Air Handling Units


An Air Handling Unit (AHU) is used to condition, regulate and circulate air as part of a heating, ventilating, and air-conditioning system.The basic function of the AHU is take in outside air, condition it and supply fresh air to a building. All exhaust air is discharged, which secures an acceptable indoor air quality.

An air handling unit; air flow is from the right to left in this case. Some AHU components shown are

1 – Supply duct

2 – Fan compartment

3 – Vibration isolator ('flex joint')

4 – Heating and/or cooling coil

5 – Filter compartment

6 – Mixed (recirculated + outside) air duct  

The AHU is a large metal box containing separate ventilators for supply and exhaust, heating coil, cooling coil, heating/cooling recovery system, air filter racks or chambers, sound attenuators, mixing chamber, and dampers. AHUs 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). Depending on the type of heat exchanger selected, the BASIC units can be utilised in buildings such as office buildings, schools, day nurseries, public buildings, shops, residential buildings, etc.A heat/cooling recovery exchanger is normally fitted to the AHU for energy savings and increasing capacity.  

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