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How to clean an air handling unit

Oct 19, 2022

Summary

Air handling units (AHUs) should be cleaned and maintained through planned, risk-based inspection and servicing to protect indoor air quality, airflow, energy efficiency and system reliability. Key tasks include checking and replacing filters according to manufacturer guidance and pressure-drop readings, clearing outdoor air inlets, inspecting ductwork and internal surfaces, cleaning coils, drain pans and accessible surfaces, and verifying that the unit operates correctly after restart.

Cleaning methods should be selected to suit the AHU, contamination type and component materials, with wet cleaning only used where drainage and component protection are suitable. For European applications, EN 15780:2025 provides cleanliness criteria and procedures for assessing and maintaining ventilation system cleanliness. Work involving internal components, electrical parts, ductwork, microbial contamination or chemicals should be carried out by trained and competent personnel.

Why do we need to clean AHUs?

Over time and through use, an air handling unit (AHU) can gather dust, debris and other contaminants that may block filters, restrict airflow, affect fans and coils, and reduce overall system performance. To support good indoor air quality, energy efficiency and hygienic operation, AHUs should be inspected, cleaned and maintained on a planned basis, with the frequency adapted to the building, operating conditions and level of contamination.

If an air handling unit isn’t cleaned and maintained regularly, the indoor air quality is likely to decrease. Regular cleaning will also:

Ensure the unit works at its full capacity.
Extend the unit’s service life
Improve efficiency
Ensure that the air quality is maintained at optimum levels.

What parts of the AHU need cleaning?

AHUs differ significantly in size, configuration, application and level of contamination, so there is no universal cleaning duration. NADCA (National Air Duct Cleaners Association) guidance focuses on the assessment, cleaning and restoration of HVAC systems, including inspection, scope of work, safety planning, cleaning methods and verification of cleanliness. For best results, use a qualified cleaning specialist who can assess the AHU and ductwork, select appropriate methods and avoid damage to components.

A cleaning and maintenance regime may include:

Inspecting filters and filter racks regularly, cleaning the racks as needed, and replacing filters in line with the manufacturer’s instructions, pressure-drop readings, operating conditions and the building’s maintenance plan. Filters should also be checked for contamination, damage, moisture and odours.
Every six months:
- Check the outdoor air inlets for damage and contamination.
- Check the air ducts for damage.
- Check the inner air duct surface for condensation and contamination.
Annually, or as defined by the maintenance plan: assess dust contamination and check for signs of microbial growth, moisture or other hygiene issues in the air handling unit and associated ventilation system.
At intervals determined by risk assessment, and more frequently in higher-risk environments, carry out a hygiene inspection and clean ducts and air system components as required. For European applications, EN 15780:2025 provides cleanliness criteria and procedures for assessing and maintaining ventilation system cleanliness over its lifetime.

If the AHU or associated outdoor air intake is located outside, keep the surrounding area clear of leaves, branches, plants, long grass and general debris. Check that outdoor air inlets remain unobstructed and that any required service clearances are maintained in line with the manufacturer’s instructions.

Wet vs dry AHU cleaning

Using a cleaning plan, the right cleaning method needs to be selected, i.e., wet or dry cleaning, to ensure no damage occurs to the components or surfaces of the AHU. A cleaning plan will include:

The ventilation system that needs to be cleaned.
The selected cleaning method(s).
The equipment required to carry out the cleaning process.
Filter efficiency.
Location of the air exhaust
Health and safety requirements.
Verification of cleaning including any notifications.

Wet cleaning, for example, would be used to clean terminal devices (once removed from the ductwork), and heat recovery units if they are installed in a place where the water from wet cleaning can be drained off. This cleaning method is more suited to removing deposits that are stuck to the surfaces, like grease or oil from installation. However, wet cleaning is not usually used to clean air ducts, as they are not watertight.

Dry cleaning, usually using compressed air and hand vacuum cleaners or mechanical brushing, can be used to clean between heat exchanger fins, filter banks, silencer surfaces and any other areas where any water from wet cleaning would damage the AHU. This method removes most of the accumulated dust and dirt that has loosely settled on the components.

Cleaning robots are also useful to clean rectangular air ducts, particularly those that have a camera attached to monitor cleaning progress.

For cleaning areas that contain or may contain microbial contamination, such as damp sections, condensate areas or outdoor air intake zones, the cleaning method and any disinfection process should be selected by competent personnel according to the contamination risk, applicable guidance, and the manufacturer’s instructions.

How do you clean an AHU?

Before cleaning or inspection begins, the AHU must be safely isolated in accordance with site procedures. Where work involves fans, coils, electrical components, access panels, ductwork, cleaning chemicals or potential microbial contamination, it should be carried out by trained and competent personnel using appropriate personal protective equipment.

Wipe down the air vents and floor vents and clear any debris that has collected. If the vents are not clear, airflow will be restricted.
· Remove and replace dirty or damaged filters with filters that are suitable for the AHU, correctly sized, properly seated and appropriate for the required filtration level and operating environment. Eurovent certified air filters can help demonstrate verified performance when the correct filter class and application are selected.
Clean accessible external surfaces, grilles and access areas using methods compatible with the AHU materials and manufacturer’s recommendations. Avoid introducing water into areas that are not designed to drain or that contain electrical components.
Inspect drain trays and condensate pans for standing water, sludge, corrosion, blockages or microbial growth, and clean them as required to allow effective drainage.
Check drain lines and traps for leaks, blockages and correct operation. Poor drainage can lead to water damage, microbial growth and reduced hygiene within the AHU.
Inspect coils and heat exchangers for dust, dirt and fouling. Clean fins carefully using suitable brushes, approved cleaning products or specialist equipment, taking care not to damage fins, coatings, sensors, electrical parts or nearby components. Wet cleaning should only be used where drainage and component protection are suitable.
Check fixings, panels, seals and access doors for corrosion, damage, air leakage or loose parts. Treat, repair or replace components in line with the manufacturer’s maintenance instructions.
Where the AHU forms part of a heating or cooling system, inspect associated pipework, insulation, valves and connections for damage, leaks or deterioration, and arrange repair by a competent HVAC professional where required.

Once cleaning and inspection are complete, confirm that panels are refitted, drains are clear, filters are correctly installed, and the unit can be safely returned to service.

After restart, check that the AHU operates normally, with appropriate airflow, pressure readings, drainage and control response. If alarms occur, airflow remains restricted, unusual noise or vibration is present, or the unit does not restart correctly, contact a qualified HVAC engineer.

Why is AHU maintenance important?

Regular AHU maintenance helps keep ventilation systems operating safely and efficiently. Clean filters, coils, drain pans and ductwork support indoor air quality, reduce unnecessary fan energy use, protect components from contamination and help extend the service life of the system.

If an AHU is taken out of service for an extended period, follow the manufacturer’s instructions for shutdown, protection and recommissioning. Avoid covering equipment in a way that traps moisture, blocks ventilation or conflicts with the unit’s maintenance requirements.

If you are not confident carrying out AHU cleaning or inspection, or if the work involves internal components, ductwork, electrical parts, microbial contamination, chemicals or access risks, hire a qualified and competent HVAC engineer or air systems cleaning specialist. They can clean and service the AHU, verify that components are working correctly, and identify any damaged parts that need repair or replacement.

FAQ

How often should an air handling unit be cleaned?

The cleaning frequency depends on the building type, operating conditions, contamination level and maintenance plan. Filters and filter racks should be inspected regularly, while hygiene inspections and duct cleaning should be based on risk assessment, with more frequent checks in higher-risk environments such as healthcare, food processing, laboratories or buildings with high dust or moisture levels.

Can I clean an AHU myself?

Basic external housekeeping, such as keeping outdoor air inlets and surrounding areas clear of debris, may be suitable for building maintenance teams. However, cleaning internal AHU components, coils, drain pans, fans, electrical areas, ductwork or contaminated sections should be carried out by trained and competent personnel. The AHU must be safely isolated before work begins, and the cleaning method should follow the manufacturer’s instructions and site safety procedures.

Why should you use Eurovent certified AHUs?

Eurovent Certified Performance for Air Handling Units provides independent third-party verification of declared product performance, helping specifiers, installers and building owners compare products using reliable data. Certified AHUs are assessed under programme rules covering selection software and relevant performance characteristics, reducing the risk that equipment underperforms because catalogue or software data is inaccurate. Where hygiene is a key requirement, selected AHUs can also be certified under the hygienic AHU option, supporting clearer specification for applications with stricter indoor air quality or hygiene needs.