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Comparing the energy performance of one building with the performance of: (1) the same building from a previous time period; (2) the performance of other buildings in the same campus or management portfolio; or (3) the average performance of buildings in a broad regional or national database.

bullhead tee

The name given to the installation of a tee fitting where the primary supply water flow enters the side tap and exits through the ends.



A condition occurring when the inlet pressure at the pump is less than the vapor pressure of the liquid being pumped, causing the liquid to vaporize into bubbles in the suction stream. The bubbles collapse on entering the pump housing with the rotating impeller, preventing the pump from effectively moving the liquid. This creates the very distinct sound of marbles being shaken in a tin can. Each pump has it own characteristic net positive suction head (NPSH) required to prevent this condition.


The smallest cooling tower subdivision with independent air and water flow. It is enclosed by exterior walls or partition walls. Each cell may have one or more fans and one or more distribution systems.

chilled water reset schedule

An automated control logic that raises or lowers the supply temperature of the chilled water leaving the chiller, in response to another variable such as outside air temperature.


Coils are used to heat and cool an airstream by transferring heat from or to another medium. They can be bare-tube type or have an extended fin surface. Coils may use water, steam, refrigerant, or electricity as a source for heat transfer.


A mechanical device used to compress a gas.


A portion of the refrigeration system in which hot refrigerant vapor is cooled by water or air, allowing the refrigerant to condense back to liquid form.




A device in which a liquid refrigerant draws heat from chilled water and vaporizes into a gaseous state.

expansion valve

A component of the refrigeration system that regulates the rate of flow of liquid refrigerant into the evaporator.


fan coil unit

A small terminal air handling unit with a fan and coil(s) to heat and/or cool the airstream.


The portion of a cooling tower that provides a large air-water interface area for heat transfer allowing a small amount of water to evaporates into the airstream, cooling the remaining water.





The rotating part of a centrifugal pump, compressor, or fan designed to move a fluid by rotational force. It is usually made up of a disc with multiple vanes attached to it.





magnetic bearing technology

A centrifugal compressor with a rotor shaft and impeller, that levitate during rotation while suspended in a magnetic field.

make-up water

Water added to a circulating water system to replace water lost by evaporation, blowdown, or leakage.


notched belt

A belt with teeth (notches) used to mechanically link two or more rotating pulleys. The notches increase grip, help cool the belt and relieve stress as the belt bends around small diameter pulleys. This improves drive efficiency.



pony motor

A smaller secondary cooling-tower fan motor, which operates instead of the primary fan motor in light load conditions.

preventive and predictive maintenance

preventive maintenance: A maintenance activity performed at a regular interval of time or run-hours, to prevent systems from failing.

predictive maintenance: Maintenance practices using specialized diagnostic equipment at regular intervals to detect the onset of deterioration of machinery with the aim of extending service life through controlling degradation.

process load

A cooling or heating load not related to maintaining occupant comfort, such as file server rooms and specialized diagnostic equipment in the health-care sector. The loads may be intermittent or continuous.

pump head

The differential pressure of a fluid generated by a pump between its inlet and outlet. This pressure may be expressed in feet of water or pounds per square inch (psi).




A substance producing a refrigerating effect by expanding or vaporizing.



The cooling of a liquid, at a constant pressure, to below the temperature at which it was condensed.


Energy removed from the refrigerant in the condenser to make the vapor 100% saturated. More heat removal causes a phase change at the same temperature, which is called latent heat. More cooling below that reduces the temperature of the liquid and it is sensible heat.


terminal unit

The final piece of HVAC equipment in the distribution system capable of modifying the temperature in a conditioned space.

trend logging / trend log

Recording system variables (e.g., temperature, volume, pressure, power) at time intervals to monitor equipment operation and help identify or diagnose problems. Logging is accomplished using portable data loggers, electrical meters, or the resident DDC system.


The amount of power necessary to melt one ton of 32°F ice into 32F water in 24 hours. A ton equals 12,000 Btu/hour.

two-way valve

A valve regulating flow between no flow and full flow. It can either modulate or operate as a two-position valve (open/closed) in response to an external input signal. There is no bypass as is found in a 3-way valve.



variable-speed drive

An electronic device that controls the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.


A belt used to mechanically link two or more rotating pulleys. The "V" shape of the belt tracks in a mating groove in the pulley (or sheave).


water-to-air heat pump

A heat pump that heats or cools air, by either taking heat from or rejecting heat to a closed circulating water loop, and then transfers the warm or cold air to a conditioned space.

water-side economizer

An economizer added to a standard chilled-water system to allow air conditioning without operating a chiller. It comprises chiller-bypass piping, associated valves, and a heat exchanger. When outside-air conditions and internal loads permit, water from the cooling tower bypasses the chiller to a heat exchanger connected to the chilled-water supply loop.

wet-bulb temperature

This is a measurement of air temperature relative to the water content of the air. A wet-bulb thermometer's bulb is covered with a wetted wick that evaporates less in humid conditions and more in drier conditions. At 100% relative humidity, the wet-bulb temperature is equal to the dry-bulb temperature. At dry-bulb temperatures where humidity is less than 100%, the wet-bulb temperature is less than the dry-bulb temperature. At any given dry-bulb temperature, lower relative humidity will result in a greater difference between the two temperatures.





PG&E, May 1997. Energy-Efficient Operations and Maintenance Strategies for Packaged HVAC Systems