Ductwork cleaning gets plenty of attention. The inside of the air handling unit itself often gets far less, even though it’s where most of the conditions for microbial growth sit together in one box: moisture, dust, warmth and airflow.
We open units every week that have had their filters changed on schedule but haven’t had the casing interior touched in years. This is what proper AHU hygiene involves.
Why the inside of the unit matters
Everything the building breathes passes through the AHU. If the internal surfaces are dirty, that dirt is in the supply air path. Poor internal hygiene shows up as:
- Musty odours reported by occupants
- Higher pressure drops across coils and filters, which costs energy
- Reduced heat transfer and poor temperature control
- Microbial growth in standing water
- Complaints that start as vague and turn into formal ones
Our guide on how your air handling unit affects indoor air quality covers the occupant side of this in more detail.
The parts that need attention
Drain pans and traps. Standing water is the single biggest hygiene risk inside an AHU. Pans should drain fully, and traps need to be correctly sized and primed, otherwise water sits and the trap either blows or sucks depending on which side of the fan it sits. We see blocked and mis-sized traps constantly on older units.
Cooling coils. Fins clog with fine dust that filters have let through, and the wet surface holds it. A dirty coil costs you capacity and pressure drop at the same time.
Filter housings and seals. A good filter in a poor housing lets air bypass it entirely. Check seals, clips and frames as well as the media. Our AHU filter maintenance guide covers grades and change intervals.
Humidifiers. Any wet component needs its own regime. We cover this in our guide to AHU humidification.
Casing interior, insulation and attenuators. Damaged internal insulation sheds fibres and holds moisture. Attenuator splitters are rarely inspected and are often filthy.
Heat recovery devices. Thermal wheels and plate exchangers collect dust across a huge surface area. On the Heathrow project the thermal wheel was a central part of the design, and units like that need the wheel surface and seals kept in condition or the recovery efficiency drops away quietly.
Where the standards sit
There isn’t one single AHU hygiene standard covering every building. What applies depends on what the building does:
- TR19 covers ductwork cleanliness and fire compliance, and it’s the reference most facilities managers know. See our guide to TR19 ductwork cleaning.
- SFG20 sets the maintenance task schedules most contracts are built around. Our SFG20 AHU maintenance article explains how the schedules work.
- HTM 03-01 applies in healthcare and is considerably more demanding. Read HTM 03-01: ventilation compliance for healthcare buildings.
- Water hygiene duties bring Legionella risk into play wherever there’s moisture. Our Legionella and your air handling unit guide covers the risk assessment side.
Food production, pharmaceutical and clean room environments carry their own requirements on top.
Building hygiene into your maintenance
Hygiene shouldn’t be a separate one-off exercise. It works best written into the planned maintenance schedule, with a deeper inspection annually and a record kept of what was found and what was done.
When we picked up the four units at Dalston Cross, the performance work started with getting the internals back into proper condition. It’s difficult to judge what a unit is capable of until it’s clean.
If your units are old enough that the internal insulation is breaking down or the casing is corroding, cleaning may not be the answer on its own. Our guide to AHU refurbishment vs replacement works through that decision.
How we could help
We maintain, refurbish and replace air handling units across the UK, from single units to plant rooms full of them. If you want your hygiene regime reviewed, or you’ve had complaints you can’t get to the bottom of, our AHU maintenance service covers inspection, cleaning and reporting.
Get in touch and we’ll arrange a site visit.
Frequently asked questions
It depends on the building and the filtration grade, but an annual deep inspection alongside routine filter changes suits most commercial buildings. Healthcare and food production usually need more.
Poor hygiene contributes to poor indoor air quality, and standing water carries a Legionella risk. Most complaints start as odours and stuffiness rather than illness, but they’re worth acting on.
Usually, yes. Clean coils and filters reduce pressure drop, which cuts fan energy and helps the unit hold temperature.
