Phoenix Journal · Ductwork
A kitchen extract duct that nobody can reach is a fire waiting for a fuel path. Getting the routing right on the drawings - velocity, access and fire containment - is where that risk is won or lost.
Case postmortem
A busy restaurant kitchen sailed through every hygiene visit it faced, yet two years after fit-out a small flare-up at the range ran the length of the extract duct, into the ceiling void and up through two floors before the brigade got hold of it.
The cooking line was not the problem. The canopy was not the problem. The fault was hidden above the plasterboard, decided long before anyone lit a burner. To save a few metres of steel, the horizontal extract run had been taken the shortest way back to the fan - squeezed tight into a boxed riser, pinched around a structural beam with a pair of hard bends, and fitted with a single access panel near the canopy and nothing else for the whole run to roof level.
By the time of the fire, that unreachable run held a heavy, hardened layer of grease. Nobody had cleaned it, because nobody could reach it. When flame from a pan found its way into the duct, it had a continuous fuel path and a sealed steel chimney to travel through. The investigation that followed did not blame the chef. It went back to the drawings.
Kitchen extract ductwork is the single most dangerous part of a commercial kitchen from a fire point of view. Fire investigators note again and again that it is where the most extensive damage occurs, because the duct interior is coated in combustible grease, oil vapour and carbonised deposits, and once alight it carries fire through the building faster than almost anything else.
Everything that made this system dangerous was a routing decision, not a cleaning failure. Three choices did the damage. First, the run was too long and too straight-to-fan, with hard 90° bends where the duct dodged the beam - and grease always builds up fastest on the underside of horizontal runs and on the heel of every bend. Second, the duct was boxed into a tight riser with no clearance, so even if you fitted an access panel you could not get an arm, a brush or a camera into it. Third, and worst, there was almost no access provision at all: one panel where the design should have carried them along the entire length.
None of that shows up in a hygiene inspection of the visible kitchen. It shows up in the ceiling void, and it shows up in a fire. When a claim lands, insurers and fire officers ask for evidence that the system was maintained to recognised best practice - and a duct that was never cleanable cannot produce that evidence. This is precisely how a design shortcut becomes an uninsured loss, and it is worth reading how quickly the cost of getting fire safety wrong escalates once a system cannot demonstrate due diligence.
The design of commercial kitchen ventilation in the UK sits under BESA's DW/172, the specification for kitchen ventilation systems. Cleaning and internal cleanliness sit under TR/19 Grease. Fire strategy sits under BS 9999. The three are meant to be read together at design stage - not bolted on afterwards.
Velocity matters more than people expect. DW/172 sets a transport velocity of roughly 6 to 9 m/s in the main duct, with lower speeds around 5 to 7 m/s at the canopy spigots. Route the duct so a run oversizes and the air slows, and grease drops out and cakes the walls; route it so the air is forced too fast and you lose the system to noise and resistance. Long, low, boxed-in horizontal runs are the classic velocity trap.
Access is not optional. TR/19 Grease requires access panels at close, regular intervals - around every 2 metres as a minimum - plus a panel at every change of direction and either side of every in-line component such as a fan or attenuator, so that every internal surface can be reached, inspected and cleaned. Panels should be on the side of the duct where possible, made of the same material as the ductwork, and secured with quick-release catches to DW144. A duct you cannot open is a duct nobody can prove is clean.
Fire containment is the third leg. BS 9999 calls for a separate, independent, non-recirculating extract for the kitchen, and it does not accept fire dampers in that extract, because grease fouls them and cooking heat can trip their fusible links. Where the duct cannot discharge straight to atmosphere from the kitchen and instead passes through other compartments, it must be fire-resisting for the same period as the compartments it crosses - tested for stability, integrity and insulation, and rated for fire from outside the duct and from grease burning inside it. All of that has to be planned into the route, because you cannot fire-rate a run you have already buried.
The failure above was cheap to prevent on paper and ruinous to fix in steel. If you are laying out a system now, these are the routing decisions that keep it cleanable, compliant and defensible for its whole life.
Questions
The duct interior collects combustible grease and carbonised deposits, and it is where fire investigators find the most extensive damage. A long, boxed-in or heavily bent route lets grease build up where nobody can clean it, and gives fire a continuous fuel path and a sealed steel chimney to travel through. Getting the route short, cleanable and properly fire-protected at design stage removes that risk before the ceilings ever close.
TR/19 Grease requires access panels at close, regular intervals - around every 2 metres as a minimum - plus a panel at every change of direction and on both sides of in-line components such as fans and attenuators. They should be positioned on the side of the duct where possible, made of the same material as the ductwork and secured with quick-release catches. The aim is simple: every internal surface must be reachable for inspection and cleaning.
No. BS 9999 does not accept fire dampers in a kitchen extract system, because grease fouls them and cooking heat can trip their fusible links, making them unreliable and impossible to keep clean. Instead, where the duct passes through other fire compartments it should be fire-resisting ductwork rated for the same period as those compartments, tested for stability, integrity and insulation, and rated for fire both inside and outside the duct.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.