Fire, insurance & risk
Ignition meets fuel, then the duct does the rest. Here's the anatomy of a duct fire - and why it moves through a building so quickly.
The short answer
A duct fire starts when an ignition source meets grease already lining the run - usually a cookline flare-up entering the canopy. It then spreads far faster than an ordinary fire because the duct is a flue: it draws air, carries an unbroken bed of fuel, holds intense heat and hides the flames. What begins as a contained flare-up becomes a fast, concealed, building-wide fire.
How they start
In a dirty extract system the fuel is already there - a grease film coats the canopy, plenum and ductwork. All a fire needs is the heat to light it, and a busy kitchen supplies that routinely. A grill flare-up or an over-heating fryer sends flame up into the canopy and straight into the duct entrance.
There is a second, sneakier route. Grease in ductwork oxidises over time and can flash at a much lower temperature than fresh oil - around 180 degrees Celsius. So a fire in another part of the building can heat a duct wall enough to ignite the grease inside it, starting a duct fire without any flame ever touching the cookline. Either way, a clean duct has no fuel to light; a dirty one is primed.
Why they spread so fast
Duct fires are not just fires that happen to be in a duct - the duct actively accelerates them. Four factors compound. First, draught: the system is built to move air, so it feeds the fire oxygen and pulls the flame front along the run. Second, a continuous fuel bed: an unbroken grease lining gives the flame nowhere to stall, and pooled grease at low points burns as a concentrated hot-spot.
Third, confinement: sealed inside metal ducting with fuel and air, internal temperatures climb very high and the fire becomes self-sustaining and hard to cool. Fourth, concealment: the run hides above ceilings and up risers, so the fire is well advanced before anyone sees it - and it crosses fire compartments as it travels, reaching other floors in minutes rather than the time a room-to-room fire would take.
Breaking the chain
Every accelerant here depends on grease. Take the fuel out of the run with regular specialist cleaning and a stray flare-up has nothing to travel along - the fire stays where it started, where the cookline suppression can deal with it. Keeping the whole run accessible and the fire dampers serviceable means the passive safeguards can still close the compartments if the worst happens.
This is the practical case for cleaning the entire system rather than the visible canopy. The speed of a duct fire comes from the parts you cannot see - the horizontal runs and risers - which is exactly what a surface clean leaves loaded.
Questions
An ignition source meets grease already lining the duct - usually a cookline flare-up entering the canopy, or a duct wall heated from outside until the oxidised grease inside flashes at around 180 degrees Celsius.
The duct acts as a flue. Draught feeds oxygen and pulls the flame along an unbroken bed of grease, confinement drives internal temperatures very high, and the run is concealed - so the fire is fast, intense and hidden.
Minutes. Because ducts run through risers and above ceilings, a duct fire can cross fire compartments and reach other storeys far faster than a fire spreading room to room.
Yes. A fire elsewhere in the building can heat a duct wall enough to ignite the oxidised grease inside, starting a duct fire with no flame ever reaching the cookline.
Remove the fuel. Regular TR19 cleaning of the whole run leaves nothing for a flare-up to travel along, and keeping dampers serviceable lets the passive safeguards contain a fire.
Phoenix Duct Clean · by the numbers
We clean the full run so a flare-up has no grease path to race along.