AHU Glossary of Terms

Based on the popularity of our recent Coil Glossary, we decided to add the following list of common terms and definitions related to air handling units (AHUs).

• Airflow Rate: The volume of air passing through the AHU per unit of time, typically measured in cubic feet per minute (CFM).

• Airflow Sensor: A device that measures the rate of airflow within the AHU ductwork to ensure proper ventilation and system performance.

• Air Handling Unit (AHU): A device used to condition and circulate air as part of a heating, ventilating, and air conditioning (HVAC) system.

• Building Management System (BMS): A computer-based control system that manages and monitors the HVAC system, including AHUs.

• Bypass Damper: A damper that allows a controlled amount of air to bypass the main airflow path in the AHU, typically used for capacity control.

• Condensate Drain Pan: A tray or basin beneath the cooling coil that collects water condensed from the air.

• Cooling Coil: A component within the AHU that removes heat from the air by transferring it to water, refrigerant or another fluid and moving it to the outdoor environment.

• Damper: A movable plate or valve within the AHU ductwork that controls airflow.

• Dampered Face and Bypass Coil: (also known as Integral Face and Bypass Coil): A coil used in cold temperatures to preheat incoming air entering the AHU, to increase efficiency and reduce the possibility of coil freezing.

• Ductwork: The system of pipes or metal channels used to distribute air from the AHU to different parts of the building.

• Economizer: Similar to a mixing box, can be used to improve the efficiency of an AHU by mixing outside air with return air to pre-heat or cool air entering the AHU.

• Fan: The mechanical device within the AHU that circulates air through the unit and into the building.

• Fan Array: A number of fans configured in stackable modules and designed to form a larger and more powerful fan system. This can also reduce the length of the AHU for tight spaces.

• Fan Wall: Multiple fans stacked and/or mounted together to form a complete “wall” of fans, in order to increase airflow while reducing noise and vibration compared to using one larger fan. This can reduce the length of the AHU for tight spaces.

• Filter: A device within the AHU used to trap dust, pollen and other particles from the air.

• Heat Exchanger: Another name for a coil, a device within the AHU that transfers heat from one source, such as a fluid, to another source, such as air.

• Heating Coil: A component within the AHU that utilizes steam or hot water to transfer heat to the air.

• HEPA Filter: Short for High Efficiency Particulate Air, this type of filter is able to remove at least 99.97% of airborne particles as small as 0.3 microns in diameter.

• Humidifier: A component that adds moisture to the air processed by the AHU.

• Make-Up Air Unit: An AHU that provides outside air to replace air exhausted from a building due to ventilation requirements.

• MERV (Minimum Efficiency Reporting Value): A rating system for filters that identifies the filter’s performance, based on the particle size it is able to capture.

• Mixed Air: The combination of outside air and return air before it enters the AHU.

• Mixing Box: A chamber within the AHU where outside air and return air are mixed together before being conditioned.

• Outside Air: Fresh air drawn from the external environment into the AHU for conditioning and distribution.

• Plenum: A space within the building structure where air circulates freely, often used for HVAC distribution.

• Return Air: The air drawn back to the AHU from the building for reconditioning.

• Return Air Temperature: The temperature of the air drawn back to the AHU from the building.

• Service Doors: Doors on the AHU that allow access to the interior for service/maintenance.

• Static Pressure: The pressure exerted by air within the ductwork of the AHU system, which affects airflow.

• Supply Air: The conditioned air delivered from the AHU into the building space.

• Supply Air Temperature: The temperature of the air delivered into the building space from the AHU.

• ULPA Filter: Short for Ultra-Low Particulate Air, this type of filter is able to remove at least 99.999% of airborne particles as small as 0.12 microns in diameter.

• Variable Frequency Drive (VFD): A type of controller that drives an AHU’s electric motor by varying the frequency and voltage of its power supply, typically to reduce energy usage.