Three duct types
The air handling unit is the start of a building's supply air - it draws fresh air, filters, heats or cools it, then pushes it into the ducts. Dirt at the AHU spreads to everything downstream, which is why it is cleaned first.
The short answer
The air handling unit, or AHU, is the box where a building's supply air begins. It draws in fresh air, passes it through filters, heats or cools it across coils, sometimes humidifies it, and a fan pushes it out into the supply ductwork that serves the rooms. Because everything downstream is fed from the AHU, contamination there spreads through the whole system - which is why the AHU and the supply ducts are surveyed and cleaned together under TR19 Air, and why the plant end matters most.
What is in the AHU
An AHU is a sequence of components, each a place dirt collects. Filters catch incoming particulates and load up over time; heating and cooling coils gather dust that bakes or dampens onto their fins; drain pans and other wet areas around the cooling coil hold moisture where bacteria and mould can grow; and the fan and casing accumulate the rest. Left alone, a fouled AHU does two things: it restricts airflow so the plant works harder and less efficiently, and it becomes a reservoir that seeds the supply air with dust, spores and microbial growth - then delivers them to every room it serves.
That is why the wet areas and filters are watched more closely than the ductwork itself. Under BS EN 15780 the recommended inspection intervals are shortest for filters and wet areas, longer for the AHU as a whole, and longer again for the ducts - tightening as the space served moves from low to high risk.
Why the supply ducts follow
The supply ducts carry whatever leaves the AHU into the occupied space, so they are held to the tighter, breathable-air cleanliness level - the supply side has always carried a stricter trigger than extract. Cleaning the AHU without the ducts, or the ducts without the AHU, leaves half the path contaminated; the survey and clean run the whole way, from the fresh-air intake through the unit and out along the supply run to the grilles and diffusers.
Access is the practical constraint. Panels roughly every couple of metres let a contractor reach and verify the full length rather than the easy sections, and the AHU needs enough access to reach filters, coils and drain pans. For how the cleanliness classes themselves are set by building use, the dry-side overview covers it - this is the short version of where to start.
What a clean covers
A TR19 Air clean of the supply side runs from the intake through the AHU - filters, coils, drain pans, fan and casing - and out along the supply ducts, returning surfaces to the cleanliness level set for the space. It finishes with verification: deposit readings or a vacuum test against the target, before and after photographs, and a post-clean report. Since March 2026 general ventilation hygiene work is reportable on the Vent Hygiene Register, the recognised scheme for evidencing that the work met the standard - so the clean produces documented compliance, not just a cleaner unit.
Questions
An air handling unit - the plant where supply air begins. It draws in fresh air, filters it, heats or cools it across coils, sometimes humidifies it, and a fan pushes it into the supply ductwork serving the building.
Everything downstream is fed from the AHU, so contamination there - dust on coils, growth in drain pans, loaded filters - spreads to every room. Cleaning the ducts without the AHU leaves the source of the problem in place.
Filters and wet areas such as drain pans, because they load and grow fastest. BS EN 15780 sets the shortest inspection intervals for those, longer for the AHU overall and longer again for the ductwork.
Yes. Supply air ends up in occupied rooms, so it carries a tighter cleanliness trigger than extract, which is judged on the contaminant it carries away.
With deposit readings or a vacuum test against the target, before and after photographs, and a post-clean report. Since March 2026 this general ventilation work is reportable on the Vent Hygiene Register.
Phoenix Duct Clean · by the numbers
We clean from the fresh-air intake through the AHU and out along the supply ducts, to TR19 Air, with a post-clean report. Call to arrange a survey.