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HVAC Common Words

ACH – Air Changes per Hour   

The number of times per hour that the volume of a specific room or building is supplied or removed from that space by mechanical and natural ventilation.

A/C – Air Conditioner

An appliance, system or mechanism designed to dehumidify and extract heat from an area. Usually this term is reserved for smaller self contained units such as a residential system.

AH or AHU – Air Handler [Unit]

A central unit consisting of a blower, heating and cooling elements, filter racks or chamber, dampers, humidifier and other central equipment in direct contact with the airflow. This does not include the ductwork through the building.

BTU – British Thermal Unit

Any of several units of energy (heat) in the HVAC industry. One BTU is the energy required to raise one pound of water one degree Fahrenheit. In the US the power of HVAC Systems is sometimes expressed in BTU/hour instead of watts.


Equipment that performs heat transfer to air when mounted inside an air handling unit or ductwork. It is heated or cooled either by electrical means or by circulating liquid or steam within it.


A component in the basic refrigeration cycle that ejects or removes heat from the system. The condenser is the hot side of an air conditioner or heat pump. Condensers are heat exchangers and can transfer heat to air or to an intermediate fluid.


A device that controls the operation of part or all of the system. Most controllers are automatic but have user input such as temperature set points (i.e. a thermostat). Controls may be analog or digital.


A component in the basic refrigeration cycle that absorbs or adds heat to the system. Evaporators can be used to absorb heat from air or from liquid. The evaporator is the cold side of an air conditioner or heat pump.