An air handling unit (AHU) is a large piece of HVAC equipment that conditions and circulates air through a building’s ductwork. A typical AHU houses a supply fan, filters, heating and cooling coils, dampers and sometimes a humidifier or heat recovery wheel, and is installed on the roof or in a mechanical room.
How an air handling unit works
- Outdoor air and return air from the building enter through dampers into a mixing box. The controls set the mix; in mild weather an economizer uses more outdoor air for free cooling.
- Filters clean the air.
- Coils heat or cool it. The cooling coil also removes moisture, which drains from a drain pan.
- A humidifier adds moisture if needed.
- The supply fan pushes the air through the ductwork to terminal units (such as VAV boxes) and diffusers.
- A return or exhaust fan draws air back, and an energy recovery wheel or plate can transfer heat between exhaust and incoming air.
Main components
| Component | Job |
|---|---|
| Casing | Insulated sections that contain the air; large units ship in pieces |
| Dampers | Control outdoor, return and exhaust air |
| Filters | Pre-filters and final filters, rated by MERV in North America |
| Coils | Chilled water or refrigerant (DX) for cooling; hot water, steam or electric for heating |
| Fans | Supply and return fans, often on a variable frequency drive |
| Drain pan and trap | Collects and drains condensate from the cooling coil |
| Energy recovery | Wheel or plate exchanger that reuses heat or coolth from exhaust air |
Types of air handling units
- Packaged rooftop unit (RTU): a self-contained AHU with its own refrigeration and often gas heat, set on a roof curb.
- Central or modular AHU: usually indoors, fed with chilled and hot water from a central plant.
- Dedicated outdoor air system (DOAS): treats 100% outdoor air for ventilation, with other units handling space loads.
- Makeup air unit (MAU): replaces air exhausted by kitchens, labs or parking garages.
- Residential air handler: an indoor fan-coil cabinet paired with an outdoor condenser or heat pump.
AHU vs RTU vs fan coil unit
| AHU | RTU | Fan coil unit | |
|---|---|---|---|
| Size | Large, serves a floor, zone or whole building | Medium to large, one zone or several | Small, one room or zone |
| Location | Mechanical room or roof | Roof | Ceiling, wall or closet in the space |
| Cooling source | Central plant or DX | Built-in refrigeration | Central chilled water or refrigerant |
| Ducts | Extensive duct system | Ducted | Little or no ductwork |
See Fan Coil Unit for more on FCUs.
AHUs on a construction project
- Plan the lift early. Rooftop units are usually set by crane onto a curb or dunnage the structural engineer has checked. Our article on planning a crane lift walks through it.
- Check access: indoor units often ship in sections to fit through doors and need clearance to pull coils and change filters.
- Build the condensate trap to the manufacturer’s height; a wrong trap on a draw-through unit stops the pan draining.
- If the permanent AHU runs during construction, protect it and the ducts from dust and change filters before turnover. Projects pursuing the LEED construction indoor air quality credit must fit minimum MERV 8 filters at return openings in that case.
- Book manufacturer startup, testing and balancing, and commissioning in the schedule.
Related terms
Related templates and guides
- Introduction to Mechanical and Electrical Systems
- HVAC Design Quality Checklist
- General Details: HVAC Systems
- Ductwork Installation Productivity Monitoring Form
Frequently asked questions
What does AHU stand for in construction?
AHU stands for air handling unit, the HVAC equipment that filters, heats or cools and moves air through a building’s ducts. On drawings it is tagged AHU with a number, such as AHU-1, and its schedule lists airflow, coils and fan data.
What is the difference between an AHU and an HVAC system?
HVAC is the whole heating, ventilation and air conditioning system: boilers, chillers, pumps, ducts, terminal units and controls. The AHU is one piece of it, the unit that conditions the air and pushes it through the ductwork.
What is the difference between an AHU and an RTU?
An RTU is a packaged air handler made for the roof, with its own compressors and often gas heat. A central AHU usually gets chilled and hot water from a plant elsewhere in the building and can sit indoors or outdoors.
Does an air handling unit cool the air?
Yes, if it has a cooling coil. The coil is supplied with chilled water or refrigerant from a chiller, condensing unit or heat pump. The AHU itself moves and conditions the air; the cooling is produced by that other equipment.
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