsepower worth of motors may operate for as few as 5 to 10 tenants who are working late.

Many tenants have terms in their leases that specify charges for using air conditioning after hours. This tends to minimize after-hours energy use because it affects the tenant's bottom line directly.

How This Wastes Energy

Operating HVAC systems in normal occupied mode when the facility is really unoccupied can waste energy in several ways. Electrical energy is wasted running fans and possibly central plant equipment. Also, the building can become overcooled or overheated to the point where the HVAC system must run more than necessary in the morning to compensate.

Possible Causes of This Problem

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: Equipment controls are overridden to allow or force operation after hours.
  2. Explanation: Equipment is operating to prevent system damage.
  3. Problem: Clock has not been reset after a power outage.
  4. Problem: System placed in manual mode at local starter.
  5. Problem: DDC system has a frozen contact that prevents equipment from being shut down at scheduled times.
  6. Problem: Setback temperatures for unoccupied hours are the same as, or close to, setpoints for occupied hours.

How to Find the Problem(s) by Inspection

Walk the entire facility both before and after normal occupancy hours to create a list of affected equipment.

Inspection Step 1

Examine the direct-digital-control (DDC) system to see if the HVAC-system control has been overridden to run after hours. A log should be kept to notify staff which tenants have requested after-hours HVAC and for how long, and to make sure they are properly billed for it. If equipment is operating with no after-hours requests, continue the inspection.

Inspection Step 2

Some conditions rightfully cause equipment to operate after hours. For example:

  • Some heating-water systems operate constantly to prevent the piping system from cooling down and springing leaks.
  • Some chilled-water pumps are activated to circulate water through coils to prevent freezing.

Inspection Step 3

Make sure the clock is set properly, whether you have a complex DDC system or simple time clocks. Check the status of the UPS or battery backup regularly.

Inspection Step 4

Make sure the local starters for each piece of equipment are in the automatic position. If not, find out who overrode them and why before resetting the starter.

Inspection Step 5

Command the equipment off via the DDC system to see if it shuts down. If it does not, a contact may be frozen, or a communications link is down. Have your DDC system technician investigate and replace any damaged parts.

Inspection Step 6

Make sure the setpoints for the unoccupied mode are properly entered. Sometimes the unoccupied setpoints are mistakenly set up as the occupied setpoints, or the two setpoints are very close. Therefore, the system can enter unoccupied mode and still need to operate to maintain that mode's setpoints. The end result is that the system seems to run continuously. Make sure realistic setback temperatures are programmed.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Status of any suspect equipment noted during the inspection process. It may not be feasible to trend log every piece of equipment.
  • Space temperatures where unoccupied setback temperatures are suspect.
  • For air handling units (AHU), trend the following:
    • Mixed-air temperature (MAT)
    • Supply-air temperature (SAT)
    • Outside-air temperature (OSAT)
    • Return-air temperature (RAT)
    • Damper position
    • AHU status
  • Any other relevant parameters such as the status of restroom exhaust fans.

Graph the trends for the suspect equipment and see how the equipment status varies as the system passes between occupied and unoccupied modes. Note that sometimes equipment runs for a while after it has been commanded to shut down. For example, chiller and boiler pumps typically operate for 15-30 minutes after they "shut down" to allow the temperature of the heat-transfer surfaces to equalize to prevent possible damage.

Example of Normal Operation

If your graph for an AHU looks like the figure below, your system is probably operating properly. (Note the noon-to-noon time scale.) The unit comes online in morning-warm-up mode at 6:00 with OSA at 0%. The space warms up from 66 to 72 during this period. The OSA damper opens at 8:00 when normal occupancy starts. MAT and SAT are controlled by the OSA damper at minimum position, with terminal units providing reheat as required. The unit shuts down at 20:00 with the OSA damper closing tight. The building temperature drifts down overnight gradually from 72 to 66 degrees.

Normal equipment schedule (noon-to-noon time scale)

Example of Abnormal Operation

If your graph for an AHU looks like the figure below (again, note the noon-to-noon time scale), something is causing the building to cool off rapidly enough that the setback-temperature setpoint has been reached and the AHU comes online to warm the building back up. Starting at 20:00, the building temperature drops rapidly from 72 to 60 degrees, which is the lower setpoint for heating during the unoccupied period. The AHU comes on at midnight for about two hours to warm the building back up to the upper limit of 68 degrees. Note that the OSA damper remains closed.

This condition could indicate a failure in the building envelope integrity (in which case your system is probably operating as intended, but in response to an abnormal load) or an exhaust fan is operating when it shouldn't. All building exhaust fans should be interlocked to shut down at the same time as the AHUs providing make-up air. In this case, the exhaust fan in the central restroom was not interlocked, so it created negative pressure in the building and bring in unconditioned air through any leaks in the building envelope. The lack of a motorized damper on the exhaust outlet can produce the same effect in buildings with more the three or four floors.

The rate of temperature decline in a tight building will depend on the envelope's thermal mass and insulation level.

Night setback activated due to an exhaust fan not being shut down (noon-to-noon time scale)

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
  • Electrician
Exterior doors are hard to open or don't close securely
  • Explanation: Envelope and/or floor-to-floor integrity is compromised.
  • Explanation: Exterior doors in another part of the building are open.
  • Explanation: Building is experiencing an unanticipated wind effect.
  • Problem: Seasonal stack effect is not under control.
  • Problem: Building is improperly pressurized (more exhaust than outside air or vice versa).
  • Problem: HVAC dampers are not operating properly.
  • Problem: Connection to other buildings is not under control.
Fan-powered VAV box runs continuously during unoccupied hours
  • Problem: Fan is in override via the DDC system.
  • Problem: Contacts on motor starter are frozen.
Heat exchanger on the steam system does not transfer enough heat.

Possible causes or explanations of this symptom. Use a printed copy as a checklist as you investigate.

  • Problem: Steam control valve has failed.
  • Problem: Temperature sensor is miscalibrated or improperly located.
  • Problem: Steam trap has failed or is closed, causing heat exchanger to flood.
  • Problem: Heat-exchanger-tube bundle or plate gasket have failed.
  • Problem: Scale has formed on heat-exchanger surfaces.
  • Problem: Water flow balance is too high or too low.
  • Problem: Heat exchanger is not properly sized for expected conditions.
Heating-water boiler operates when OSAT is above 60 degrees

Introduction

Heating-water systems typically have adjustable setpoints for outside-temperature "lockout." When the outside-air temperature (OSAT) exceeds this setpoint, boilers and pumps are disabled. Operating the heating-water boiler when the OSAT is above 60 degrees may waste energy.

How This Wastes Energy

Heating-water systems may be kept active in warm weather to reheat zones that are below their comfort setpoint. Terminal units may overcool their zones for a number of reasons discussed below. Reheating mechanically cooled air wastes both heating and cooling energy.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: A process load (e.g., domestic water heating) is connected to the system.
  2. Explanation: Hot water is circulated through distribution piping to prevent seals from leaking.
  3. Problem: OSAT lockout is not programmed or active.
  4. Problem: Operator has overridden OSAT lockout to overcome a heating problem in an HVAC zone.
  5. Problem: OSAT sensor is miscalibrated or improperly located. OSAT sensor could be a shared data point from another DDC controller and communication between controllers was lost.
  6. Problem: Contacts in control relays at the boiler control panel are frozen or burned.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect your design drawings and "walk" the piping loop of the heating-water system. A process load, like a domestic-water heater, may be connected to the system requiring constant, year-round hot water. There is no low-cost solution to this situation. A capital project may be required to install dedicated equipment to server the process load(s).

Inspection Step 2

When the system is shut down and allowed to cool, gaskets and other seals sometimes leak when the system is reactivated. Building operators often intentionally keep the system hot and active to prevent excessive leakage. This is sometimes considered a "low-cost" solution for the situation compared to reworking all the piping joints. Make sure this is not the case by talking with senior facility engineers before turning the system off. There is no low-cost solution to this situation. A capital project may be required to reseal the heating-water piping.

Inspection Step 3

Inspect the override logs of the direct-digital-control (DDC) system to verify that the lockout setpoint is operational and has not been turned off. Verify that the original setpoint is set as commissioned or as described in the operating plan.

Inspection Step 4

If the lockout setpoint has been overridden, determine the cause before correcting the problem. Some reasons for overriding may be:

  • A malfunctioning control valve on an air-handling unit (AHU) may be causing the unit to cool the supply air below its setpoint. Use the DDC system to inspect all the AHU supply-air temperatures (SATs) to determine if this is the case. If a unit is cooling the supply air below its setpoint, physically check the control valve to see if it is stuck open.
  • A terminal unit such as a variable-air-volume (VAV) box is oversized for the zone it serves and cools the space below its setpoint even at minimum primary airflow. Review the zone airflow requirements and determine if it is possible to rebalance the terminal unit. Otherwise, the unit needs to be replaced with a smaller unit.

Inspection Step 5

Inspect the OSAT sensor location and installation. Is it located where it provides an accurate reading? Check to see if it shares a DDC system point with another controller. If communication was lost between controllers, the reset logic may be using the last OSAT value transmitted. Some boiler installations have dedicated boiler control panels that regulate operations, including a lockout using a dedicated OSAT sensor. (Buildings with these controls may have a status and alarm point interfaced to the DDC system.) Inspect the sensor for proper location and calibration, and the local control panel for frozen contacts.

Inspection Step 6

Inspect the control relays at the boiler control panel for possible frozen or burned contacts.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • OSAT
  • Heating-water supply temperature (HWST)
  • Heating-water return temperature (HWRT)
  • Boiler status
  • Boiler circulating pump status (pressure or amps)
  • Terminal-unit heating demand (typically heating-water valve position).

Graph the HWST and HWRT with respect to the OSAT, boiler status, and pump status.

Example of Normal Operation

The graph below shows a reset schedule based on OSAT. As the OSAT nears the lockout temperature (60 degrees in this example), the differential between HWST and HWRT gets smaller. The heating-water temperatures should drop and equalize above the anticipated OSAT lockout setpoint. The boiler status should shift to off and the pump should stop about 15 to 30 minutes after the boilers shut down. The boiler also has a flow switch that must be energized before it can be started, so the pump must start before the boiler. If your graph looks similar to this, the system is operating properly.

Normal boiler operation

Example of Abnormal Operation - Reheat is Active

The graph below shows a heating system that operates when the OSAT is above 60 degrees because a terminal-unit-heating valve is calling for heating. The HWRT is 4 degrees below the HWST, which is larger than the temperature difference due to piping losses alone (typically 2 degrees). If there is a terminal-coil valve calling for heating, determine if the unit is cooling the space to below its setpoint. This sometimes occurs in an internal zone that has high occupancy but low heat load.

Abnormal boiler operation: reheat is active

Example of Abnormal Operation - Operator Override

The graph below shows a heating system operating when the OSAT is above 60 degrees, but with no terminal-unit heat load, as indicated by the 2-degree difference between the HWST and HWRT. Experience will tell what your specific temperature difference should be when the terminal unit is not providing heating. The operator also adjusted the minimum HWST up by 10 degrees to 130 degrees, possibly in an attempt to mitigate system leakage or to compensate for a frozen contact or relay. Go back to Inspection Step 1 to determine which problem is the cause.

Abnormal boiler operation: boiler stays active

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
Heating-water pump does not slow down sufficiently as heating load decreases
  • Explanation: Pump is undersized for the connected load.
  • Problem: VSD control is either in manual mode, overridden, or being bypassed.
  • Problem: Sensor for differential water pressure is miscalibrated or improperly installed.
  • Problem: Coil control valves do not follow coil loading.
  • Problem: The piping system has a short circuit.
Heating-water pump operates significantly more hours than boiler
  • Explanation: Load is below the minimum boiler capacity.
  • Explanation: Pump sequence calls for heating water to circulate periodically to maintain system integrity during the cooling season.
  • Problem: Pumps operate too long after boiler shuts down.
  • Problem: Pumps operate in manual mode at the local controller or are overridden in the DDC system.
  • Problem: Contacts on motor starter are frozen.
Heat pump starts and stops frequently
  • Problem: Refrigerant is short.
  • Problem: The heat pump is stopped due to a lack of condenser water flow, which triggers the high-pressure-limit switch for the refrigerant.
  • Problem: The heat pump is stopped due to the high temperature of water entering the condenser, which triggers the high-pressure-limit switch for the refrigerant.
  • Problem: Deadband for zone temperature is too narrow.
  • Problem: Cooling-water pump is cycling. Compressor cycles because a high-limit pressure sensor is being tripped.
  • Problem: Load is below the minimum heat pump capacity.
  • Problem: Compressor draws too much current due to wear and tear.
  • Problem: Control valve for condenser water has failed.
  • Problem: Control valve for condenser water opens too slowly.
Humidity is too high or too low
  • Problem: Multiple units are on line at once causing a lack of overall control.
  • Problem: Humidity sensor is miscalibrated or improperly located.
  • Problem: Envelope integrity is compromised.
  • Problem: HVAC system design does not match current load requirements, equipment is undersized or loads have increased or decreased.
Large amounts of steam are escaping the flash-tank vent
  • Problem: Heat-recovery heat exchanger lacks cooling medium.
  • Problem: Steam trap has failed or is open.
  • Problem: Orifice plates are eroding.
  • Problem: Steam-trap-bypass valve is open or leaking.
Mixed-air temperature is near outside-air temperature during cooling mode

Introduction

Elevated mixed-air temperature (MAT) can be a problem when the system is in cooling mode and the return-air temperature (RAT) is lower then the outside-air temperature (OSAT). If the system is working properly, the MAT should be closer to the RAT (which is cooler) than to the OSAT.

As you try to determine the cause of this symptom, remember that each facility has its own unique HVAC system-design criteria. Some facilities have air-side economizers based on dry-bulb temperature and some economizers controlled by enthalpy or humidity. Other facilities have HVAC systems with fixed outside-air (OSA) quantities.

There is no set standard for how much outside air should be taken in at the minimum damper position. Each system has its own requirement based on associated exhaust systems and pressurization needs.

This analysis would not hold true in a VAV system if the MAT is near the OSAT when it is cold out. A system's design airflow rate is based on cooling, not heating. Therefore, in heating mode the system will require very little total airflow, so the OSA percentage is higher than it is in cooling mode. Under this condition the MAT can even be equal to the OSAT.

How This Wastes Energy

An elevated MAT can force the cooling system to use more energy to reach the required supply-air temperature (SAT).

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: System is controlling humidity.
  2. Explanation: System is designed to have a high percentage of outside air.
  3. Problem: Air in the mixed-air plenum is stratified.
  4. Problem: Setpoints for supply-air temperature are overridden.
  5. Problem: Damper or actuator has failed.
  6. Problem: Return-air fan is not on or not operating at expected conditions.
  7. Problem: Return air path is restricted by a closed mechanical or fire damper.
  8. Problem: The static-pressure sensor has failed, preventing the return fan from providing enough return air.

How to Find the Problem(s) by Inspection

Inspection Step 1

Hospitals and large computer/printing spaces may have HVAC systems with humidity control, so their economizers, if present, are usually controlled by enthalpy and not dry-bulb temperature. Because an enthalpy economizer looks at the total heat present in the air, an elevated MAT may not necessarily be a problem. Make sure the enthalpy controller is properly calibrated. If it is properly calibrated and OSA relative humidity is low, you still have a problem so continue the inspection.

Inspection Step 2

Inspect the design documents to determine how much OSA the unit should be pulling in. Sometimes comfort systems are also used as make-up air for certain process-exhaust loads such as in kitchens, laundries, and locker rooms. If this is the case, your system could be operating properly. Continue the inspection just to be prudent.

Inspection Step 3

Make sure the temperature sensor within the mixing box it is not located directly in front of the OSA louver. The mixed air also must be thoroughly blended to prevent stratification in the mixing plenum. Temperature sensors with long sensing tubes that traverse the entire mixing-box outlet can help you accurately measure temperature. Special fans can also be used to mix the air.

Inspection Step 4

Inspect the override logs of the DDC system to verify that the MAT, SAT, space-pressure setpoint, or return-fan speed-control logic have not been altered. If the setpoints have been overridden, find out who changed them and ask why before resetting them. Determine the original intended setpoints, such as minimum OSA, either as designed or as specified in the operating plan. If the reason these settings were overridden has been resolved, you can reset the system to the proper setpoints.

Inspection Step 5

Inspect the mixing-box dampers of the air-handling unit (AHU) for proper operation and position. Return-air (RA) and OSA dampers should force the air directly toward each other and produce well-mixed air. Test linkages and dampers for binding problems.

Inspection Step 6

Check the static pressure of the mixing box. Is the access door harder to open than normal? If so, make sure the return fan is operating properly. The supply fan might be trying to pull all its supply air through the OSA damper.

Inspection Step 7

If the return-air fan is rotating in the correct direction but not providing adequate airflow, check the upstream return-air ducting for a closed fire damper or obstruction. Internal insulation sometimes separates from the duct and gets caught in dampers or turning vanes. Inspect the zone or building pressure with respect to the adjacent zone or outside. Check the building pressure at both the street and the roof levels. Normally, the building pressure should be slightly positive at the street level. If it is either negative or excessively positive, check the other fan systems serving the same area. Review the alarm log to see if any other fans have tripped offline. It is likely that a fan is not functioning somewhere.

Inspection Step 8

Inspect the static-pressure sensor for proper calibration. If it is miscalibrated or has failed, the return fan will not provide enough return air to the supply fan.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Return-air temperature (RAT)
  • Mixed-air temperature (MAT)
  • Supply-air temperature (SAT)
  • Economizer damper position, if positive feedback is available.

Graph the temperatures and see how the MAT varies with respect to the SAT, the RAT and the OSAT. See if the MAT curve changes slope when the dampers go to minimum position.

Example of Normal Operation

The graph below shows that MAT tracks RAT closely (OSA damper set at 20%) when the OSAT is warmer. When the OSAT is cooler than RAT, the MAT should equal the OSAT (100% economizer cooling). This illustrates normal, efficient operation of the system. If your graph looks like this, the problem should be solved.

Normal System Operation

Example of Abnormal Operation - Temperature Problem

The graph below shows that MAT tracks OSAT closely (OSA damper set at 80%) when the OSAT is warmer than the RAT. If your graph looks like this, you could be drawing in too much OSA. You will need to start the process over. If the graph profile changes over time, another fan system may be affecting the space with the problem. It could also be caused by external wind pressure, which is discussed below.

Example of High MAT

Trend log the following additional points:

  • Supply- and return-fan status
  • Supply- and return-fan speed, if on a variable-speed drive (VSD). (Make sure that you trend the VSD output and not the DDC-system output signal to the VSD.)
  • The same points on adjacent AHUs, if they serve the same area.
  • Building static pressure with respect to the exterior.

Graph the parameters and see how the building pressure varies with respect to each fan's operation. Look for variations in the building differential pressure with respect to OSAT, fan operation, and general wind conditions. A pressure problem can be caused by an HVAC fan, wind, or temperature differential between the lobby and top floor (known as stack effect).

Example of Normal Operation - Controlled Building Pressure

The graph below shows normal operation of the building system with controlled building pressure under all conditions. If your graph looks similar to this, the problem has been resolved. Consider adding alarm setpoints on your building pressure so that in the future you will be notified of the problem before it affects energy usage.

Normal Operation with Controlled Building Pressure

Example of Abnormal Operation - Pressure Problem

The graph below shows how the building pressure becomes negative due to the stack effect as the OSAT increases. Excessive air will be pulled through the OSA damper, increasing the MAT. OSA can also be pulled in through perimeter doors, and even through curtain walls in certain cases. You have either a fan-control problem or a large break in the building envelope at the roof. Inspect the roof area for open doors, or open isolation dampers in pressurization fans if the building is a high-rise.

Abnormal Operation with Negative Pressure

Labor Skills Required to Find and Resolve the Problem

  • DDC-system operator/programmer
  • Service mechanic
  • Architect, if envelope problems are involved
Mixed-air temperature is near outside-air temperature during heating mode

Introduction

A depressed mixed-air temperature (MAT) can be a problem when the system is in heating mode and the return-air temperature (RAT) is higher then the outside-air temperature. The MAT should normally be closer to the RAT (which is warmer) than to the outside-air temperature (OSAT).

How This Wastes Energy

A depressed MAT causes the heating system to expend more energy to reach the required supply-air temperature (SAT). The heated return air is wasted to the outside.

Possible Causes of Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: System is controlling humidity.
  2. Explanation: HVAC system demand is low so airflow is low and a high percentage of OSA is required to maintain minimum OSA requirements.
  3. Explanation: System is designed to have a high percentage of outside air at full load.
  4. Problem: Setpoints for supply-air temperature are overridden.
  5. Problem: Temperature sensor for mixed air is miscalibrated or improperly located.
  6. Problem: Damper or actuator has failed.
  7. Problem: Return air fan is not operating under conditions at which it should. Return air path may be restricted by a closed mechanical or fire damper.
  8. Problem: Building is not properly pressurized because a fan in one of the two (or more) AHUs serving a space has failed.

How to Find the Problem(s) by Inspection

Inspection Step 1

Review the design criteria for the system to make sure the air-handling unit (AHU) is not being controlled by humidity requirements. This can cause outside-air (OSA) to become a much larger fraction of mixed air than what you would normally expect in an office HVAC system.

Inspection Step 2

Review the test-and-balance reports to make sure the system does not have a high OSA requirement, for example, to supply make-up air for a process-exhaust system. Sometimes an office AHU is designed to accommodate the make-up air for a small kitchen hood, or maybe a locker-room-exhaust system.

Inspection Step 3

If it is a VAV system, look at the system load as a percentage of its design capacity. As the system airflow is reduced, the percentage of OSA increases even though more air is not being drawn in. In a VAV system with 20% OSA that is operating at 20% of its design flow, MAT can actually equal the OSAT. If this is the case, make sure there is some type of freeze protection installed (glycol in the water-based system(s) or a freezestat) to prevent breaking coils.

Inspection Step 4

Inspect the override logs of the DDC system to verify that the supply-air setpoints have not been altered. Determine the original setpoints as commissioned or as specified in the operating plan. If the setpoints have been overridden, find out why before correcting the problem. Some reasons for override may be:

  • A process load is connected to the system requiring a constant low SAT.
  • An AHU cannot meet its zone loads at higher SAT due to an air-balance problem.

Inspection Step 5

Make sure the MAT sensor is installed and calibrated properly. If the sensor is not located properly in the air stream, it could provide incorrect readings to the DDC system. The mixed air must be thoroughly blended to prevent stratification in the mixing plenum. Temperature sensors with long sensing tubes that traverse the entire mixing box outlet can help overcome this issue. Special fans can also be used to mix the air.

Inspection Step 6

Inspect the dampers and their actuators. Cycle the dampers via the DDC system and make sure they are responding properly and do not bind.

Inspection Step 7

Check the pressure in the mixed-air plenum. Is the door harder to open than normal? If so, there may be a problem with the return-air fan or path that is preventing return air from entering the mixing box. This will force the supply fan to pull in more OSA through the OSA dampers, increasing the negative pressure in the mixing box.

Inspection Step 8

Check the zone or overall building pressure with respect to the adjacent zone or outside. Check the building pressure at both the street and the roof levels. Normally, the building should be slightly positive at the street level. If it is either negative or excessively positive, check the other fan systems serving the same area. It is likely that a fan is offline somewhere.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Return-air temperature (RAT)
  • Mixed-air temperature (MAT)
  • Supply-air temperature (SAT)
  • Economizer damper position, if positive feedback is available.
  • Status of the supply and return fans, and fan speed if on a VSD. (Be sure to trend log the VSD output and not the DDC system output signal to the VSD.)
  • The same points on adjacent AHUs, if they serve the same area.
  • Building static pressure with respect to the exterior (differential) and OSAT.

Graph the temperatures and see how the MAT varies with respect to the SAT, RAT, and OSAT. See if the MAT curve changes slope when the dampers go to minimum position. Graph the other parameters and see how the building pressure varies with respect to each fan's operation. Look for variations in the building differential pressure related to OSAT, fan operations, and the general wind conditions. A pressure problem can be caused by an HVAC fan, wind, or temperature differential between the exterior and interior, known as stack effect.

Example of Normal Operation Based on Temperature

The graph below shows the MAT tracking 2 degrees below the SAT. (The 2-degree rise due to fan heat is normal.) The OSA damper modulates as required to maintain the 58-degree MAT. This graph illustrates normal, efficient operation of the system. If your graph looks like this, the problem should be solved.

Normal operation: normal mixed-air temperature

Example of Abnormal Operation Based on Temperature

The graph below shows the MAT and SAT tracking the OSAT, with only minimal offset due to the RAT. If your graph looks like this, you might be drawing in too much OSA, unless it is a VAV system operating at minimum fan speed. In the latter case, the reheat system at the VAV boxes must compensate for the heat wasted from the return air. If the graph profile changes over time, another fan system may be affecting the space being served by this fan system. It could also be caused by external wind pressure, which is covered in the second trend log task. Recheck the damper actuators for proper response to a MAT command from the DDC system.

Abnormal operation: low mixed-air temperature

Example of Normal Operation Based on Pressure

The graph below shows normal operation of the building system with controlled building pressure over all conditions. In this case, the return-fan speed is controlled by the supply-fan speed with a predetermined offset. The building pressure in the morning is below the setpoint of +0.05" wg due to the reduced amount of OSA introduced. If your graph looks similar to this, the problem has been resolved. Consider adding alarm setpoints on your building pressure so you will be notified in the future before the problem affects energy use.

Normal building pressurization

Example of Abnormal Operation Based on Pressure

The chart below shows abnormal operation of the system. In this case, the return-fan speed is still controlled by the supply-fan speed with a predetermined offset. Note the difference in pressure range on this graph compared to the previous graph. In this case, the building can barely achieve a positive pressure. In this graph, the building pressure is negative at the start of the day due to stack effect and the small amount of OSA being introduced. As the OSAT rises, the fans increase speed bringing in more OSA which increases building pressure. The rise in OSAT also reduces the stack effect. At 10:00, the pressure equalizes, so perimeter doors will no longer let OSA into the building.

The minimum return-fan speed is most likely set too high and, when combined with the building exhaust system or leakage at the roof, the airflow out exceeds the OSA brought in by the supply fan. Return to Inspection Step 8.

Abnormal building pressurization

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
  • Architect, if envelope problems are involved
Multiple boilers operate when load is low
  • Problem: Boiler control schedule is malfunctioning or is overridden via the control system.
  • Problem: Deadband for bringing on the next boiler is too narrow.
  • Problem: Large temperature fluctuations cause another boiler to start.
  • Problem: Relay contacts are frozen.
  • Problem: Piping is configured such that a water flow at a single pump activates multiple flow switches that activate separate boilers.
  • Problem: Temperature sensor for hot-water supply is miscalibrated or improperly located.
Multiple chillers operate when load is low
  • Problem: Chiller control schedule is malfunctioning or is overridden via the control system.
  • Problem: Deadband for bringing on next chiller is too narrow.
  • Problem: Large temperature fluctuations cause another chiller to start.
  • Problem: Relay contacts are frozen.
  • Problem: Piping is configured such that a single pump activates multiple flow switches that activate separate chillers.
  • Problem: Temperature sensor for chilled-water supply is miscalibrated or improperly located.
One pump is on when two are needed, load is not met
  • Problem: Pump is locked out via the DDC system.
  • Problem: Pump is locked out locally.
  • Problem: Motor has failed.
  • Problem: Pump has failed.
  • Problem: Motorized valve has failed, end switch fails to activate the pump starter.
  • Problem: Flow imbalance causes one pump to deadhead and shut down due to amperage overloading.
Only one boiler is on when two are needed, load is not met
  • Problem: Second boiler is locked out via the DDC system.
  • Problem: Failed flow switch prevents second boiler from starting.
  • Problem: Boiler fails to start because a failed or closed valve prevents the water-flow switch from making contact.
  • Problem: Natural gas is not turned on at second boiler.
  • Problem: Pump has failed.
  • Problem: Flow is imbalanced.
Only one chiller is on when two are needed, load is not met
  • Problem: Second chiller is locked out via the DDC system.
  • Problem: Failed flow switch prevents second chiller from starting.
  • Problem: Chiller fails to start because a failed or closed valve prevents the water-flow switch from making contact.
  • Problem: Pump had failed.
  • Problem: Flow is imbalanced.
Outside-air dampers do not close to minimum setting during hot or cold weather

Introduction

Air-side economizers are one of the most effective methods to conserve energy. They allow outside air to be used for cooling when temperatures are favorable.

There are typically two methods for economizer control: dry-bulb temperature or enthalpy. Enthalpy economizing is only used when humidity must be controlled, such as in hospitals or where the outside air is excessively humid. The controller measures the difference between the enthalpy of the outside air (OSA) and return air (RA), and selects the one with the lowest value. In the Pacific Northwest, economizer control is most often based on dry-bulb temperature.

Anytime the outside-air temperature (OSAT) is below the return-air temperature (RAT), economizing is possible. If the OSAT is above the RAT, then the economizer dampers should close to the minimum OSA setting.

On the colder side of the range, OSA also drops to a minimum flow when the mixed-air temperature (MAT), or supply-air temperature (SAT), cannot be maintained at setpoint.

Minimum damper positions, which relate to minimum OSA flow rate, can be established using several different criteria:

  • Damper position is fixed with the system at full flow, permitting the minimum design amount of OSA to enter the air-handling unit (AHU). In a variable-air-volume (VAV) system, the amount of air will vary with fan speed but the damper position will remain the same.
  • Damper position is controlled by an air-flow-monitoring station. This method provides constant OSA flow regardless of fan speed.
  • Damper position is controlled by a carbon-dioxide (CO2) sensor. The damper position is controlled to keep the CO2 concentration below a preset level. Depending on the occupancy in the space, the dampers may close to allow less OSA than the first two options, and are only limited by the need to maintain space pressure at the designed level.
  • Damper position is based on the thermal balance between the RAT and OSAT, which mix to produce the MAT. The damper is adjusted to keep the MAT below the RAT by 20% of the temperature difference between the OSAT and RAT. If the RAT is at 80 degrees, and the OSAT is at 30 degrees, setting the dampers to provide 20% OSA would produce a MAT of 70 degrees:

    RAT - 0.20×(RAT - OSAT) = MAT

    80 - 0.20×(80 - 30) =

    80 - 0.20×50 =

    80 - 10 = 70

  • Damper position is controlled by building pressure. The economizer can be used to control building pressure. This type of control logic is found usually in high-rises or buildings with frequently used envelope penetrations such as overhead doors. The amount of OSA is usually well beyond the minimum required by local codes for nonsmoking environments.

Another option is to have a dedicated OSA damper separate from the economizer dampers. This damper has two positions, for occupied and unoccupied periods. This allows the economizer dampers to cycle from fully open to fully closed, avoiding uncertainties associated with how much flow is passing through a preset minimum damper position.

How This Wastes Energy

Excess OSA through an AHU forces the unit coils, in both cooling and heating, to compensate and use more energy to achieve the desired SAT setpoint.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: Damper position is controlled by a signal and not the actual damper position.
  2. Problem: Damper or temperature setpoints are overridden via DDC system.
  3. Problem: Damper linkage or actuator has failed.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect the design drawings to determine which of the above methods is used to control the OSA dampers. If your dampers are fixed and can't modulate, you may not have a problem. See below for a description of how to verify correct operation with trend logs.

Inspection Step 2

Inspect the DDC system controls to see if the minimum damper setting has been overridden, possibly to compensate for another problem or condition. Determine who made the change and why before resetting the damper back to its original mode. Some possible reasons are:

  • The damper position has been changed to allow more OSA in order to flush out an area or suite that has just been renovated with new carpet and paint.
  • A remodel within the facility has additional exhaust requirements such as an exhaust hood or locker rooms, and OSA was increased in order to maintain overall building pressure.
  • The minimum OSA flow rate was increased to respond to a localized complaint about indoor air quality (IAQ) instead of expending capital to modify or replace ducts or terminal units.

Inspection Step 3

Inspect the damper itself and the actuator. Make sure there are no physical obstructions in the damper blades. Have your DDC operator cycle the damper while observing its operation. The fan system should be operating when this is being done to simulate actual operating conditions. Due to pressure differentials imposed on the blades, a bound-up damper linkage or an actuator can prevent a damper from fully cycling while the system is operating. This may not be evident when the unit is down and there is no differential pressure across the damper blades.

Inspection Step 4

Check when the sensors affecting the damper controls were last calibrated. They may be OSAT, MAT, SAT, or CO2 sensors. Also make sure each sensor is located appropriately. You can quickly check for an obvious problem by using a handheld thermometer and comparing the value to the control-system reading.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Mixed-air temperature (MAT)
  • Supply-air temperature (SAT)
  • Return-air temperature (RAT)
  • OSA damper position

Graph the OSA damper position with respect to the other four points.

If the OSAT varies enough to allow the economizer to shift between free cooling and minimum position, look for repetitive cycles in the damper position based on OSAT.

Verify that the OSA damper closes fully when the AHU is scheduled off. Conversely, verify that the OSA damper opens properly when the AHU is scheduled on. Typically, this damper action should only occur when the fan is not running. Trying to fully close, or open, a damper against normal fan pressure can exert excessive force on the linkage and actuator, causing failure.

Example of Normal Operation

The graph below illustrates normal, efficient operation of the damper system. The damper modulates its position as required by the control logic to maintain the MAT roughly 2 degrees below the SAT, which is attributable to fan heat.

Normal damper operations

Example of Abnormal Operation

The graph below illustrates possible abnormal operation with the damper fixed at 80% OSA. The MAT and SAT track the OSAT closely, offset by only about 5-7 degrees. If your graph looks similar to this, you need to re-inspect the system starting at Inspection Step 1.

Abnormal operation with damper at 80% OSA

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
Parking-garage fans do not shut off or slow down as expected

Introduction

Each facility has its own unique garage ventilation system. There may be multiple floors with matching fans or a single fan controlled by a single carbon monoxide (CO) sensor. Review the as-built construction documents to familiarize yourself with your particular system's design, then inspect the facility for compliance with the design. You should conduct the inspection during occupied hours, preferably mid-morning when CO levels are low, and during the afternoon exodus, when there are a large number of vehicles running at the same time.

Fan control may be any of the following types:

  • Constant speed
  • Two-speed motor
  • Variable-speed drive

Acceptable CO levels in a parking garage are typically10 parts per million (ppm) or less, depending on location. High CO levels in outside air (OSA) will elevate that accordingly. (Seattle has an average of 4 ppm in its downtown district, according to the Puget Sound Clean Air Agency (PSCAA).) Above 10 ppm, exhaust fans should either start or speed up above the baseline. The allowable peak concentration is 35 ppm in an 8-hour period. Most building occupants spend very little time in a parking garage but attendants and valets can spend the entire day there, so levels should be kept below 10 ppm whenever possible. These values are examples; check your local codes for specific requirements in your area.

How This Wastes Energy

Excessive operating hours, or running fans at elevated speeds when CO levels are within acceptable limits, wastes fan energy and reduces the lifespan of the equipment.

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable requirement of your system's current condition or configuration that would require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Problem: A car is running near the sensor.
  2. Problem: CO sensor is miscalibrated or improperly located.
  3. Problem: CO setpoint is overridden via the DDC system.
  4. Problem: VSDs are in manual mode, are being bypassed, or are in smoke-removal mode.
  5. Problem: Outside air intake is introducing CO, causing elevated CO levels.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect the CO sensors visually to assure they are installed in an appropriate location and there is not a parked car running near a sensor.

Inspection Step 2

Inspect the maintenance logs to see when the CO sensors were last calibrated. Check the uniformity of readings with an empty garage. A test gas with specific carbon monoxide levels can be used to test and calibrate sensors.

Inspection Step 3

Inspect the override logs of the direct-digital-control (DDC) system to verify that the operating schedule and setpoints have not been altered. Alternatively, check the time clock for proper pin settings if no DDC system is present.

Inspection Step 4

Check the fans' variable-speed drives (VSDs) (or starter if constant speed), to ensure that they are in automatic mode. They may be in local mode with a specified speed programmed. Verify that they are not placed in bypass mode if a bypass is present. If the latter two conditions exist, make sure you know the reason before returning it to automatic mode. Make sure the VSD or starter is not accessible to the public. A lockable wire-mesh cage allows protection as well as visibility during weekly walkthroughs.

Inspection Step 5

Check the OSA-intake location to verify that elevated CO levels are not caused by a vehicle with its engine running at the loading dock. OSA intakes for below-grade parking lots must often be located at street level due to architectural and structural restrictions. This may cause random spikes in your CO levels that will typically last from 5 to 30 minutes, sometimes longer. If they occur at roughly the same time during the day, it may be easy to determine the cause.

How to Confirm the Problem(s) by Trend Logging

Trend log the following data points if they are connected to the DDC system. If your system has standalone controls that are not tied into your DDC system, you will need data loggers to take trend logs.

  • Exhaust-fan speed
  • CO level at each sensor
  • CO level at the OSA intake

Graph the data and see how the fan speed or run time varies with respect to the CO levels.

Example of Normal Operation

The graph below illustrates normal, efficient operation of a system with the fan running at 20% of speed for good ventilation during occupied hours. If your graph looks similar to this, the problem should be solved.

Normal operation

Example of Abnormal Operation

The graph below illustrates abnormal, inefficient operation of the system with the fan running at 60% of speed during occupied hours, regardless of CO level. Due to the constant airflow from continuous fan operation, the CO level increases only slightly during the peak exiting period. If your graph looks like this then the fan control has been overridden either in the control programming or locally at the VSD.

Abnormal operation with constant fan speed

Labor Skills Required to Find and Resolve the Problem(s)

  • DDC system operator/programmer
  • Service mechanic
Perimeter heating is excessive
  • Problem: Supply-air temperature is low.
  • Problem: The reset schedule for supply-air temperature is absent or inactive.
  • Problem: Outside-air temperature is below design temperature.
Position of modulating damper does not change for a long time

Introduction

Modulating dampers are most often used in air-handling units (AHUs) to control an economizer cycle for free cooling. They modulate to maintain some preset temperature for optimum energy efficiency. They can also be used for controlling space pressure on a floor or in an entire building. This may be at an entrance to a return airshaft or even as discharge dampers on an exhaust fan for low-pressure control applications.

As you determine the cause of this symptom, remember that each facility has its own unique HVAC system design criteria. Air-side economizers can be controlled by dry-bulb temperature, enthalpy, or humidity, depending on the needs of the facility. Other facilities have HVAC systems with fixed outside-air quantities and do not use modulating dampers.

How This Wastes Energy

A failed or static modulating damper can prevent air-side economizers from operating properly for maximum energy efficiency. Outside-air (OSA) flow can be either too high or too low. It can also have a negative effect on building pressure, allowing for wind or stack pressure to force high levels of infiltration where it is not desirable. Either condition can also cause noticeable problems with indoor air quality (IAQ).

Possible Causes of This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Problem: DDC controls are overridden.
  2. Problem: Damper linkage is bound.
  3. Problem: Actuator has failed.
  4. Problem: Temperature sensor for mixed air is miscalibrated or improperly installed.
  5. Problem: Sensor for differential air pressure is miscalibrated or improperly installed.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect the direct-digital-control (DDC) system controls to see if they have been overridden, possibly to compensate for another problem or condition. Find out who overrode these controls and why before resetting the system back to automatic mode. Some possible reasons are:

  • Sometimes the economizer on an AHU is manually set to 100% OSA to flush out an area or suite that has just been renovated with new carpet and paint.
  • In new building construction that follows Leadership in Energy & Environmental Design (LEED) standards, this can occur for several weeks before occupancy to flush out all volatile organic compounds that off-gas from man-made materials.
  • Contractors sometimes use the building AHUs at 100% OSA and full reheat to dry gypsum wallboards and remove excess moisture from the plastering compound. LEED standards provide guidelines for this.

Inspection Step 2

Inspect the damper linkage while the system is operating. Make sure getting into the mixing box is easy. Do not try to force open an access door against high pressure; you could get trapped inside. Have your DDC operator cycle the dampers while you observe the action. Make sure all shafts rotate easily, and replace linkages and bushings as required. Inspect damper-blade seals and replace them if they are worn or damaged.

Inspection Step 3

Replace any failed or underpowered actuators. Underpowered actuators cannot modulate the dampers under normal conditions due to a lack of torque. Make sure the actuator can modulate continuously and is not just 2-position. Sometimes actuators are simply replaced with what is on hand and not the correct unit.

Inspection Step 4

Check the location and calibration of the mixed-air sensor. Make sure the air in the mixing box is not stratifying. Typically, OSA should enter from the top and warm return air (RA) should enter from the end or side to assure good mixing. Parallel dampers are typically used in mixing boxes. The OSA and RA blades should open pointing toward each other for good mixing.

Inspection Step 5

If your economizer dampers use pressurization logic, make sure the sensor is calibrated and appropriately located. A sensor that reads low or is subject to local exhaust conditions can keep the OSA damper open wider than it should or in a fixed condition.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Return-air temperature (RAT)
  • Mixed-air temperature (MAT)
  • Supply-air temperature (SAT)
  • Economizer damper position, if positive feedback is available

Graph the temperatures and see how the MAT varies with respect to the SAT, RAT, and OSAT.

The graph below illustrates normal, efficient operation of the damper system. The damper modulates as required in order to keep the MAT 2 degrees below the SAT. If your graph looks like this, the problem should be solved.

Normal modulating damper operation

If your graph looks like the one below, your dampers are not modulating properly. The dampers could be stuck at any percentage, not just at the 60% shown in this graph. The important symptom to note is that the damper does not change position as the OSAT changes. You will need to re-inspect the system.

Abnormal operation: modulating damper stuck at 60%

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
Pressure-relief valve on the hydronic system pops
  • Problem: There is excessive heat transfer from the steam system.
  • Problem: Relief valve is set improperly.
  • Problem: System expansion tank is flooded and has no room for added volume when temperature is increased.
  • Problem: Residual system pressure is set too high, relief valves lifts when system pump starts.
Pressure in the steam system cycles
  • Problem: Boiler-firing control has failed.
  • Problem: Pressure-reducing valve has failed.
  • Problem: Relief valve has failed Problem Large load has a cycling-control valve.
Program logic for optimal system start is not working, times do not vary
  • Problem Optimal-start program is overridden to address occupant or process needs.
  • Problem Control logic is improperly programmed.
Return and/or exhaust fan does not shut off with supply fan
  • Problem: Interlock is overridden via the DDC system.
  • Problem: Local VSD or starter is in manual mode.
  • Problem: Relay contacts are frozen.
Simultaneously high hot deck temperature and low cold deck temperature

Introduction

This condition can exist in two types of older HVAC systems-multizone and dual-duct. These systems have been generally prohibited by energy codes for several years and are typically found in older buildings.

Typically, the air stream is split into two separate ducts and either heated or cooled by a coil. The air is then blended, right at the unit outlet in a multizone system or at the terminal unit in a dual-duct system. The coils are designed to operate with no water-flow control.

A multizone or dual-duct single-fan system has a single mixed-air temperature (MAT) that is routed to both decks in a blow-through configuration, while a dual-duct dual-fan has two MATs, one for the hot deck and one for the cold deck. These are typically draw-through configurations.

In a dual-fan arrangement, there are two economizers, one for each deck. These would need to be analyzed much as a single-duct system would be, to make sure that proper outside-air (OSA) levels and temperatures are maintained.

To complicate matters, these systems can have variable-speed drives (VSDs) which, if properly programmed, can save even more energy.

How This Wastes Energy

Air flows can get out of balance over time due to alterations and changes in internal heat loads. The fast and easy solution is to either raise the hot-deck temperature, or drop the cold-deck temperature to overcome the problem. The farther apart these temperatures are, the more energy is wasted in both the heating and cooling systems due to excessive blending at the terminal units.

Possible Causes of This Problem

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable requirement of your system's current condition or configuration that would require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Step:

  1. Explanation: DDC system setpoints or reset schedules have been overridden.
  2. Problem: Temperature sensor is miscalibrated or improperly located.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect the override logs of the direct-digital-control (DDC) system to verify that the setpoints or reset schedules have not been altered. Verify the original setpoints as commissioned or as specified in the operating plan. If the setpoints have been overridden, find out why before correcting the problem. Some reasons for overriding may be:

  • A process load is connected to the system requiring a constant temperature. This can drive either the hot-deck temperature up or the cold-deck temperature down.
  • The system can't meet its normal zone loads at design deck temperatures due to an air balance problem or a physical change in a zone, such as increased occupancy or IAQ complaints. Even a change in adjacent landscaping can have a large affect on solar heat gain in a perimeter zone.
  • Inspect the plan for recent changes in air distribution at the zone levels.

Inspection Step 2

Inspect the deck-temperature probes for calibration and proper location.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • Outside-air temperature (OSAT)
  • Hot-deck supply-air temperature (HDSAT)
  • Cold-deck supply-air temperature (CDSAT)
  • Return-air temperature (RAT)
  • OSA damper position

Graph the data. Look for similarities in the curves. If the HDSAT or CDSAT is constant, the reset schedule may be overridden.

Example of Normal Operation

The graph below shows efficient operation of a multizone (single-fan) system. Both the CDSAT and HDSAT are reset based on OSAT. As a result, the hot deck provides a 25-degree rise across the coil. If your graph looks like this, the problem should be solved.

Normal operation with SAT reset logic

Example of Abnormal Operation

The graph below shows the CDSAT held steady at 55 degrees with no reset, which forces the HDSAT higher to compensate. The economizer cycle is operating properly keeping the MAT 2 degrees below the CDSAT. If your graph looks like this, your multizone (single-fan) system is probably wasting considerable energy.

Operation with no SAT reset logic

A dual-duct-dual-fan system is analyzed in the same way, except there is a separate MAT for the hot deck, as well as a VSD if it is a variable-air-volume (VAV) system.

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
Space calls for cooling and VAV box damper is at minimum position
  • Explanation: HVAC unit is in heating mode.
  • Problem: Local thermostat thumbwheel is overridden or has a limited span.
  • Problem: Thermostat is not located in the zone the VAV box serves.
  • Problem: Damper is overridden via the DDC system.
Space temperature does not set back or set up during unoccupied hours
  • Problem: Space-temperature resets are overridden or not present.
Space is too warm or cool
  • Explanation: Setpoints are based on the needs of a process load and not on occupant comfort.
  • Explanation: Occupants are located in under- or over-conditioned pockets or their metabolism is non-standard.
  • Problem: Thermostats are improperly located.
  • Problem: Air is not distributed or balanced properly.
  • Problem: An uninsulated floor or ceiling is adjacent to an unconditioned space.
  • Problem: Open windows are producing cross-flow ventilation.
Static pressure in duct stays significantly higher than setpoint
  • Problem: Fan control logic is improperly programmed.
  • Problem: Operator has manually overridden fan control to compensate for a lack of airflow in one HVAC zone.
  • Problem: Sensor for differential air pressure is miscalibrated or improperly installed.
Static pressure in duct varies significantly during occupied hours
  • Explanation: Reset logic for static pressure in duct is active.
  • Problem: Control-loop feedback is too sensitive and the system hunts.
  • Problem: Large VAV box is cycling, causing system to follow.
  • Problem: Primary duct leaks excessively.
Steam pipe is hammering
  • Problem: Steam piping is not properly sloped for drainage.
  • Problem: Steam trap has failed or is closed.
  • Problem: System warms up too rapidly.
  • Problem: Steam trap is undersized.
  • Problem: Steam trap is the wrong type and is unable to handle startup loads.
  • Problem: Steam trap bypasses are not open during startup.
Supply-air temperature is constant at all hours, even with reset controls in place
  • Problem: Reset-control logic is overridden.
  • Problem: Reset-control logic is improperly programmed.
Supply-air temperature is high during warm weather
  • Explanation: HVAC zone may require high supply-air temperature to compensate for an envelope problem, e.g., no insulation or leaky wall.
  • Explanation: A process exhaust load is served by the system and requires warm air to prevent cooling below the setpoint.
  • Problem: DDC has no control over AHU setpoints or supply-air temperature cannot be reset.
  • Problem: Setpoint of supply-air temperature has been overridden.
  • Problem: Temperature sensor is miscalibrated or improperly located.
Temperature of combustion exhaust is significantly higher than 300 degrees
  • Problem: Too much air is entering the combustion chamber (combustion-air register is improperly set or air leaks into firebox).
  • Problem: Water-side has scaling.
  • Problem: Gas-side is fouled.
  • Problem: Absence or failure of feedwater (heating-water) heat recovery.
  • Problem: Absence or failure of heat recovery from combustion air.
  • Problem: Burner orifice is improperly sized.
Temperature of condensate return is high
  • Problem: Steam trap has failed or is open.
  • Problem: Orifice plates are eroding.
  • Problem: Steam-trap-bypass valve is open or leaking.
Temperature of condenser water is significantly higher than setpoint, even during cool weather
  • Problem: Logic for temperature control and 3-way bypass is not coordinated.
  • Problem: Cooling-tower fan control is set in manual or is overridden.
  • Problem: System water flow is not balanced.
  • Problem: Cooling tower fans are not operating.
  • Problem: Temperature sensor for condenser water is miscalibrated or improperly installed.
Two pumps are on when only one needed, pressure setpoint is not met
  • Explanation: Pumps operate in parallel with VSDs.
  • Problem: Flow is imbalanced.
  • Problem: Differential pressure sensor is miscalibrated or improperly located.
Two pumps are on when only one is needed, temperature setpoint is not met
  • Explanation: Pumps operate in parallel with VSDs.
  • Problem: Pump is locked on via the DDC system.
  • Problem: Pump is in manual mode at local starter.
  • Problem: Terminal units lack balance valves.
  • Problem: Temperature sensor is miscalibrated or improperly located.
VAV box reheat is active, damper is open to maximum position

Introduction

This condition can exist in almost any variable-air-volume (VAV) system regardless of VAV box type or source of reheat. Please refer to your operation-and-maintenance (O&M) manuals to determine what type of VAV boxes you have installed.

An air handler delivers air to all of the zones it serves at the same temperature as determined by the control-program logic. Some zones may require cooling while others, usually on the building perimeter, may require heating. The air temperature is set to satisfy the zones needing cooling, and the air delivered to the zones needing heating will be reheated by an electric or hydronic coil in the VAV box for that zone. Before the heating coils are energized, the damper in the VAV box should be set to the minimum position. As more heat is needed, the airflow should be as low as possible to minimize the volume of cooled air that has to be heated.

There are conditions (noted below) where having a damper open at 100% does not necessarily mean that airflow is 100%. Also, there are special situations where reheat is used for humidity control and different control logic may apply.

Reheat systems using heating water have no minimum requirement for coil air velocity, whereas electric reheat coils require a minimum air velocity to prevent the coil from overheating. This is typically around 70 cfm/kW. Electric reheat coils can also be provided with stages of 1 kW through 5 kW. They can also be modulated using special controllers. These can greatly reduce electrical demand compared to staged reheat, which is often in 5 kW increments.

How This Wastes Energy

The VAV-box primary air damper (from the AHU) should be at its minimum position before the reheat energizes. It is possible for the damper to be open more than minimum. This allows simultaneous heating and cooling which is prohibited by most, if not all, energy codes.

Possible Causes for This Symptom

The table below shows some of the possible causes of this symptom. The cause of a symptom can be an energy-performance problem that can be fixed, or it may be explained by an unavoidable aspect of your current system that would probably require a capital project to change. Follow the steps described after the table to determine the possible cause of this symptom. If you find a problem, perform the suggested trend logging to confirm that the problem exists and, later, that you have solved the problem.

Inspection Steps:

  1. Explanation: AHU is in heating mode, zone needs additional heat.
  2. Problem: DDC controls are overridden or reset, keeping damper open.
  3. Problem: VAV box controller has failed.
  4. Problem: Inlet static pressure is not adequate to maintain minimum air flow rate.
  5. Problem: Excessive static pressure prevents the damper from closing.

How to Find the Problem(s) by Inspection

Inspection Step 1

Inspect the direct-digital-control (DDC) system to see if the AHU is in cooling or heating mode. If it is in heating mode, then the zone reheat is providing the additional heat that the AHU can't provide. This can occur during morning warm-up.

Inspection Step 2

If the damper control has been overridden in the DDC programming, determine why before correcting the problem. Some reasons for override may be:

  • The space has a high minimum make-up-air requirement such as for a fume or exhaust hood.
  • The damper was overridden during the cooling season to meet a temporary need for space temperature and was not subsequently reset.
  • Tenant complaints about poor air circulation, or "dead-air" conditions.

The first condition has no simple solution and may actually be part of the original design. The second and third conditions require a review of the space-cooling loads to ensure that proper primary air is available under all conditions. The diffuser distribution should also be checked to make sure the supply air is not short-circuiting to the return-air plenum.

Inspection Step 3

Inspect the damper operator to make sure it is operating properly. Cycle the damper using the DDC system while observing its response locally. If there is no change in position, the damper operator may have failed. A DDC technician can determine whether this is the case. Make sure there are no obvious air leaks at the flex connection to the VAV box.

Inspection Step 4

The AHU may have a pressure-control problem that is starving the VAV box of primary air, forcing the damper to be wide open just to meet minimum airflow requirements. Inspect the AHU static pressure (SP) setpoint (in inches of water, typically denoted as "wg) and variable-speed-drive (VSD) speed to see if the static pressure design conditions are being met. This condition can also be caused by excessive duct leakage or a closed fire damper upstream of the VAV box.

Inspection Step 5

In conjunction with Inspection Step 4, the AHU static pressure may be too high and the VAV box damper actuator does not have enough torque to overcome it. This typically occurs in VAV boxes close to the air handler where the duct static pressure is the highest.

How to Confirm the Problem(s) by Trend Logging

Trend log the following:

  • AHU supply-air temperature (SAT)
  • AHU static pressure (SP)
  • VAV box SAT, if available
  • Damper position
  • Heating coil percent capacity (feedback and not output from the DDC system)

Graph the heating coil percent with respect to the other four points. Look for correlations in the trends.

If the damper position is constant when you would expect variation, then the motor operator or DDC-box controller (including the pressure transducer and its tubing) may be the problem.

If the damper position is constant, the duct static pressure is not satisfied, and the AHU VSD is at 100%, then the system capacity is inadequate, or there may be a large duct leak somewhere in the system. (Contractors have been known to remove terminal units and leave the medium-pressure ducting wide open for added ventilation during renovation projects.)

If the AHU SP is being maintained and the VSD is cycling as needed to maintain the setpoint, there may be a closed fire damper upstream of the VAV box. Inspect any other VAV boxes being fed by the same branch duct (downstream of the last fire damper) to see if they exhibit similar problems.

Example of Normal Operation

If your graph looks similar to the one below then your system is operating as it should and the problem is resolved. The damper is at minimum position while reheat is active. When the thermostat calls for cooling, the reheat goes to zero and the damper opens as needed to satisfy the space requirements. Note that the fan static pressure is maintained throughout the day.

Normal VAV-box operation

Examples of Abnormal Operation

VAV boxes are notorious for having many different modes of failure. Below are two examples of what a trend log of abnormal operation might look like, but your own trend-log graph will probably have its own unique features.

If your graph looks similar to one below then your system is not operating as it should and the problem is unresolved. The reheat is energized at 50% once the damper rises above minimum setpoint. Airflow from the AHU to the pressure-sensor location is not a problem since it can maintain static pressure. Inspect the static pressure at the VAV box locally to make sure it has adequate pressure. There may be a partially closed damper upstream, so check other VAV boxes fed by the same branch ducting to see if they have the same problems.

Abnormal Operation: reheat not de-energized

If your graph looks similar to the one below then your system is not operating as it should and the problem is unresolved. The reheat is energized at 50% once the damper rises above minimum setpoint. Airflow from the AHU to the pressure-sensor location is a problem since it cannot maintain static pressure. The damper has opened to 100% in an attempt to maintain the space setpoint. This chart depicts a reheat system that uses heating water. If the system has electric reheat, this graph shows at least 70 cfm/kW of heating capacity, otherwise the reheat would be at zero. Look for fan-speed restrictions, open duct-inspection ports, large leaks, or removed terminal units in a tenant renovation area. Checking above the ceiling for air noise is a good way to locate a duct leak.

Abnormal operation: AHU SP setpoint not met

Labor Skills Required to Find and Resolve the Problem

  • DDC system operator/programmer
  • Service mechanic
VAV box reheat won't activate
  • Problem: Contacts on electric heater are frozen.
  • Problem: Valve on hot-water coil has failed.
  • Problem: Air flow over coil is inadequate.
  • Problem: Thermostat has failed.
VSD does not vary the discharge-air fan speed
  • Explanation: VSD is being used to compensate for pressure drop over time due to filters.
  • Problem: VSD is overridden via DDC for constant speed.
  • Problem: VSD is set locally at constant speed or to bypass mode.
  • Problem: Static-pressure sensor is miscalibrated or improperly located.
  • Problem: Fan can't meet the system demand.
  • Problem: System demand is less than the minimum stable fan capacity.
  • Problem: There is a significant duct leak.
VSD varies the fan speed erratically
  • Problem: Static-pressure sensor is miscalibrated or improperly located.
  • Problem: Static-pressure logic is causing the system to hunt.
  • Problem: Fan can't meet system demand and is operating in an unstable condition.
  • Problem: System demand is less than the minimum stable fan capacity.
  • Problem: There is a significant duct leak.
Water level in sump is high
  • Problem: Pump controller is switched off.
  • Problem: Float-level controller has failed.
  • Problem: Pump has malfunctioned-motor, coupling, or clogged impeller.
Condensing pressure or temperature are significantly higher than setpoint for air-cooled unit

This table lists possible causes or explanations of this symptom. Use a printed copy as a checklist as you investigate.

  • Explanation - Improperly sized or located enclosure is causing air-side to short-cycle.
  • Problem - Fan motor is not operating.
  • Problem - Condenser coils are dirty.
Cooling-tower fans are always on low, even in hot weather

Possible causes or explanations of this symptom. Use a printed copy as a checklist as you investigate.

  • Explanation - Cooling load demanded by chiller is low.
  • Problem - VSD control on cooling-tower fan is set to manual or is overridden.
  • Problem - Two-speed motor or pony motor has failed.
  • Problem - Relay contacts on the motor starter are frozen.
Fan-powered VAV box runs continuously during unoccupied hours

Possible causes or explanations of this symptom. Use a printed copy as a checklist as you investigate.

  • Problem - Fan is in override via the DDC system
  • Problem - Contacts on motor starter are frozen.
Boiler starts and stops frequently

This table lists possible causes or explanations of this symptom. Use a printed copy as a checklist as you investigate.

  • Explanation - Load is below the minimum boiler capacity.
  • Problem - Flow switch is malfunctioning.
  • Problem - Water-temperature high-limit switch is set too low.
  • Problem - Deadband between on/off is too narrow.
  • Problem - Boiler is overfiring or the flue or turbulence-inducing inserts in fire tubes may be clogged.
  • Problem - Heating-water pump is cycling. Boiler cycles via the system flow switch.
  • Problem - NG pressure at the manifold is low.
  • Problem - NOx controls are malfunctioning.
  • Problem - Flow of induced-draft fan is inadequate.
Chilled-water pump operates significantly more hours than chiller

Possible causes or explanations of this symptom. Use a printed copy as a checklist as you investigate.

  • Explanation - Load is below the unit's minimum capacity.
  • Explanation - Pump sequence calls for chilled water to circulate during cold weather to prevent freezing coils.
  • Problem - Pumps operate too long after chiller shuts down.
  • Problem - Pumps operate in manual mode at the local controller or are overridden in the DDC system.