Three duct types
A ventilation system moves air two ways: supply pushes fresh air into rooms, extract pulls stale air out. Because one feeds what people breathe and the other carries contaminants away, they are held to different cleanliness levels.
The short answer
Any mechanical ventilation system moves air in two directions. Supply ductwork carries fresh or conditioned air from the plant into the rooms people occupy; extract ductwork pulls stale, moist or contaminated air out and sends it to a fan and discharge. Because supply air ends up in people's lungs while extract air is on its way out of the building, the two sides are surveyed and cleaned to different cleanliness levels - and neglected for different reasons.
Why supply is held to a tighter level
Supply ductwork is the last thing the air passes through before it reaches a room, so anything living in it - dust, pollen, bacteria, mould spores - is delivered straight to the occupants. That is why the cleanliness trigger has always been stricter on the supply side: historically a mean deposit above sixty microns on a supply or recirculation surface defines a cleaning requirement, against a far more tolerant level for extract. BS EN 15780 sets the modern dust-accumulation limits the same way, tightening as the space it serves gets more sensitive - a general office falls in the medium class, a hospital theatre or lab in the high class.
Recirculation ductwork - which mixes return air back into supply - sits with the supply side for exactly this reason, though it carries a slightly more generous limit than primary supply. The principle is constant: the closer a duct is to delivering breathable air, the cleaner it has to be.
Why extract is cleaned for different reasons
Extract ductwork is judged by the contaminant it removes, not by air quality delivered downstream, so its tolerance is looser - the historic trigger for extract was around one hundred and eighty microns. But looser does not mean ignorable, because what the extract is carrying decides the risk. A kitchen grease extract is an extract duct whose contaminant is combustible grease, so it is governed by TR19 Grease and its fire-risk regime entirely. A general extract carrying dust and moisture is a TR19 Air matter, where the concern is build-up, airflow and, in wet runs, microbial growth.
So the same word - extract - covers very different jobs. The clean follows the contaminant: grease on one run, dust and damp on another, and each with its own standard and its own evidence.
What each side needs in practice
In practice a good survey treats the system as a whole and applies the right threshold to each part: supply and recirculation held to the tighter class for the space served, extract judged on its contaminant, wet areas and filters watched more closely because moisture breeds growth. Access panels roughly every two metres let a contractor reach and verify the full run rather than the easy sections, and a post-clean report with before and after readings and photographs proves each part met its own level. For the detail on how the classes are set, the dry-side overview covers it - this page is about why the two directions differ.
Questions
Supply carries fresh or conditioned air from the plant into occupied rooms; extract pulls stale, moist or contaminated air out to a fan and discharge. One feeds what people breathe, the other carries air away.
Because its air ends up in people's lungs. Historically a mean deposit above around 60 microns defines a cleaning need on supply or recirculation surfaces, tighter than the extract trigger, and BS EN 15780 sets dust limits the same way.
Yes - it is an extract whose contaminant is combustible grease, so it falls under TR19 Grease and its fire-risk regime, not the general TR19 Air dust standard that covers other extract runs.
With the supply side, because it mixes return air back into the supply. It carries a slightly more generous limit than primary supply but is still held to the tighter, breathable-air principle.
A survey treats the system as a whole and applies the right threshold to each part - supply and recirculation to the tighter class, extract judged on its contaminant, with wet areas and filters watched more closely.
Phoenix Duct Clean · by the numbers
We survey and clean supply and extract ductwork to the right TR19 Air thresholds, with a post-clean report. Call to arrange an assessment.