Fire, insurance & risk
Both come from cooking fat, but they're different fires with different defences. Here's what separates them - and why you need to guard against both.
The short answer
A grease fire and a duct fire both come from cooking fat, but they are different hazards. A grease fire burns on the cookline or in the hood, where you can see it and where wet-chemical suppression can hit it. A duct fire burns inside the run, hidden and travelling, and is stopped by clean ductwork and working dampers - not by the cookline system. Controlling one does not control the other.
The grease fire
A grease fire is a Class F fire - burning cooking oils and fats - and it starts where the cooking happens: a pan, a fryer, a chargrill, or the hood and plenum just above. Because it is out in the open, it can be tackled at source. Commercial kitchens use fixed wet-chemical suppression over the cooking line and hood, backed by Class F extinguishers, which smother the burning oil and cut the fuel and heat. Water must never be used, as it throws burning fat outward.
Prevention is about the cookline: controlling oil temperatures, not overfilling fryers, keeping the cooking area and hood clean, servicing the suppression system, and training staff. Get these right and most grease fires are caught within seconds of igniting, before they reach anything else.
The duct fire
A duct fire is a different animal. It burns inside the ductwork, fed by the grease lining the run plus the draught through the system. It is concealed above ceilings and up risers, reaches very high internal temperatures, and travels - carrying fire through the building as it goes. The cookline suppression that guards the hood may reach the immediate duct entry, but pooled grease deep in the run is where nozzle coverage is weakest, so it often cannot put out a fire already established along the ductwork.
That is why a duct fire is prevented differently. The controls are cleaning the whole run to TR19 so there is no continuous fuel, keeping access panels so every section can be reached, and maintaining fire-rated ductwork and working fire dampers so a fire that does start cannot cross compartments. Passive protection and cleanliness, not the cookline suppression, are what contain it.
Where they meet
The two are linked: a grease fire that is not caught quickly can reach the canopy and become a duct fire, at which point the cookline defences are behind it and the concealed run takes over. That is exactly why you cannot rely on suppression alone. A serviced suppression system protects the cooking line; a clean, accessible, well-dampered duct protects the building if the fire gets past it.
In short, treat them as two jobs. Keep the cookline controlled and its suppression serviced, and keep the whole extract run cleaned to TR19 and accessible. Skip either and you have left one of the two fires unguarded.
Questions
A grease fire burns on the cookline or in the hood, where it can be seen and suppressed at source. A duct fire burns inside the ductwork run, hidden and travelling, and is controlled by clean ductwork and working dampers instead.
With fixed wet-chemical suppression over the cooking line and hood, backed by Class F extinguishers, which smother the burning oil. Water must never be used on a grease fire.
Not reliably. Cookline suppression protects the hood and immediate duct, but a fire already established deep in the run, where nozzle coverage is weakest, needs a clean duct and fire dampers to be contained.
Yes. A cookline grease fire that is not caught quickly can reach the canopy and enter the ductwork, becoming a duct fire that the cookline defences can no longer stop.
For grease fires: control oil temperatures, keep the cookline and hood clean, service the suppression system, train staff. For duct fires: clean the whole run to TR19, keep access panels, and maintain fire dampers.
Phoenix Duct Clean · by the numbers
We clean the whole extract run to TR19 so a cookline fire has nowhere to become a duct fire.