What is an air handling unit (AHU)? Components, types and filters

What an AHU is for

Most commercial buildings above a certain size cannot rely on windows for fresh air and temperature control. An air handling unit does it centrally: it takes in outdoor air, filters it, brings it to the right temperature and delivers it through ducts to the occupied spaces. Offices, hospitals, schools, swimming pools, cleanrooms and large retail units are typical users. For the buildings where a central unit is the right answer see air handling units and ductwork.

The main components

Fans. Supply fans push air into the building; extract fans pull it out. Modern units use variable-speed fans so airflow follows demand.

Filters. Usually two stages, a coarser pre-filter protecting a finer main filter. They are the part that wears out and the part most often neglected.

Heating and cooling coils. Water or refrigerant coils heated or cooled from a boiler, heat pump, chiller or direct-expansion outdoor unit.

Heat recovery. A plate heat exchanger or thermal wheel transfers heat between the outgoing and incoming air so less heating or cooling energy is needed to condition the fresh air.

Dampers, attenuators and controls. Dampers set the mix of fresh and returned air, attenuators reduce fan noise in the ducts, and controls tie the unit to temperature, carbon dioxide or building management system signals.

Supply-only, supply-and-extract and recirculating units

A supply-only unit brings air in but relies on leakage or separate fans to push it out, which wastes the opportunity for heat recovery. A supply-and-extract unit with heat recovery is the usual choice where energy matters. Recirculating units reuse most of the room air and bring in only a minimum of fresh air, and suit spaces where fresh air demand is low.

How to read a filter class

Under ISO 16890, filters are tested with particles between 0.3 and 10 micrometres and placed in one of four groups: ePM1, ePM2.5, ePM10 or Coarse. A filter earns a group by capturing at least half of the particles of that size, and the rating is published as a percentage rounded to the nearest 5%, so "ePM10 70%" means it captured about 70% of PM10 particles. ISO 16890 replaced the older EN 779 G and F classes, so a data sheet still quoting only G4 or F7 predates it.

The right class depends on the building and the outdoor air quality. A hospital theatre and a warehouse office need different filtration, and finer filters raise fan energy because they resist airflow more.

Where the money goes

Cost follows airflow, the coils and heat recovery fitted, the filter class and the installation and commissioning around it. Published equipment prices are in our air handling unit cost guide, and the decision between refurbishing an old unit and replacing it is covered on the AHU vertical page. For the legal duty behind fresh air supply see commercial ventilation requirements.

Sources and last checked

Checked on 18 October 2026. Regulations and guidance are revised, so confirm the current edition before you rely on it.

• Mann+Hummel: the ISO 16890 standard for classifying air filters
• Ventilationland: ISO 16890 and MVHR filters (EN 779 classes superseded)