An air handler, or air handling unit (often abbreviated to AHU), is a device used to condition and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handling unit is usually a large metal box containing a blower, heating or cooling element, filter racks or chambers, sound attenuators, and dampers. Air handling units typically connect to ductwork that distributes the air through the building and returns it to the Air handling units.
Air handlers may need to provide heating, cooling, or both to change the supply air temperature depending on the location and the application. Large commercial air handling units contain coils that circulate hot water or steam for heating, and chilled water for cooling. The hot water or steam is provided by a central boiler, and the chilled water is provided by a central chiller. Downstream temperature sensors are typically used to monitor and control 'off coil' temperatures, in conjunction with an appropriate motorized control valve prior to the coil.
Air filtration is almost always present in order to provide clean dust-free air to the building occupants. It is typically placed first in the AHU in order to keep all its components clean. The panel filter is cheaper to replace and maintain, and thus protects the more expensive bag filters.