Phoenix Journal · Ductwork
The grease lining your extract ductwork does not need a spark to catch fire. Understanding the temperatures at which it ignites - and how ageing deposits change the odds - is what separates a controlled kitchen from a serious blaze.
Every commercial kitchen runs a quiet experiment in fire chemistry, and the ductwork is where the results collect.
When you fry, grill and sauté, you drive a fine mist of oil and fat into the extract system. Some of it condenses on the canopy, the filters and the internal walls of the ducting, building a layer that thickens week by week. That layer is fuel. The only question that really matters for fire safety is how hot it needs to get before it lights - and, crucially, whether it needs a flame to do so at all.
There are two very different ways that stored grease catches fire, and they behave nothing alike. The first is flame-driven ignition, governed by the flash point and fire point of the oil. The second is auto-ignition - combustion that starts with heat alone, no spark and no naked flame required. Knowing where each threshold sits, and what pushes it lower, tells you exactly why a clean duct is a safe duct. Let us put the two mechanisms side by side.
This is the fire most people picture. Oil or grease is heated until it releases enough vapour to catch when a flame or spark reaches it. For most cooking oils the flash point sits around 315°C - the point at which the vapour will flash alight momentarily. A little higher is the fire point, where the flame sustains itself and keeps burning. Below both, at roughly 200°C and up depending on the oil, is the smoke point: not a fire, but the visible early warning that the fat is breaking down and heading towards trouble.
In a working kitchen this matters most at the cooking line and the first stretch of ductwork, where naked burners, chargrill embers and stray sparks live cheek by jowl with grease-laden air. Those same greasy deposits that condense on your canopy can, once liquefied by heat, run back towards the source - which is why grease dripping from your canopy is never just a housekeeping nuisance.
The good news
The catch
This is the mechanism the title of this article is really about, and it is the one that catches operators off guard. Auto-ignition is the temperature at which a substance bursts into flame from heat alone, with no spark and no external fire to light it. For liquid cooking oil that figure is high - somewhere around 400°C to 435°C depending on the oil. That sounds reassuringly out of reach, until you look at what happens to grease once it has been sitting in a duct.
Residual and carbonised deposits behave far worse than fresh oil. Spontaneous ignition of aged grease films is reported from roughly 310°C to 360°C - well under the figure for liquid oil. Worse still, thick layers of old, oxidised grease can generate their own heat through slow exothermic oxidation, effectively pre-warming themselves toward that threshold. This is the same self-heating process that lets an oil-soaked cloth combust in a bin, and it is why grease that has escaped the system to settle elsewhere - the sort of build-up behind grease marks on your ceiling - is a genuine hazard, not a cosmetic one.
The good news
The catch
Both mechanisms point to the same conclusion: the fire risk is the grease, and the control is removing it before it accumulates. In the UK the benchmark is BESA’s TR19® Grease specification, published in 2019 as the dedicated standard for kitchen extract fire-risk cleaning. It sets grease deposits a mean average limit of 200 microns across the system before a full clean is due, with any single reading of 500 microns or more demanding immediate localised attention. After cleaning, a competent technician verifies the result below 50 microns using a deposit thickness test or wet film thickness test, so you hold objective evidence rather than a gut feeling.
Frequency follows use: broadly, a heavy-use kitchen running 12 to 16 hours a day is looked at every three months, moderate use every six, and light use every twelve, with the exact interval set by a cleanliness risk assessment rather than a calendar guess. Work should be carried out by someone holding the BESA Grease Hygiene Technician qualification and documented properly - the record is what satisfies your insurer and your responsible-person duty if the worst ever happens.
Flash point and auto-ignition are two doors into the same fire, and a commercial kitchen leaves both of them ajar. Flame-driven ignition needs a spark you might just control; auto-ignition needs only heat and time, and ageing deposits keep lowering the bar. You cannot change the chemistry of fat, but you can remove the fuel - and once the metal is clean, there is simply nothing left to reach either threshold.
Questions
It depends on the state of the grease. Fresh liquid cooking oil auto-ignites at roughly 400°C to 435°C, while its flash point - where vapour lights from a spark - is around 315°C. Aged, carbonised deposits in ductwork are the real concern, as they can spontaneously ignite from about 310°C to 360°C and can even self-heat towards that point through slow oxidation.
Flash point is the lowest temperature at which a substance gives off enough vapour to ignite momentarily when a flame or spark is present. Auto-ignition temperature is higher and needs no external spark at all - the material simply combusts from heat alone. In a kitchen, flash point matters near naked flames at the cooking line, while auto-ignition is the hidden risk deep in heated ductwork.
Fire needs fuel, and in an extract system the fuel is the grease itself. Cleaning to the TR19® Grease standard keeps deposits below the 200 micron mean threshold and verifies the finish below 50 microns, so there is little left to reach any ignition temperature. It also removes the thick, oxidised layers that can self-heat, and gives you documented proof of compliance for your insurer.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.