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, the AHU is connected to air distribution ductwork; alternatively, the 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 specified duty and the ambient weather conditions. In some buildings, Air Handling Units are used only to supply fresh air for ventilation and extract stale air.