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. Usually, an air handler is a large metal box containing a blower, heating and/or cooling elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to ductwork that distributes the conditioned air through the building, and returns it to the AHU. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served, without ductwork.
Small air handlers, for local use, are called terminal units, and may only include an air filter, coil, and blower; these simple terminal units are called blower coils or fan coil units. A larger air handler that conditions 100% outside air, and no recirculated air, is known as a makeup air unit (MAU). An air handler designed for outdoor use, typically on roofs, is known as a packaged unit (PU) or rooftop unit (RTU).
In large commercial air handling units, multiple blowers may be present, typically placed at the end of the AHU and the beginning of the supply ductwork (therefore also called "supply fans"). They are often augmented by fans in the return air duct ("return fans"), pushing the air into the AHU.
Smaller air handlers may contain a fuel-burning heater or a refrigeration evaporator, placed directly in the air stream. Electric resistance and heat pumps are used too. Evaporative cooling is possible in dry climates too.
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.
It is typically placed first in the AHU in order to keep all its components clean.
A heat recovery heat exchanger, of many types, may be fitted to the air handler for energy savings and increasing capacity.
Common control components include temperature sensors, humidity sensors, sail switches, actuators, motors, and controllers.
The fan compartment can be further isolated by placing it on a spring suspension, which will mitigate the transfer of vibration through the floor.
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