Canopy, filters & fan
A filter is the cheapest place in the whole system to stop grease. When it is neglected, wrongly specified or clogged, it stops doing that job - and the grease it should have caught travels straight into the plenum, ductwork and fan, turning a simple sink task into a slow, disruptive and far more costly system clean.
The short answer
A filter is the cheapest place in the whole system to stop grease. When it is neglected, wrongly specified or clogged, it stops doing that job - and the grease it should have caught travels straight into the plenum, ductwork and fan, turning a simple sink task into a slow, disruptive and far more costly system clean.
The detail
A grease filter works by capture efficiency - a good baffle catches around 85 to 90 per cent of airborne grease at the correct airflow. That figure is not fixed. When a filter blinds with baked grease, air stops turning cleanly through its channels and forces through the gaps instead, and the droplets that should have struck the vanes are carried past. A clogged filter does not just work less well; it actively passes grease downstream.
The wrong filter, or the wrong airflow, has the same effect. Baffles only reach their rated efficiency at the right face velocity, so a canopy where the fan is undersized, or where mesh has been swapped for baffle without a redesign, never captures what it should. Whatever the cause, grease that is not stopped at the filter has only one place to go: into the system behind it.
Once past the filter, grease settles first in the plenum directly behind it, the higher-risk chamber that TR19 Grease singles out for more frequent attention. From there it coats the ductwork along its whole length and finally loads the extract fan's blades and housing. Each of those surfaces is harder to reach and slower to clean than a filter, and each adds to the fire load carried through the building.
The cost curve is steep. A filter is cleaned in minutes at a sink; a plenum needs the canopy stripped; a duct needs access panels, specialist equipment and often out-of-hours working; a fan needs isolating and dismantling. Letting a filter fail quietly converts the cheapest job in the system into the most expensive ones, and shortens the interval the whole system can safely run.
What it means for you
The lesson is that filter care is system protection, not housekeeping. Keeping filters clean, correctly specified and running at the right airflow holds grease at the one point where removing it costs almost nothing. It is the highest-leverage maintenance in the whole kitchen, precisely because everything downstream depends on it.
It also reframes what a weak or greasy filter is telling you. A canopy that drips, a duct that grimes up fast between cleans, or a fan that greases quickly is often a filter problem in disguise - the grease reaching those places got past a filter that was blinded, wrong or starved of airflow. Fixing the filter fixes the symptom at source.
When the filter is doing its job, the periodic system clean stays small and predictable. When it is not, the plenum, duct and fan load faster, the TR19 interval shortens, and the fire and insurance risk rises across the whole system. The filter is small and cheap, but what it protects - and what it exposes when neglected - is the entire extraction path.
The service behind the guide
Questions
A blinded filter stops capturing grease, so the droplets it should have caught carry past into the plenum, ductwork and fan. It does not just work less well - it actively passes grease downstream.
First into the plenum behind the filters, then along the whole length of ductwork, and finally onto the extract fan's blades and housing - each harder to reach and slower to clean than the filter.
Baffles only reach their rated efficiency at the correct airflow. An undersized fan, or mesh swapped for baffle without a redesign, means grease is never fully captured and passes into the system.
A filter is cleaned in minutes; a plenum needs the canopy stripped, a duct needs access panels and specialist work, and a fan needs isolating and dismantling. A neglected filter turns the cheapest job into the most expensive ones.
Keep filters clean, correctly specified and running at the right airflow. That holds grease at the one point where removing it costs almost nothing and keeps the whole system's cleaning interval predictable.
Phoenix Duct Clean · by the numbers
We clean the whole path - canopy, filters, plenum, ductwork and fan - and verify the result to TR19 Grease. Call or email for a clear quote and a certificate you can show your insurer.