Phoenix Journal · Extraction
A spotless canopy tells you nothing about the metres of hidden ductwork it feeds - and that gap is where fire risk, corrosion and invalidated insurance quietly build up.
A stainless canopy that gleams under the kitchen lights tells you almost nothing about the state of the system it feeds - and that gap is where the real risk lives.
Most kitchen teams keep the visible metalwork spotless. The canopy gets wiped down at the end of every shift, the baffle filters go through the dishwasher, and the front edge shines. It looks clean because it is clean - on the surface. The trouble is that the canopy is the doorway to a system that runs for many metres out of sight: up through the plenum, into the riser, along horizontal ductwork above the ceiling, through the fan housing and out to the discharge point on the roof.
Every gram of vaporised fat that the filters do not catch travels into that hidden run and condenses on the cold internal walls. You never see it, so it is easy to assume it is not there. In reality, a canopy that looks immaculate can sit at the mouth of ductwork carrying a thick, tacky, highly flammable coating - and that is the damage this article is about.
The relevant benchmark in the UK is TR19® Grease, the specification published by the Building & Engineering Services Association (BESA) for managing the fire risk of grease accumulation in kitchen extract systems. It applies to the whole system, not just the bit you can reach - from the canopy, through the ductwork, to where the system leaves the building.
What matters is that the standard is written around measurement, not appearance. A technician takes readings in microns using a deposit thickness test or wet film thickness test. TR19® Grease works to a mean deposit of under 200 microns across the system, treats any single reading above 500 microns as needing immediate localised cleaning, and expects a verified post-clean level below 50 microns. A micron is a thousandth of a millimetre, so these are coatings you would struggle to spot with the naked eye at the canopy - and coatings that are thicker still, undisturbed, in the parts nobody ever opens up.
This is exactly why a clean-looking canopy is such a poor guide. The one section that gets wiped daily is the one section least likely to be carrying a dangerous deposit. The build-up concentrates where the air slows and cools - the horizontal runs, the bends, the fan - and those are the places access panels every two metres exist to reach. If your ductwork has no panels, or they are painted over and sealed above a false ceiling, nobody has measured what is in there at all.
The headline risk is fire. Grease-lined ductwork behaves like a fuse. If a flare-up at the hob reaches the extract, the flames have a continuous fuel path running through the fabric of the building, and a fire that should have stayed at the range can travel the full length of the duct in seconds - into ceiling voids, plant rooms and floors above. Grease build-up in extract systems is one of the main reasons kitchen fires spread rather than staying contained.
Fire is not the only thing eating away out of sight. Old grease is acidic, and left long enough it corrodes the ductwork itself, pitting seams and joints until sections need replacing rather than cleaning. It also rots slowly, which is how a kitchen ends up with a persistent stale smell drifting back down through a canopy that looks perfectly clean - if that sounds familiar, it is worth reading why a canopy can be clean but the system still smells. And a warm, greasy, undisturbed duct is close to ideal habitat for insects and rodents, which is a separate problem that builds up quietly behind clean-looking equipment.
Then there is the commercial damage, which tends to arrive at the worst possible moment. UK insurers expect evidence of TR19®-compliant cleaning as a condition of fire cover. If a claim follows a kitchen fire and you cannot produce dated certificates and grease-level readings showing the system was maintained to standard, the insurer can reduce or refuse the payout - and can treat the policy as breached. A spotless canopy is not evidence of anything. A signed report with before-and-after micron readings, a photographic record and a schematic of what was accessed is.
The honest answer is that you cannot judge it from the kitchen floor, and neither can anyone who only cleans what they can see. Knowing the true condition of your system means opening it up along its length, measuring deposits at points across the whole run, and cleaning to a verified figure rather than to a shine. A proper visit ends with paperwork you can hand straight to your insurer.
A few practical pointers help you stay ahead of the hidden build-up:
The point is not that a clean canopy is bad - it is that it is not the measure that matters. The measure that matters is what an instrument reads inside the ductwork, and that is only ever visible once the system is properly opened, cleaned and certified.
Questions
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TR19 Grease works to a mean grease deposit of under 200 microns across the system, and treats any single reading above 500 microns as needing immediate localised cleaning. After a clean, technicians verify the result is below 50 microns using a deposit thickness or wet film thickness test. These are figures measured with an instrument, not judged by eye.
Yes. UK insurers generally expect evidence of TR19-compliant cleaning as a condition of fire cover. If a fire occurs and you cannot produce dated certificates and grease-level readings showing the system was maintained to standard, the insurer can reduce or refuse the claim and may treat the policy as breached. A clean-looking canopy is not evidence; a signed report with before-and-after readings is.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.