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Kitchen extraction cleaning - duct fire case study
A fire that starts in a kitchen can, if it gets into the extraction ductwork, travel far beyond the kitchen - along the grease-lined duct, through the building, floor to floor and into voids. A duct fire is not contained to where it starts. Here is how a duct fire can spread through a whole building, following the grease through the ductwork, and why keeping the duct clean is what stops the fuel being there. This is general commentary.
The short answer
This is a general account - drawn from how duct fires are known to behave rather than one named incident - of how a fire in kitchen ductwork can spread through a whole building. It illustrates why a grease-laden duct is so dangerous. This is general commentary; a fire risk assessment addresses the specifics of any premises. How the fire gets into the duct. A kitchen fire often starts at the cooking - a flare-up on the range, a chip-pan or fryer fire, ignited fat. The extraction is right above, pulling the air (and flame, sparks and heat) upward - so a cooking fire can be drawn up into the extraction system: through the canopy and filters and into the ductwork. If the ductwork is grease-lined, the fire can ignite that grease - and now the fire is burning inside the duct. So the first step is the fire getting from the cooking into the ductwork, where the grease is. The grease is the fuel. Once the fire is in the duct, the grease lining the internal surfaces is its fuel. A grease-laden duct is coated in combustible grease along its length - so a fire in the duct has a continuous fuel supply to feed on and follow. The grease is what lets the fire take hold and travel in the duct: it burns along the grease, and the more grease, the more intense and sustained the fire. (A clean duct, by contrast, would have little grease to burn - so a fire reaching it would find little fuel.) So the grease lining is the fuel that makes a duct fire possible and lets it spread. The duct is the path through the building. The ductwork runs through the building - often a long route, up through floors, through service voids and risers, to discharge at roof level (especially in a multi-storey building). So the duct provides a ready-made path for the fire to travel through the structure: burning along the grease inside the duct, the fire follows the duct's route - up through floors, into and through the voids and risers the duct passes through, far from the kitchen. So the fire spreads with the duct, through the building. Spreading through the whole building. Following the grease along the building-spanning duct, the fire can spread far beyond the kitchen: to other floors (the duct carrying it up), into concealed voids and risers (where the duct runs, and where fire can spread unseen and reach the building's structure), and through the fabric - so a fire that started at a cooking range can end up involving multiple floors and the structure of a whole building. This is why a duct fire is so dangerous: it turns a localised kitchen fire into a building-wide one, via the grease-lined duct. The defence: a clean duct. The key defence against this spread is keeping the ductwork clean - removing the grease fuel. If the duct is clean (little grease), then even if a fire reaches it, there is little for it to feed on or carry through the building - so the fire is far less able to take hold and spread in the duct. Fire dampers (closing to block the duct) and fire-rated construction also help contain a duct fire, but removing the fuel (the grease) by cleaning is the fundamental defence. So how a duct fire spread through a whole building: it got into the grease-lined duct, fed on the grease, and followed the duct's route through the structure - and keeping the duct clean, by removing the grease fuel, is the key way to stop it. This is general commentary on duct fire spread.
Key points
From the cooking into the duct
The story of a duct fire usually begins at the cooking, and the first critical step is the fire getting from there into the ductwork. Many kitchen fires start at the cooking line - a flare-up on the range, a chip-pan or fryer fire, hot fat igniting. The extraction canopy sits right above the cooking, pulling the air upward - so it is also positioned to pull the flame, sparks and intense heat of a cooking fire upward, into the extraction system. So a cooking fire can be drawn up through the canopy and filters and into the ductwork.
Once the fire (or its flame and heat) reaches the ductwork, if that ductwork is grease-lined, the fire can ignite the grease - and now there is a fire burning inside the duct. This is the pivotal transition: the fire has moved from the open cooking area (where it might be tackled or contained) into the enclosed ductwork (where it is hidden, hard to reach, and - if the duct is greasy - well fuelled). So the extraction, by drawing the fire upward into a grease-lined duct, can turn a cooking fire into a duct fire. This is why the state of the duct matters so much: a grease-lined duct is ready to catch and carry a fire that reaches it. So from the cooking into the duct - how the fire reaches the ductwork - is where the spread begins. So the fire reaches the ductwork from the cooking. This is general commentary.
The grease is the fuel
Once a fire is burning in the ductwork, the grease lining the duct is its fuel - and a grease-laden duct gives the fire a continuous, generous fuel supply to feed on and follow. A neglected duct is coated with combustible grease along its internal surfaces, throughout its length. So a fire in such a duct is surrounded by fuel: it burns along the grease, feeding on it, and the more grease there is, the more intense and sustained the fire becomes. The grease is what lets the fire take hold in the duct and keep going - without fuel, a fire reaching the duct would struggle to sustain itself.
This is the crucial role of the grease: it is the fuel that makes a duct fire possible and lets it spread. A grease-lined duct is, in effect, a fuel-lined channel running through the building - and a fire in it follows the fuel. By contrast, a clean duct has little grease, so a fire reaching it would find little to burn - far less able to take hold or travel. So the difference between a grease-laden duct and a clean one is the difference between a duct that feeds and carries a fire and one that starves it. This is why the grease is the heart of the danger, and why removing it (cleaning) is the fundamental defence (covered later). So the grease is the fuel - a grease-lined duct feeds the fire. So a grease-lined duct feeds the fire. This is general commentary.
The duct is the path
The second thing that makes a duct fire spread through a building is that the ductwork provides a ready-made path - an enclosed route running through the structure that the fire follows. The extraction ductwork does not stay in the kitchen: it runs from the kitchen through the building to discharge outside - often a long route, and in a multi-storey building typically up through the floors, through service voids and risers (the vertical shafts carrying services between floors), to the roof. So the duct threads through the building, connecting the kitchen to distant parts of the structure.
A fire burning in the duct follows this route: fed by the grease, it travels along the inside of the duct, and wherever the duct goes, the fire can go - up through floors, into and along the voids and risers the duct passes through, far from the kitchen where it started. So the duct is the path: an enclosed channel that carries the fire through the building. And because the duct runs through concealed spaces (voids, risers) and up through floors, the fire it carries can reach parts of the building far from and above the kitchen - and can spread within the concealed spaces (where fire spreads readily and unseen). So the duct is the path for the fire through the building - the route along which a duct fire travels. So the duct is a route for the fire through the building. So a route for the fire through the building. This is general commentary.
Through the whole building
Put the fuel and the path together, and you have how a duct fire spreads through a whole building: fed by the grease and following the duct's route, the fire travels far beyond the kitchen - floor to floor, into voids, through the structure. Following the grease along the building-spanning duct, the fire can reach: other floors (the duct carrying it up through the building, so floors above the kitchen become involved); concealed voids and risers (where the duct runs - and where fire spreads readily, unseen, and can reach the building's structure and other systems); and the building fabric (the fire breaking out of or spreading from the duct into the surrounding construction).
So a fire that began at a single cooking range can, via the grease-lined duct, end up involving multiple floors and the structure of a whole building - a localised kitchen fire turned building-wide. This is what makes a duct fire so dangerous and so feared: it is not contained to the kitchen, but uses the extraction as a hidden path to spread fire through the building, potentially endangering everyone in it (including people far from the kitchen who may have no idea a fire started there). The spread through concealed voids is especially dangerous - fire moving unseen through the building's hidden spaces. So through the whole building - floor to floor, into voids, far from the kitchen - is how a duct fire spreads. So it spreads floor to floor, into voids, far from the kitchen. This is general commentary; a fire risk assessment addresses the specifics.
The defence is a clean duct
So the key defence against a duct fire spreading through a building is to keep the ductwork clean - removing the grease fuel, so that even if a fire reaches the duct, there is little for it to feed on or carry through the structure. The whole mechanism of duct-fire spread depends on the grease: it is the fuel that lets the fire take hold in the duct and travel along it. Take the grease away (by cleaning the duct), and the fire loses its fuel - so even if a fire reaches a clean duct, it finds little to burn, and is far less able to take hold or spread through the building. So a clean duct is the fundamental defence: it removes what the fire needs. So remove the fuel, stop the spread.
This is why keeping the extraction ductwork clean is so important - it is not just about preventing a fire starting, but about denying a fire the fuel and path to spread through the building if one occurs. Cleaning the duct to the TR19 Grease standard, at a frequency matched to the cooking, keeps the grease removed - so the duct is not a fuel-lined channel through the building but a clean one with little to burn. Fire dampers (closing to block the duct) and fire-rated duct construction also help contain a duct fire, and are part of the defences - but removing the fuel by cleaning is the fundamental measure (a fire damper helps contain a fire; a clean duct helps ensure there is little fire to contain). So the defence against a duct fire spreading through a whole building is a clean duct: remove the grease fuel, and stop the spread. So the defence is a clean duct - remove the fuel, stop the spread. This is general commentary on duct fire spread; a fire risk assessment addresses the specifics.
Questions
Usually from a fire at the cooking - a flare-up or chip-pan fire - which the extraction, sitting right above and pulling air upward, can draw up through the canopy and filters into the ductwork. Many kitchen fires start at the cooking line (a flare-up on the range, a fryer or chip-pan fire, igniting fat). The extraction canopy is directly above the cooking, pulling the air upward - so it can also pull the flame, sparks and intense heat of a cooking fire upward, into the extraction system, through the canopy and filters and into the ductwork. If the ductwork is grease-lined, the fire can then ignite that grease, and a fire is burning inside the duct. So the extraction, by drawing the fire upward, can turn a cooking fire into a duct fire - moving it from the open kitchen into the enclosed, and if greasy, well-fuelled, ductwork. So a fire gets into the ductwork by being drawn up from the cooking by the extraction. So it is drawn up from the cooking into the duct. This is general commentary.
Because the grease lining the duct is fuel and the duct runs through the building - so the fire feeds on the grease and follows the duct's route, floor to floor and into voids, far from the kitchen. A duct fire spreads through a combination of fuel and path. The fuel is the grease lining the duct: a grease-laden duct gives the fire a continuous fuel supply along its length, so it burns along the grease and keeps going. The path is the ductwork itself: it runs through the building - often up through floors, through voids and risers, to the roof - so the fire, following the grease inside the duct, travels along that route through the structure. Together, the grease feeds the fire and the duct carries it - so a fire that started at a cooking range can spread to other floors, into concealed voids, and through the building, far from the kitchen. So a duct fire spreads far because the grease fuels it and the duct routes it through the building. So it spreads far via the grease fuel and the duct path. This is general commentary.
Yes - if it gets into ductwork that runs up through the building, the fire can travel along the duct to other floors and into the voids and risers it passes through, well above the kitchen. In a multi-storey building, the extraction ductwork typically runs up through the floors, through service voids and risers, to discharge at roof level. So a fire burning in the duct can follow that vertical route upward - reaching floors above the kitchen and spreading within the voids and risers the duct passes through. This means a fire that started at a cooking range on one floor can end up involving floors above it and the concealed spaces between - far from where it started, and often endangering people who have no idea a fire began in a kitchen below. This is a large part of why a duct fire in a multi-storey building is so dangerous. So yes - via the ductwork, a kitchen fire can reach other floors. So yes - it can travel up the duct to other floors. This is general commentary; a fire risk assessment addresses the specifics.
The grease is the fuel - it is what lets a fire take hold in the duct and travel along it; a grease-laden duct feeds and carries the fire, while a clean duct has little for it to burn. Once a fire is in the ductwork, the grease lining the internal surfaces is its fuel: a grease-laden duct is coated in combustible grease along its length, so a fire in it has a continuous fuel supply - it burns along the grease, and the more grease, the more intense and sustained the fire. So the grease is what makes a duct fire possible and lets it spread through the building. A clean duct, by contrast, has little grease - so a fire reaching it would find little to burn, far less able to take hold or travel. So the grease is central: it is the fuel that turns the duct into a fire path through the building. This is why removing the grease (cleaning the duct) is the key defence. So the grease is the fuel that feeds and carries the fire. This is general commentary.
It removes the grease fuel - so even if a fire reaches the duct, there is little for it to feed on or carry through the building, greatly reducing its ability to take hold and spread. The whole mechanism of duct-fire spread depends on the grease being there to fuel it. Keeping the ductwork clean (cleaning it to the TR19 Grease standard, at a frequency matched to the cooking) removes that grease - so the duct is not a fuel-lined channel through the building but a clean one with little to burn. Then, even if a fire reaches the duct, it finds little fuel, so it is far less able to take hold or spread through the structure. So a clean duct is the fundamental defence against a duct fire spreading: it denies the fire its fuel. Fire dampers and fire-rated construction help contain a duct fire too, but removing the fuel by cleaning is the basic measure. So keeping the duct clean removes the fuel, stopping the spread. So a clean duct denies the fire its fuel. This is general commentary.
Fire dampers help contain a duct fire (closing to block the duct), but they are not a substitute for a clean duct - removing the grease fuel is the fundamental defence, and the two work together. Fire dampers are devices in the ductwork that close (triggered by heat) to block the duct and help stop a fire spreading through it - a valuable containment measure. But they are one layer, not the whole answer: they help contain a fire, whereas keeping the duct clean helps ensure there is little fire to contain in the first place (by removing the grease fuel that lets a fire take hold and spread). A grease-laden duct with fire dampers still has the fuel for an intense fire; a clean duct denies the fire its fuel throughout. So fire dampers and clean ductwork are complementary defences - the dampers contain, the cleaning removes the fuel - and relying on dampers alone, while letting the duct stay grease-laden, leaves the fundamental fuel problem unaddressed. So fire dampers help, but a clean duct (removing the fuel) is the fundamental defence. So no - dampers help but don't replace a clean duct. This is general commentary; a fire risk assessment addresses the specifics.
A grease-lined duct is the fuel that lets a fire spread through a building. We clean the ductwork throughout to TR19 Grease, removing the grease - so even if a fire reaches the duct, there is little for it to feed on or carry through the structure. Ask us to keep your ductwork clean and starve a fire of its fuel. This is general commentary on duct fire spread.
