An air handling unit, or AHU, is the box of equipment that moves air through a building’s ventilation system. It draws in fresh air, filters it, heats or cools it and pushes it through the ductwork. Because all that air passes through the unit, anything that builds up inside it ends up in the air people breathe. This guide explains the parts of an AHU that need cleaning and how cleaning fits into TR19® Air.
What’s inside an AHU
Units vary, but most include some or all of:
- Filters that catch dust, pollen and debris from incoming air
- Heating and cooling coils, rows of fins that warm or chill the air
- Drain trays under cooling coils, which collect condensation
- Fans that move the air through the unit and ductwork
- Heat recovery sections, on some units
- Internal surfaces, the inside walls and floor of the casing
What goes wrong
Dust and debris build up on filters, coils and fan blades. Clogged filters cut airflow and make the fan work harder. Dirty coils don’t heat or cool as well. The biggest hygiene concern is often the drain tray. If it doesn’t drain properly, standing water collects, and that can lead to slime, mould and smells being carried into the building. Debris can also gather on internal surfaces and in the ductwork beyond.
The signs aren’t always obvious from outside the unit. Rooms that feel stuffy, uneven temperatures, musty smells when the system starts up, or a fan that sounds strained can all point to a unit that needs attention. Regular inspection catches these problems before people start to complain.
What an AHU clean involves
- Inspect the unit and record its condition, with photos.
- Check filters and replace them if the maintenance plan requires it.
- Clean coils carefully, without damaging the fins.
- Clear and clean drain trays and check the drain runs.
- Clean fans, fan housings and internal surfaces.
- Record what was cleaned and anything that needs attention.
The unit is isolated before work starts, and the work is planned around building use so that areas aren’t left without ventilation at busy times. Duct Doctor offers AHU cleaning alongside ventilation duct cleaning, with a photo report showing the unit before and after.
Need it done? Duct Doctor cleans and tests kitchen extract and ventilation systems across London and the South East, with a certificate and photo report after every clean. Get a fixed quote.
TR19 Air and how often
TR19® Air covers the hygiene of general supply and extract ventilation systems, including AHUs. It takes a risk-based approach: systems are inspected, and cleaning is carried out when inspection shows it’s needed (BESA, Ventilation Hygiene Standards). Buildings with vulnerable occupants, such as hospitals and care homes, or units in dusty locations, may need more frequent attention. See TR19 Air explained.
Ductwork and dampers beyond the unit
A clean AHU feeding dirty ductwork won’t deliver clean air. Supply ducts can hold dust and debris, and extract ducts carry whatever the building gives off. Inspection should cover the ductwork as well as the unit. Fire dampers in ventilation ducts should be tested at least annually, according to BESA, and dust can stop them closing (BESA).
Keeping records
Keep cleaning reports, photos and filter change records together in the building’s maintenance log. BESA advises that certificates should record the date, location and technician (BESA). Good records help show building users, insurers and auditors that the ventilation is being looked after.
Quick answers
How often should an AHU be cleaned?
TR19 Air uses inspection to decide. Filters need regular checks, and the rest is cleaned when inspection shows it’s needed.
Why does my AHU smell?
Standing water in the drain tray is a common cause. It should be cleared and the drain checked.
Can an AHU be cleaned while the building is open?
Usually, with planning. The unit is isolated for the work, so timing is agreed with the building manager.
Related services
More advice
- TR19 Air Explained: Ventilation Hygiene for Buildings
- Fire Damper vs Smoke Damper: The Difference
- School Kitchen Extract Cleaning: Holiday Planning
- Care Home Kitchen Ventilation and Extract Cleaning
- All advice
This guide is general information. It isn’t legal or insurance advice: check your own policy wording and lease, and speak to a qualified adviser about your fire risk assessment.