Air handling unit AHU
To help develop this article, click 'Edit this article' above.
Air handling units can be supplied in a range of sizes, and with a variety of capabilities, but typically they comprise an insulated box that forms the housing for; filter racks or chambers, a fan (or blower), and sometimes heating elements, cooling elements, sound attenuators and dampers (that can be operated manually or automatically to regulate or prevent specific air flows). In some situations, such as in swimming pools, air handling units might include dehumidification.
Generally, air handling units will be connected to the ductwork within the building that supplies air to and extracts air from the interior, but they can be used to supply and extract air direct to a space, or they may be located on a roof (rooftop units or RTU).
Air handling units can be used to re-circulate a proportion of ‘stale’ air within a building, mixing this with fresh air to reduce the amount of air conditioning that is required. They can also include heat recovery, recovering heat from return air and using it to warm the supply air.
Fans may be single speed, may have a range of set speeds, or may be variable frequency drive. Flow rates may also be controlled by inlet vanes or outlet dampers.
Air handling units generate noise (and vibration) which can be disruptive, and this can be compounded where ductwork passes between acoustically separate spaces. In this case, acoustic attenuators might be used. In addition, vibration can be generated. This vibration can be isolated by inserting flexible sections between the unit and ductwork and by isolating the unit from the building structure.
Where air handling units are located outside buildings, they can be the source of complaints by neighbours because of the noise and vibration they generate and sometimes because of odours they expel (typically where they are serving kitchens). It is important that these units are properly designed, installed and maintained to minimise this disturbance.
‘Wet’ systems, that include water evaporative cooling, can present a hazard to health. Businesses using such systems are required to carry out a risk assessment and put in place procedures to ensure there is no risk to public health.
 Related articles on Designing Buildings Wiki
- Air conditioning.
- Air conditioning inspection.
- Chiller unit.
- Chilled water.
- CIBSE Case Study London Olympic Aquatics Centre.
- Cooling systems for buildings.
- Cooling tower design and construction.
- Constant air volume.
- Evaporative cooling.
- Fan coil unit.
- Mechanical ventilation.
- Natural ventilation.
- Thermal comfort.
- Underfloor air distribution.
- Variable air volume VAV.
- Variable refrigerant flow VRF.
 External references
Featured articles and news
Here is our list of the top 25 buildings in London. Do you agree with our selection?
A blueprint for the construction industry from Canada's west coast.
China's elaborate idea for a mass-transport system has been abandoned.
Read this article about the theories that characterise life in the hyperreal post-modern city.
Polyisocyanurate (PIR) insulation and how it was tested.
"We can’t sustain low density suburbs, density isn't a choice, it's a necessity." - Read our interview with the award-winning social housing architect Peter Barber.
Conservation area designation can be crucial, but treatment of individual parks varies considerably.
ICE publish new NEC4 Design, Build and Operate contract.
Report states $2 trillion is needed over the next 10 years to fix American roads.
What is the client's strategic brief for construction projects?
Read the story behind the world's most iconic festival stage, Glastonbury's Pyramid Stage.
First ever BREEAM Communities innovation credit is claimed by Temple Farm Development.
Read the story of Ronan Point, another disastrous event which had profound consequences for the construction industry.
CIOB to help conservation specialists gain recognition for their expertise with launch of new Certification Scheme.
What is ACM cladding, what is it used for, and is it banned?