An air handler (or air handling unit, often abreviated to AHU) is a device used as part of an HVAC system. Usually, an air handler is a large unit containing a blower, heating and/or cooling elements, filter racks or chambers, sound attenuators and dampers, but not including the ductwork through the building.

Air handlers usually contain a large squirrel cage blower driven by an induction electric motor. The blower may operate at a single speed, offer a variety of pre-set speeds, or be driven by a Variable Frequency Drive so as to allow a wide range of speeds. Some residential systems use a brushless DC electric motor, which contains internal drive equipment that allows the blower to automatically compensate for static pressure and operate much more efficiently than is possible with a blower powered by an induction motor, even a multi-speed one. Additionally, variable pitch fans can be used to alter the air flowrate.

If used to supply heat, the air handler may contain a fuel-burning heater or it may simply contain coils that are heated using circulating water or steam with the heat provided by a central boiler.

If used for cooling, the unit may contain a refrigeration system or simply a coil cooled by building chilled water provided by a chiller unit or a direct refrigerent coil within the unit.

Air filtration may be mechanical, HEPA, electrostatic, or a combination of techniques may be used.

Humidification may be provided. Air handlers commonly also provide provisions to allow the introduction of fresh air into and the exhausting of stale air from the building. An air-to-air heat exchanger may be fitted for this purpose in higher-efficiency units.

Control may be as simple as an off/on thermostat or as complex as a building automation system using BACnet or LonWorks.

Constant Volume Air-Handling Units
The less efficient type of air-handler is a "Constant Volume Air Handling Unit," or CAV. The fans in CAVs do not have variable-speed controls. Instead, CAVs open and close dampers and water-supply valves to maintain temperatures in the building's spaces. They heat or cool the spaces by opening or closing chilled or hot water valves that feed their internal heat exchangers. Generally one CAV serves several spaces, but large buildings may have many CAVs.

Variable Volume Air-Handling Units
A more efficient unit is a "Variable Air Volume (VAV) Air-Handling Unit," or VAV. VAVs supply pressurized air to VAV boxes, usually one box per room or area. A VAV air handler can change the pressure to the VAV boxes by changing the speed of a fan or blower with a variable frequency drive or (less efficiently) by moving inlet guide vanes to a fixed-speed fan. The amount of air is determined by the needs of the spaces served by the VAV boxes.

Each VAV box supply air to a small space, like an office. Each box has a damper that is opened or closed based on how much heating or cooling is required in its space. The more boxes are open, the more air is required, and a greater amount of air is supplied by the VAV air-handling unit.

Some VAV boxes also have hot water valves and an internal heat exchanger. The valves for hot and cold water are opened or closed based on the heat demand for the spaces it is supplying. These heated VAV boxes are sometimes used on the parameter only and the interior zones are cooling only.

A minimum and maximum CFM must be set on VAV boxes to assure adaquate ventilation and proper air balance.

VAV Hybrid Systems
Another variation is a hybrid between VAV and CAV systems. In this system, the interior zones operate as in a VAV system. The outer zones differ in that the heating is supplied by a heating fan in a central location usually with a heating coil fed by the building boiler. The heated air is ducted to the exterior dual duct mixing boxes and dampers controlled by the zone thermostat calling for either cooled or heated air as needed.