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).
Air handler components
Air handlers typically employ a large squirrel cage blower
driven by an AC induction electric motor
to move the air. 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 air flow rates. Flow rate may also be controlled by inlet vanes or outlet dampers on the fan. Some residential air handlers (central 'furnaces' or 'air conditioners') use a brushless DC electric motor
that has variable speed capabilities.
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.
Heating and/or cooling elements
Depending on the location and the application, air handlers may need to provide heating, or cooling, or both to change the supply air temperature.
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.
is almost always present in order to provide clean dust-free air to the building occupants. It may be via simple low-MERV
pleated media, HEPA
, or a combination of techniques. Gas-phase and ultraviolet air treatments may be employed as well.
It is typically placed first in the AHU in order to keep all its components clean.
is often necessary in colder climates where continuous heating will make the air drier, resulting in uncomfortable air quality and increased static electricity
. Various types of humidification may be used:
- Evaporative: dry air blown over a reservoir will evaporate some of the water. The rate of evaporation can be increased by spraying the water onto baffles in the air stream.
- Vaporizer: steam or vapour from a boiler is blown directly into the air stream.
- Spray mist: water is diffused either by a nozzle or other mechanical means into fine droplets and carried by the air.
In order to maintain indoor air quality, air handlers commonly have provisions to allow the introduction of outside air into, and the exhausting of air from the building. In temperate climates, mixing the right amount of cooler outside air with warmer return air can be used to approach the desired supply air temperature. A mixing chamber is therefore used which has dampers
controlling the ratio between the return, outside, and exhaust air.
A heat recovery heat exchanger, of many types, may be fitted to the air handler for energy savings and increasing capacity.
are necessary to regulate every aspect of an air handler, such as: rate of air flow, supply air temperature, mixed air temperature, humidity, air quality. They may be as simple as an off/on thermostat
or as complex as a building automation
system using BACnet
, for example.
Common control components include temperature sensors, humidity sensors, sail switches, actuators, motors, and controllers.
The blowers in an air handler can create substantial vibration and the large area of the duct system would transmit this noise and vibration to the occupants of the building. To avoid this, vibration isolators (flexible sections) are normally inserted into the duct immediately before and after the air handler and often also between the fan compartment and the rest of the AHU. The rubberized canvas-like material of these sections allow the air handler to vibrate without transmitting much vibration to the attached ducts.
The fan compartment can be further isolated by placing it on a spring suspension, which will mitigate the transfer of vibration through the floor.