Phoenix Journal · Ductwork
A small, well-handled fryer fire turned into a three-storey blaze because a grease-lined extract duct gave it a clear run through the building. Here is exactly how it happened - and how to stop it on your own site.
Case postmortem
It started with a single fryer flare-up in a busy first-floor kitchen, and within twenty minutes fire crews were fighting flames on the roof three storeys up - not because the building was poorly built, but because a grease-lined extract duct handed the fire a clear run through the structure.
The kitchen itself did what a kitchen is supposed to do. Staff hit the fryer with a fire blanket, the suppression nozzles discharged, and the flames on the range were out in seconds. Everyone assumed it was over. What nobody could see was that the flash of heat had already travelled up into the extract canopy and lit the film of carbonised grease coating the inside of the ductwork.
From there it stopped being a kitchen fire and became a building fire. The duct ran vertically from the kitchen, through a riser, past a plant room, and out through the roof plant - crossing floors that had nothing to do with catering. Grease burns hot and it burns along a surface, so the fire followed the fuel. By the time smoke was showing on the second floor, the flames were effectively behind the walls, moving through a void that no extinguisher in the building could reach.
This is the pattern behind a large share of serious commercial kitchen fires in the UK, and it is worth understanding in detail, because almost every part of it is preventable.
The ignition source was ordinary. Fryer fires, flare-ups from chargrills and radiant heat off the range are a normal hazard of a working kitchen, and a competent suppression system is designed to deal with them. The problem was never the spark. It was the fuel waiting in the duct and the route that fuel gave the fire.
Three failures stacked up, and each one on its own would have been survivable.
The ductwork was never being cleaned to standard. The site had a cleaning contract, but it covered the canopy, the filters and the first accessible metre of duct - the parts you can see. The long vertical run, the bends and the fan housing had not been opened in years. Grease does not sit still; it drips, pools and bakes onto the steel into a hard, combustible layer. The recognised UK benchmark, set out in BESA's TR19® Grease specification, is that grease deposits should not exceed a mean average of 200 microns between cleans. Behind the access panels on this system, deposits were many times that - thick enough that a competent surveyor would have condemned the run on sight.
Nobody was measuring, so nobody knew. There was no post-clean verification, no deposit readings, no photographic record of the internal surfaces. TR19® Grease expects cleaning to be verified down to a residual thickness of less than 50 microns and the condition of the system to be documented section by section. Without that evidence, "we get the extract cleaned" was a comfortable assumption rather than a fact. The Responsible Person under the Regulatory Reform (Fire Safety) Order 2005 is legally obliged to manage exactly this risk, and you cannot manage what you have never seen.
The duct defeated the building's compartmentation. Modern buildings are divided into fire compartments so that a fire in one area is held there long enough for people to get out and for crews to arrive. A grease-lined duct that runs through several of those compartments quietly punches a hole through that protection. Once the grease ignites, the fire cascades along the duct route and can break out into any compartment the duct passes through. Guidance from the London Fire Brigade and fire-engineering bodies is clear that kitchen extract ductwork leaving its compartment should be fire-rated all the way to discharge, and that ordinary fire dampers should not be relied on in greasy extract, because grease fouls the fusible links and jams them open. On this building, the duct was plain galvanised steel for its entire length. There was nothing between the fire and the rest of the structure.
Put those together and the outcome was almost scripted. A small, controllable fire found a continuous ribbon of fuel running through the heart of the building, inside voids where it could not be fought, crossing fire barriers that had been silently cancelled out.
None of what follows is exotic. It is the ordinary discipline that a properly managed commercial kitchen is expected to have in place, and it is the difference between a fryer fire that is out in seconds and a fire that closes a building for a year.
There is a commercial edge to this as well as a safety one. Most buildings and contents insurers now expect evidence of a TR19®-compliant cleaning regime, and a claim can be reduced or refused where a kitchen cannot show that its extract system was being inspected and cleaned to standard. The certificate that proves you are safe is often the same certificate that protects your cover.
The mechanism is worth spelling out, because it is the part most people miss. A kitchen fire is dangerous in the room it starts in. A duct fire is dangerous everywhere the duct goes. Grease deposits give the fire a continuous fuel path, the duct carries that path through voids and risers you cannot fight from inside the rooms, and every compartment wall the duct crosses becomes a potential breakout point. We look at exactly this behaviour in more detail in our piece on how fire spreads through building voids and ductwork, and the same logic explains why a clean, well-designed extract route matters so much more than its length in the building would suggest.
The uncomfortable truth from this case is that the kitchen team did nothing wrong on the night. They caught the fryer fire quickly and correctly. What failed them was months of unseen neglect in the ductwork and a design that let a duct quietly cancel the building's fire barriers. Both are fixable, and both are cheaper to fix than to survive.
Questions
The flames on the range were extinguished, but the heat had already ignited the grease coating the inside of the extract duct. That grease is a continuous fuel path, so the fire moved along the ductwork into risers and voids that could not be reached from the rooms. The kitchen fire was over in seconds; the duct fire was a separate, hidden event that carried the fire through the building.
BESA's TR19 Grease specification sets a mean-average limit of 200 microns of grease deposit between cleans, and after cleaning the system should be verified to less than 50 microns residual. Beyond 200 microns the fire risk rises sharply. Because the dangerous build-up is usually in the hidden vertical runs and fan housing, you cannot judge it by looking at the canopy - it has to be measured behind access panels.
The recognised UK approach bands the frequency by daily cooking hours: heavy use of around 12 to 16 hours a day suggests cleaning about every 3 months, moderate use of 6 to 12 hours about every 6 months, and light use of 2 to 6 hours about every 12 months. A fryer and chargrill operation running all day is a heavy-use site. The interval should be written into your fire risk assessment, not left to memory.
Possibly not. Most buildings and contents insurers now expect evidence of a TR19-compliant cleaning regime, and a claim can be reduced or refused where a kitchen cannot prove its extract system was being inspected and cleaned to standard. Keeping dated cleaning certificates, deposit readings and photographs is what protects both your building and your cover.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.