Mixed-air temperature is near outside-air temperature during heating mode

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