Three duct types
It moves air, not grease, so nothing forces the issue. But dry ductwork fills with dust, hurts air quality and efficiency, and TR19 Air sets a measurable standard for it.
The short answer
Dry ductwork - the supply, recirculation and general extract that moves air through offices, function rooms and corridors - gets forgotten because nothing catches fire on a normal day. But it quietly fills with dust, pollen and microbial growth, circulating them through the building, restricting airflow and raising energy costs. Its standard is TR19 Air, and its risk is real, just quiet.
What dry ductwork is
Where kitchen extract carries grease and laundry ducting carries lint, dry ductwork carries air - fresh supply in, stale extract out, sometimes recirculated around. It serves the parts of a building people occupy all day, and it is usually out of sight above ceilings and in risers. Out of sight is exactly the problem: because it does not smell, drip or scorch, it slips off the maintenance schedule while the grease system stays on it.
Yet the surfaces involved are vast. The internal surface area of a ventilation system is roughly a tenth of the floor area it serves, which is a great deal of hidden surface for dust, pollen, bacteria and mould spores to settle on and be blown off into occupied rooms.
The standard
Dry ventilation is cleaned to TR19 Air, the BESA specification first published in 2024 and aligned with BS EN 15780. It classifies systems by use into low, medium and high cleanliness quality. A hospital operating theatre or laboratory is high; a general office, hotel or school is medium; an infrequently occupied boiler room or store is low. Each class carries a dust-deposit target - a medium-risk system, for instance, works to around three grams per square metre, with higher allowances for recirculation and secondary ductwork - so cleanliness is measured, not guessed, and verified with readings and photographs.
Access matters as much as method: good practice puts access panels along the run so the whole system can be reached and cleaned, not just the sections near a grille.
Why it is worth doing
The case for cleaning dry ductwork is threefold. First, indoor air quality: dirty ducts recirculate dust, allergens and microbial growth that affect the people breathing the air, and clean ventilation is a workplace expectation under the Workplace regulations. Second, efficiency: a fouled system makes fans and air handling units work harder, shortening their life and raising running costs. Third, and easily missed, fire: a heavy accumulation of dust is itself a combustible load, so a neglected dry system is not entirely free of fire risk either.
None of these forces itself on your attention the way a grease fire does, which is exactly why dry ductwork is the system everyone forgets - and why putting it on a schedule, class-rated and verified, is the quiet win.
Questions
The supply, recirculation and general extract ductwork that moves air rather than grease - serving offices, function rooms, corridors and plant. It is distinct from kitchen grease extract and laundry ducting.
TR19 Air, the BESA specification first published in 2024 and aligned with BS EN 15780, which classifies systems by use as low, medium or high cleanliness quality with dust-deposit targets.
Because it does not smell, drip or catch fire on a normal day, it drifts off the maintenance schedule - even though the internal surface area is roughly a tenth of the floor area it serves and collects a lot of contamination.
Poor indoor air quality from recirculated dust, allergens and microbial growth; reduced efficiency and higher energy costs as fans work harder; and a heavy dust load that is itself a combustion risk.
By cleaning the whole system to its cleanliness class, then confirming with dust-deposit readings and before-and-after photographs, using access panels along the run to reach every section.
Phoenix Duct Clean · by the numbers
We clean ventilation ductwork to TR19 Air - class-rated and verified. Ask us to assess the air-side system you may be forgetting.