Fire, insurance & risk
Cooking coats your ductwork in combustible grease. Here's why that layer is a fire risk long before anything ignites - and what cleaning takes back out.
The short answer
Grease lining a kitchen extract duct is stored fuel. Cooking condenses fat onto every inside surface, meal after meal, until the run is coated in a combustible layer. All it then needs is heat - a grill flare-up or a fryer running too hot reaches the temperature at which that grease ignites. A duct that has not caught fire yet is not safe; it is loaded and waiting.
Why grease burns
Cooking releases fat, oil and grease as airborne vapour. As that vapour is drawn into the extract system and cools, it condenses and sticks to the canopy, the filters, the plenum and the full length of the ductwork. No filter catches all of it, so the layer grows steadily out of sight.
Oil in liquid form does not burn - its vapour does. Heat it and it smokes, then reaches its flash point, then ignites. Cooking-oil autoignition sits in the region of 315 to 435 degrees Celsius depending on the blend, and grease baked onto a duct wall is combustible in the same range. A flare-up on the grill or an over-heating fryer clears that bar easily. Worse, once cooking oil has caught, its ignition temperature can drop - so it can re-ignite after it appears to be out, unless it is properly cooled.
The fuel load nobody sees
The deposit does not have to be thick to be dangerous. A barely visible film can carry fire, which is why TR19 Grease caps the mean layer at around 200 microns - about half a business card - and calls for an immediate spot-clean anywhere it reaches 500. The danger is the surface area: a full run of ductwork coated in even a thin skin of grease is a large, continuous fuel bed.
Commercial kitchen extract is a recognised leading source and accelerant of non-domestic fires - roughly a quarter of accidental fires in non-domestic buildings trace back to cooking appliances, and a greasy system turns a contained flare-up into a travelling fire. The reassuring thought that it has been fine for years is not safety; it is an untested fuel store.
What removes the risk
You cannot stop grease being produced, but you can stop it accumulating. Regular specialist cleaning of the whole run - not just the canopy - strips the fuel back out of the system before it reaches a dangerous depth, and daily wiping of canopies and filters slows the build-up in between. Cleaning to TR19 Grease, with readings to prove the layer is back under control, is how you turn a loaded duct into a safe one.
The parts you cannot see are the ones that matter most: the horizontal runs and vertical risers behind ceilings and up walls hold the highest, most hidden fuel load. That is exactly where a canopy-only clean leaves the danger untouched.
Questions
Grease is fuel. It condenses on the inside of the extract system and builds into a combustible layer. A flare-up or over-heating appliance can reach the temperature at which it ignites, turning the whole greasy run into a fire path.
Cooking-oil autoignition is roughly 315 to 435 degrees Celsius depending on the oil, and grease baked onto ductwork burns in a similar range. Grill and fryer flare-ups easily reach these temperatures.
Not much. A thin, barely visible film can carry fire. TR19 Grease caps the mean layer at about 200 microns and requires an immediate spot-clean at 500 microns.
Not necessarily. An uncleaned duct is stored fuel that simply has not met an ignition source yet. The absence of a fire so far is not evidence the system is safe.
No. The highest, most hidden fuel load is in the ductwork and risers behind ceilings and walls. A canopy-only clean leaves that untouched, so the risk remains.
Phoenix Duct Clean · by the numbers
We clean the full extract run and prove the grease is gone - before a flare-up finds it.