Phoenix Journal · Fire Safety
A building is built to hold a fire in one compartment. Voids and ductwork are how that plan quietly fails, and in a catering building the grease lined extract duct is the most dangerous path of all.
A building is designed to contain a fire in the compartment where it starts, holding it there long enough for people to get out. Voids and ductwork are how that plan quietly fails. A fire that should have stayed in one room follows the ceiling void, the service riser and the extract duct, and turns up two floors away before anyone expected it. In a catering building, the extract duct is the most dangerous path of all.
Fire compartmentation divides a building into fire resisting boxes. Compartment walls and floors are built to hold back fire for a set period, commonly thirty to two hundred and forty minutes, tested for stability, integrity and insulation. The theory is simple: contain the fire, protect the escape routes, buy time. The weakness is that no building is a set of sealed boxes. Ducts, pipes, cables and voids all have to pass through those fire resisting walls and floors, and every one of those penetrations is a potential hole in the barrier. Where a service crosses a compartment line, the fire resistance of the wall or floor has to be reinstated around it, or the duct itself becomes the route fire takes to bypass the whole scheme.
How the breach is sealed
For most ventilation ductwork, the answer at a compartment line is a fire damper: a device set into the duct where it penetrates a fire resisting wall or floor, held open in normal use and driven shut when fire is detected. Many use a fusible link that melts at around seventy two degrees Celsius, dropping the blades under spring tension; others are motorised and closed by the alarm. Installed and maintained to the BESA guide DW145 and tested at least annually, a fire damper restores the fire resistance of the barrier the duct passes through, keeping fire in one compartment.
Kitchen extract is the critical exception, and it catches people out. Fire dampers are not permitted in grease bearing extract ductwork, because grease would foul the mechanism and stop it closing when it mattered most. So a kitchen extract duct that leaves the kitchen compartment cannot rely on a damper at all. Instead it must be fire rated ductwork, built to hold the same fire resistance as the compartments it passes through, or run in a dedicated protected shaft. It is tested both for a fire outside the duct and for a fire inside it, precisely because the grease lining makes an internal fire so likely.
This is why a grease laden extract system is treated as such a serious fire path. A fire that starts at the cookline or the filters spreads into and up the duct, fed by the oil and carbonised grease coating the inside, and burns far hotter than a fire in a clean ventilation duct. Radiant heat through the duct wall can ignite combustible material in the ceiling void around it, and grease weeping from duct seams can start fires in the spaces the duct passes through. High profile restaurant fires have followed exactly this route: a kitchen fire climbing the ducting to roof level, spreading damage far beyond the room where it began.
Where fire finds the gaps
The dangerous spaces are the ones nobody looks at: the void above a suspended ceiling, the vertical riser carrying services between floors, the gap where a duct passes through a wall that was never properly fire stopped. Fire and hot smoke move through these concealed routes quickly, crossing compartment lines that look intact from the room side, which is how a contained fire becomes a whole building fire in minutes. Kitchen extract ties directly into this hidden network, so understanding the link between ventilation and fire compartmentation is what turns an abstract idea about fire resisting walls into a practical question about where your ducts go once they leave the room and what protects the barriers they cross.
Starve the path of fuel
Fire rated ductwork and correct fire stopping are the passive defences, but they assume the duct itself is not a reservoir of fuel. A clean extract system removes the very thing that lets a duct fire start, intensify and travel: the grease. Cleaning the whole system to standard, canopy, filters, ductwork and fan, on an interval matched to how hard the kitchen runs, is not just a hygiene job; it is the most direct way to stop the extract becoming the path fire takes through the building. That starts with a proper look at the system, which is what a canopy cleaning fire risk assessment provides, setting the cleaning frequency against the real grease load rather than a guess, so the duct never becomes the route that defeats the compartmentation around it.
Questions
Ductwork and voids cross the compartment walls and floors meant to contain a fire, giving flame and hot smoke a concealed route between rooms and floors. Grease lined kitchen extract is worse still, because the grease fuels an internal fire that burns hot and travels up the duct.
Because grease would foul the damper mechanism and could stop it closing in a fire. Instead of a damper, a kitchen extract duct leaving the compartment must be fire rated ductwork, or run in a protected shaft, so it resists fire without a moving part that grease could disable.
Yes. Grease is the fuel that lets a duct fire start, intensify and travel. Cleaning the whole system to standard removes that fuel, so even if a fire reaches the extract, there is far less to carry it through the building.
Phoenix Duct Clean · by the numbers
Grease is what lets a fire travel up your extract duct. Phoenix cleans canopy, ductwork and fan to TR19 Grease on an interval matched to your kitchen, so the duct carries air, not fire. UK-wide, overnight.