Fire, insurance & risk
Reach the ductwork and a fire has a flue to travel. Here's how it moves through the building - and what's meant to stop it.
The short answer
Once flames reach the ductwork, a dirty extract system behaves like a flue: enclosed, drawing air and lined with fuel. Fire runs along the grease layer, and because ducts pass through fire-rated walls, floors and risers, it can leap the barriers meant to contain it - reaching roof voids and other floors in minutes. The spread is fast, hidden and hard to fight.
The flue effect
A duct is designed to move air, which is exactly what a fire wants. When a flare-up reaches the canopy and enters the ductwork, the grease coating the inside becomes a continuous fuel path. Flame and hot gases travel along it, and the draught through the system feeds the fire oxygen as it goes. Instead of one contained blaze on the cookline, you get a fire moving through the building inside the ducting.
Grease in a duct is more dangerous than fresh oil in a pan, because it oxidises over time and can flash at a much lower temperature - guidance puts it around 180 degrees Celsius. That means a fire in a completely different part of the building can heat a duct wall enough to ignite the grease inside it, and the extract then carries the fire onward.
Breaching the barriers
Buildings are divided into fire compartments - walls and floors rated to hold fire back for a set time so people can escape. Every point where a duct passes through one of those barriers is a potential breach. A grease fire travelling the ductwork bypasses the rated walls and floors and can spread across multiple storeys through riser shafts, corridor penetrations and ceiling voids far faster than a fire creeping through rooms.
Fire dampers are the safeguard: fitted where ducts cross fire barriers, they close automatically - usually on a fusible link that releases around 72 degrees - to seal the duct and restore the compartment. But a damper choked with grease or seized in its frame cannot close, and general ventilation ductwork has no fire resistance of its own. Clean ductwork and working dampers are what keep a duct fire from becoming a building fire.
Containing the spread
You limit fire spread through ductwork the same way you limit the fire load: by keeping the whole run clean and the dampers serviceable. Stripping grease from the entire system removes the fuel path, so a cookline flare-up has nothing to travel along. Keeping access panels in place lets both the ductwork and the fire dampers be inspected and cleaned, so nothing is hidden or seized.
This is why cleaning only the visible canopy misses the point. The parts of the run that carry fire between floors - the horizontal ducts above ceilings and the vertical risers - are precisely the parts a surface clean never reaches.
Questions
The duct acts as a flue. Fire that reaches the canopy travels along the grease lining the ducting, fed by the draught, and passes through the walls and floors the duct penetrates - carrying the fire through the building.
Grease in ductwork oxidises over time and can flash at a much lower temperature - around 180 degrees Celsius - so even a fire elsewhere in the building can heat the duct wall enough to ignite the grease inside.
Fire dampers fitted where ducts cross fire-rated barriers close automatically to seal the duct. But a grease-choked or seized damper cannot close, and plain ductwork has no fire resistance of its own - so a clean run matters too.
Very fast. Through riser shafts, corridor penetrations and ceiling voids, fire can cross multiple floors in minutes, bypassing the compartment walls and floors meant to hold it back.
No. The ducts that carry fire between floors run above ceilings and up risers. A canopy-only clean leaves those greasy runs - the actual fire path - untouched.
Phoenix Duct Clean · by the numbers
We clean the whole run and keep dampers accessible - so a flare-up has nowhere to travel.