Air handling units (AHUs) are used to supply and circulate air around a building, or to extract stale air as part of a building’s heating, ventilating, and air conditioning (HVAC) system. 

Essentially, an Air Handling Unit system comprises a large insulated metal box that contains a fan, heating and/or cooling elements, filters, sound attenuators and dampers. In most cases, AHUs are connected to air distribution ductwork. Alternatively, an AHU can be open to the space it serves. 

Supply air passing through the AHU is filtered and is either heated or cooled, depending on the specified duty and the ambient weather conditions. Air Handling Units can also be used to supply fresh air for ventilation and to extract stale air. 

For heating or cooling, AHU coils may be connected to central plant such as boilers or chillers, receiving hot or chilled water for heat exchange with the incoming air. Alternatively, heating or cooling may be provided by electric heating elements or direct expansion refrigeration units built into the air handler. 

When AHU systems are used to extract stale air from the building, a controlled proportion of this air may be recirculated to avoid having to condition all supplied air. AHUs can also incorporate heat recovery exchangers to extract heat from the air being expelled and use it to heat incoming supply air. 

Air Handling Units vary considerably in size, capacity and complexity, depending on the job they are designed to perform.