Phoenix Journal · Ductwork
Most serious kitchen fires do not begin at the cooking line - they begin in the grease coating your extract system, and travel from there. Here is the real sequence, and where to break it.
Fire risk explained
Most kitchen fires do not begin at the hob - they begin above it, in the warm, greasy dark of the extract system, and by the time anyone smells smoke the fire is already travelling.
Ask most operators where a serious kitchen fire starts and they will point at the fryer or the chargrill. That is where the spark comes from, but it is almost never where the damage is done. The real fuel load sits out of sight, coating the inside of your canopy, filters, ductwork and fan. Grease that has been drifting up on cooking vapour for months is the difference between a flare-up you beat out with a wet-chemical system and a fire that runs the length of the building. Understanding the sequence - how a small ignition becomes a structural fire - is the whole point, because every step in that chain is one you can break with cleaning.
A fire needs only three things - heat, fuel and oxygen - and a commercial kitchen extract system serves up all three in one place. Here is the sequence, step by step, from the first pan of oil to a fire in the roof void.
What makes ductwork fires so serious is that every factor works against you at once. The fuel is spread over a long, enclosed run rather than concentrated in one spot, so a wet-chemical or CO₂ system covering the cooking line does nothing for a fire that has already moved past it. The fan pulls air through the burning duct, feeding oxygen to the flame and drawing it deeper into the building. And because it is all hidden inside metalwork, nobody sees it start - the first sign is often smoke appearing on a floor above the kitchen.
The numbers back this up. Around 70% of fires in commercial kitchens are linked to the extract ventilation system and the fat and grease inside it, and industry bodies estimate that roughly 90% of catering fires are intensified once deposits in the grease extract ductwork ignite. UK fire services recorded well over a thousand fires in food and drink premises in a single recent year. The pattern is consistent - the ignition is small and ordinary, but the grease turns it into something that writes off a kitchen.
There is a financial edge to this as well. Insurers now treat kitchen extract cleaning as a condition of cover, not a nicety. Payouts on fires traced to poorly maintained extract ducts run into tens of millions of pounds a year across the UK, and a claim can be reduced or refused outright if you cannot show the system was being cleaned to a recognised standard. The recognised standard here is BESA’s TR19® Grease specification, which sets out how deposit levels are measured and how often a system of your usage should be cleaned - heavy use pointing toward every three months, moderate use every six, and lighter use annually.
It is worth remembering that grease does not only travel upward. The same cooking process that loads your ductwork also sends fats down the sink, which is why grease trap management sits alongside extract cleaning as part of the same underlying problem - fat that is left to accumulate where it should not. Deal with one and ignore the other and you have only solved half of it. The fire risk, though, lives in the duct, and that is where a proper clean earns its keep.
The reassuring part of all this is that a grease fire is a chain of events, and you only have to break one link to stop it. You cannot stop fat coming off the cooking line, and you cannot promise there will never be a flare-up. What you can control is the fuel sitting in the duct. Strip that back to bare metal on a schedule matched to how hard you cook, keep the certificate and the before-and-after photographs, and the worst-case sequence never gets past step four. That is the difference between an incident and a catastrophe, and it costs a fraction of either.
Questions
The ignition normally comes from the cooking line - a fryer boil-over, a chargrill flare or a burner left running. The damage, though, is done in the extract system above it, where months of grease have coated the canopy, filters and ductwork. That hidden fuel is what turns a manageable flare-up into a fire that spreads through the building, which is why around 70% of kitchen fires are linked to the extract system rather than the appliance itself.
The grease lining inside the duct behaves like a fuse. Once hot gases from a flare-up ignite the deposit in the canopy, the flame follows the greasy coating along the inside of the ductwork, drawn deeper by the extract fan and fed with oxygen as it goes. Because it is all enclosed in metal, it moves unseen through walls, ceilings and roof voids before breaking out into the structure - often on a floor well away from the kitchen.
It depends on how hard you cook. BESA's TR19 Grease specification links frequency to usage - heavy use points toward cleaning roughly every three months, moderate use every six months, and lighter use annually. The system also uses a measured deposit threshold of 200 microns as the level that needs attention, so a proper clean should measure grease levels, strip back to bare metal and leave you with a certificate and photographs 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.