What is an air handling unit? How it works, components and service life

What is an air handling unit?

An air handling unit is the central component of a mechanical ventilation system. It draws in outdoor air, filters it, brings it to the required temperature and sometimes humidity, and supplies it to the building through air ducts. At the same time, it extracts the used air and exhausts it outside, usually after recovering the heat from it.

It is commonly abbreviated to AHU; in Dutch it is called a luchtbehandelingskast, or LBK. An AHU installation usually refers to the entire system: the unit itself, the ductwork, the grilles and the controls. You will find air handling units in almost every non-residential building, from offices and schools to hospitals, data centres and factories.

Why is such a unit needed? Modern buildings are made as airtight as possible to save energy. That is good for the energy bill, but without mechanical ventilation, CO2 levels and humidity in a meeting room or classroom rise within an hour. The air handling unit ensures a continuous supply of fresh air, without losing the heat along with the used air.

How does an air handling unit work?

The easiest way to understand how it works is to follow the air. Two airflows pass through the unit: fresh air coming in and used air going out.

• Intake. A fan draws in outdoor air through a grille and a shut-off damper, which closes when the installation is switched off.
• Filtration. Filters remove dust, pollen and particulate matter from the air. There is often a coarser pre-filter and a finer main filter.
• Heat recovery. In an air handling unit with heat recovery, the extracted indoor air transfers its heat to the incoming outdoor air via a heat recovery wheel or a heat exchanger. In summer, this can work the other way round to provide cooling.
• Heating or cooling. A heating coil or cooling coil brings the air to the required supply temperature. During cooling, moisture condenses and is drained via a condensate tray.
• Humidification. In buildings where humidity matters, such as hospitals and museums, a humidifier adds moisture.
• Supply and extract. The supply fan blows the treated air through the ducts into the building. The extract fan draws the used air back to the unit and then outside.

The controls determine how much air the unit moves. Older installations often run at a constant airflow: always the same amount of air, whether the building is full or empty. Modern installations use variable airflow, with CO2, temperature or pressure sensors making the fans run faster or slower. This saves a lot of energy, because an office is only fully occupied for part of the day.

What are the components of an air handling unit?

Air handling unit components vary by manufacturer and application, but the following are almost always included. Together, they determine how clean, efficient and reliable the unit is.

For the components with the greatest impact on energy use and air quality, we have dedicated pages: on circular air filters, on cleaning heat recovery wheels and heat exchangers and on replacing old fans with energy-efficient EC fans.

Types of heat recovery

There are three common ways to recover heat. A heat recovery wheel rotates slowly through the extract and supply airflows, transferring heat and some of the moisture; its efficiency is high, but there is a small chance of odours transferring between the airflows. A plate heat exchanger keeps both airflows completely separate, which makes it suitable for healthcare and laboratories. A run-around coil system uses two separate coils with a fluid circuit between them, so the supply and extract do not need to be located side by side. All three lose a great deal of efficiency when contaminated, which is why cleaning the heat recovery system is a standard part of good maintenance.

Filter classes

Since 2018, filters have been classified according to ISO 16890, which replaced the old G and F classes. The standard looks at the proportion of particulate matter a filter captures, in the groups ePM10, ePM2.5 and ePM1; the smaller the number, the finer the particles. Coarse pre-filters fall into the Coarse group. The right combination for your unit depends on the outdoor air and how the building is used. You can read more about this on our circular air filters page.

What types of air handling units are there?

• With or without heat recovery. Almost all new units have heat recovery; some older units still lack it.
• Full fresh air or with recirculation. A unit with 100 percent outdoor air supplies only fresh air. A mixed-air or recirculation unit adds back part of the extracted air, which saves energy but is not permitted everywhere.
• Indoor or outdoor installation. A rooftop unit needs a weatherproof casing; a unit in a plant room does not.
• Compact or modular. Compact units come ready-made from the factory. Modular units are assembled from sections, tailored to the building and the available space.
• Standard or hygienic. Hygienic versions have smooth, easy-to-clean interior walls and are used in healthcare, laboratories and the food industry.

Which type suits offices, healthcare or industry?

In offices, energy and comfort are what matter: a unit with heat recovery and demand-controlled ventilation based on CO2 delivers the most there. In healthcare facilities, hygiene and pressure relationships come first, so 100 percent outdoor air and a hygienic design. In industry, the process and the air volume determine the choice, and robustness plays a bigger role than the last percent of efficiency. For existing buildings, there is another factor: the space for the unit is fixed, which limits the options when replacing it.

Air handling unit, air conditioner or ventilation unit: what is the difference?

In short: an air conditioner treats the air in a room, a ventilation unit refreshes the air, and an air handling unit does both, for an entire building. You can read more about this distinction in our article on climate control and ventilation.

In many offices, the air handling unit works together with fan coil units or induction units in the rooms themselves. The unit supplies the fresh, pre-treated air, while the units in the ceiling or below the window fine-tune the temperature in each room. They are part of the same system and also need maintenance: contaminated units distribute air poorly and waste energy. We clean these too as part of the climate system.

How long does an air handling unit last?

In practice, a service life of twenty to twenty-five years is often assumed for the unit as a whole. However, that figure says little, because the components wear at their own pace. Filters last months, belts a few years, fan motors and coils longer. The casing often lasts longest, provided corrosion is tackled in time.

• The number of operating hours per year
• The quality and regularity of maintenance
• The environment: moisture, salt or aggressive air accelerate corrosion
• The quality of filtration, which determines how much dirt reaches the rest of the unit
• Whether the unit is installed indoors or outdoors

A unit that is technically ageing does not necessarily need replacing. Reconditioning restores an existing unit to the performance of a new one, while the ductwork stays in place.

What maintenance does an air handling unit need?

Air handling unit maintenance falls into two categories: technical maintenance and hygiene maintenance. Standards such as EN 15780 and EN 12097 provide a framework for how often a system should be inspected and cleaned, depending on how the building is used.

• Checking and replacing filters, based on the measured pressure drop
• Cleaning the unit, coils and condensate tray
• Checking and cleaning the heat recovery system
• Checking fans, bearings and any belts
• Testing dampers, controls and sensors
• A hygiene inspection, particularly in healthcare, education and the food industry

How often this is needed varies greatly from building to building. A hospital requires a stricter regime than an office. You can find more detail in our article on maintaining an air handling installation, or see how we clean air handling units.

Cleaning, reconditioning or replacement: how do you know what is needed?

• Energy consumption rises while building use remains the same
• Complaints about draughts, temperature, odours or stuffiness increase
• More noise or vibration from the plant room
• Visible contamination, mould or corrosion in the unit
• A condensate tray that does not drain properly
• Recurring faults

Replacement is often costly and operationally unattractive, especially if the new unit is larger than the available space allows. In many cases, reconditioning is the smarter choice. And if the unit is still technically sound, professional cleaning is usually the first step.

Which step is right can rarely be seen from the outside. An inspection maps out the hygiene, corrosion and technical condition of the components, often including measurements of airflow and energy consumption. That turns the choice between cleaning, reconditioning and replacement into a well-founded decision rather than a gut feeling.

Making an existing air handling unit more sustainable

An older unit can often be made considerably more efficient without replacing it. That is good for more than just the energy bill: since 1 January 2023, office buildings in the Netherlands must have at least energy label C, and air handling is one of the largest factors in that. The most effective measures are:

• Replacing old fans with EC fans, which use up to 30 percent less energy
• Adding heat recovery, or having the existing heat recovery system cleaned and repaired
• Sealing leaking ducts with Aeroseal, so the fan does not move air that is lost along the way
• Switching to demand-controlled operation, so the unit does not run at full power when the building is empty