What does air handling unit mean?
An air handling unit is, in English, also known as an Air Handling Unit (AHU). It serves not a single room like a split air conditioner or fan coil, but a zone or the whole building through a duct system. Its most important difference is that it takes in fresh air from outside, conditions it and supplies it indoors; for this reason it combines ventilation and air conditioning in one unit.
How does an air handling unit work?
Air passes through the sections inside the unit in sequence. In a typical unit, the order is as follows:
01Fresh air and mixing dampers
Outside air and the air returning from the building are mixed in the desired ratio by dampers. In 100% fresh air units, return air is not used.
02Filters
Pre-filter and fine filter stages trap dust in the air; in places that need hygiene, such as hospitals, HEPA filters are added. Filter replacement timing is monitored by differential pressure; for details: when is the AHU filter replaced.
03Heat recovery
Plate exchangers, rotary wheel (rotor) exchangers or run-around systems transfer the energy of the exhaust air to the fresh air, lowering operating costs.
04Cooling and heating coils
Water coils work with water from the chiller and boiler. In DX coil units, the coil works like the evaporator of an outdoor unit. In cooling, the moisture in the air condenses on the coil and is removed through the drain.
05Humidifier
In winter, dry air is brought to the desired humidity with steam or adiabatic humidifiers; for capacity, you can use the humidifier calculator.
06Fan and silencer
The fan pushes the air into the ducts; efficient EC plug fans are common in new units. Silencers reduce the sound traveling into the duct.
Maintenance or overhaul for your air handling unit
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Types of air handling units
| Type | Feature | Where it is used |
|---|---|---|
| Standard (comfort) unit | Mixed-air or fresh-air, with water coils | Offices, shopping malls, schools |
| Hygienic air handling unit | Cleanable inner surfaces, HEPA, sealing | Hospitals, operating rooms, pharmaceutical and food production |
| Air handling unit with heat recovery | Plate or rotary exchanger | Buildings with high fresh air demand |
| DX coil air handling unit | Outdoor unit (VRF or condenser) connection for cooling | Buildings without a chiller |
| Rooftop (packaged) unit | Refrigeration cycle and fans in a single package on the roof | Supermarkets, shopping malls, factory halls |
We covered the specific requirements of hospital air handling units in our article hygienic air handling unit, and rooftop units on our rooftop air conditioner page.
Air handling unit maintenance
Air handling unit maintenance includes filter replacement, coil and drain pan cleaning, fan and motor checks, belt tension (on belt-driven fans), damper and valve tests, and freeze thermostat and humidifier checks. For the detailed list, see our article air handling unit maintenance checklist, and for the scope of service, our air handling unit maintenance page.
Frequently asked questions
What is the difference between an air handling unit and an air conditioner?
An air conditioner conditions a single room with its indoor unit. An air handling unit takes in fresh air from outside, filters it, cools or heats it and distributes it through ducts to a zone or the whole building.
What does AHU mean?
AHU is short for Air Handling Unit; in Turkish it is called klima santrali or hava işleme ünitesi.
How does an air handling unit cool?
Mostly with coils fed by chilled water from a chiller. In DX coil units, cooling is provided by a connected outdoor unit.
What is a hygienic air handling unit?
These are units with easy-to-clean inner surfaces, high sealing and HEPA filters, for areas such as hospitals, operating rooms and clean rooms.
When is the air handling unit filter replaced?
It is replaced when the differential pressure across the filter reaches the final value recommended by the manufacturer; this period varies with use and environment.