Canopy, filters & fan
A canopy can look spotless and still front a system loaded with grease. Appearance is a poor guide, because the grease that matters sits behind the filters and out of sight - in the plenum, the duct and the fan. TR19 Grease exists precisely because the eye cannot judge what the system actually holds.
The short answer
A canopy can look spotless and still front a system loaded with grease. Appearance is a poor guide, because the grease that matters sits behind the filters and out of sight - in the plenum, the duct and the fan. TR19 Grease exists precisely because the eye cannot judge what the system actually holds.
The detail
The visible canopy - the polished stainless front and the filters you can see - is the one part of the system kept clean by daily housekeeping. It is also the only part most people ever look at, which creates a powerful illusion: if the bit you can see is bright, the system feels clean. The trouble is that almost none of the fire-carrying grease sits where you can see it.
Directly behind the filters is the plenum, a hidden chamber that TR19 Grease singles out as a higher-risk area needing more frequent cleaning. Grease that gets through the filters settles here first, unseen from the kitchen. Beyond it the ductwork runs through the building, coating internally along its whole length, and the fan loads at the far end. None of this shows on the gleaming face of the canopy.
This is why TR19 Grease measures rather than looks. It sets a mean grease limit of 200 microns between cleans and requires post-clean deposits below 50 microns, checked with a Wet Film Thickness Test or Deposit Thickness Test at defined points - behind the filters, at intervals along the duct, upstream of the fan and at the discharge. Those points are chosen precisely because they are where the eye cannot reach.
A single reading tells the story a clean face cannot. A canopy wiped to a shine can sit in front of a plenum or duct measuring well over the mean limit, sometimes past the 500-micron level that demands immediate attention. The measurement, not the mirror finish, is what reveals whether the system is genuinely safe or simply presentable.
What it means for you
The takeaway is to distrust a clean-looking canopy as proof of anything. It proves the kitchen wipes its hood, which is worth having, but it says nothing about the plenum, duct or fan. The only honest answer to whether the system is clean comes from inspecting and measuring the hidden parts, not from admiring the visible one.
This is where a proper inspection earns its place. Opening access panels, looking behind the filters and taking thickness readings turns a guess into a fact, and often reveals a system far greasier than its front suggested. It also sets the real cleaning frequency, because the measured rate of grease build-up - not the look of the canopy - is what should drive the schedule.
For anyone responsible for a kitchen, the shift is from appearance to evidence. A post-clean verification report, with measurements and photographs of the hidden surfaces, is what demonstrates the system is clean - to you, to a fire risk assessment and to an insurer. The canopy looking clean is pleasant; the report is what actually counts.
The service behind the guide
Sibling guides
Plenum cleaning above the filters · Grease gutters and capture trays · A schedule you can prove
Questions
Because the fire-carrying grease sits behind the filters, in the plenum, duct and fan - out of sight. Daily housekeeping keeps the visible front bright while the hidden parts load with grease.
First in the plenum directly behind the filters, then along the whole length of ductwork, and finally on the extract fan. None of this shows on the visible canopy face.
By inspection and measurement, not appearance. Deposits are checked with a Wet Film Thickness Test or Deposit Thickness Test at defined points and compared against the 200-micron mean and 50-micron post-clean limits.
Because the eye cannot judge the hidden parts of the system. Measurement at points behind the filters, along the duct and at the fan reveals what a clean-looking canopy conceals.
A post-clean verification report with thickness measurements and photographs of the hidden surfaces. That evidence, not a shiny canopy, is what satisfies a fire risk assessment and an insurer.
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.