Fire safety & extraction
The grease cooking throws into the air lines the ductwork and becomes a ribbon of fuel threaded through the building.
Every commercial kitchen produces grease, and most of the attention it gets is about hygiene: greasy filters look bad and harbour bacteria. But the more serious story is about fire. The grease that cooking throws into the air does not vanish; it lines the inside of the extraction system, and there it becomes a continuous ribbon of fuel threaded through the fabric of the building.
High temperature cooking, frying, grilling, chargrilling, roasting, releases microscopic droplets of fat and oil into the air along with the heat and smoke. The extraction system is designed to capture that vapour and carry it away, drawing it through the canopy and filters and up into the ductwork. The filters and the airflow do their job, but they cannot remove all of it, and what is left condenses and settles on every internal surface it passes: the canopy plenum, the fan, and the long runs of ducting hidden above ceilings and inside risers. Layer by layer, service after service, a coating of solidified grease builds up where nobody can see it, and that coating is highly combustible.
Why the build up is dangerous
Grease deposits turn the extract system into a fire risk in two distinct ways. The first is simply that grease is fuel. Fire needs fuel, heat and oxygen, and a kitchen supplies heat and oxygen in abundance; the grease lining the ductwork supplies the third element in a form that ignites readily and burns fiercely. A flare up on the cookline, a fryer reaching its auto ignition temperature, or a spark drawn into the canopy can set that lining alight. The second, more insidious effect is that grease also blocks airflow and insulates heat, so a fouled system forces the extract fans to work harder and run hotter, raising temperatures across the whole system and pushing it closer to the point of ignition even before any flame appears.
What makes a grease fire in ductwork so much worse than a fire on the cookline is the path the fuel provides. Once the grease lining ignites, the fire does not stay put; it runs along the inside of the ductwork, following the fuel through the building at speed and out of sight. It breaks out into ceiling voids, wall cavities, roof spaces and adjoining premises, arriving in places firefighters cannot easily reach and where the fire may be well established before anyone realises it has spread. Fires that get into the ductwork are notoriously difficult to extinguish, and the damage is frequently severe enough to close the business. This is why fire and rescue services trace the majority of serious commercial kitchen fires back to the extract system.
There is a compliance and financial edge to all of this as well. Because the fire risk from grease is so well established, insurers commonly require evidence that the extract system is cleaned to the recognised standard, and a fire in an uncleaned or only partially cleaned system can leave a claim unpaid and the responsible person facing prosecution. The grease that looks like a mere hygiene issue is, in law and in insurance terms, a fire safety issue that carries real liability.
Removing the fuel
The only reliable way to control this risk is to remove the fuel, and that means cleaning the whole extract system, not just the parts that are easy to reach. Daily wiping of surfaces and washing of filters is necessary hygiene but does nothing about the grease deep inside the ductwork, which is exactly where the dangerous fuel load sits. Periodic professional cleaning to a measurable standard strips the grease from the canopy, plenum, fan and full length of ducting, removing the combustion load and resetting the fire risk. How frequently that has to happen depends on how hard the kitchen cooks, which is the whole point of matching how often to clean a kitchen extraction system to the intensity of the cooking rather than to a generic calendar.
Cleaning and suppression together
Removing the grease is the foundation of kitchen fire safety, but it works best paired with a means of tackling ignition at the source. A fixed suppression system over the cookline responds automatically when a fire starts at the appliances, cutting the fuel and power and smothering the flames before they can reach the ductwork, which is why understanding how commercial kitchen fire suppression systems work completes the picture. The two defences address different halves of the same problem: cleaning removes the fuel that lets a fire spread, and suppression stops the ignition that starts it. Keep the grease out of the ductwork with regular, thorough, documented cleaning, back it with a maintained suppression system, and the single biggest fire risk in the building is brought firmly under control.
Questions
Grease is highly combustible. As it builds up inside the extraction ductwork it becomes a continuous fuel load running through the building, so a cookline flare up or overheating fryer can ignite it and the fire then spreads along the duct.
In two ways. It provides the fuel a fire needs, and it blocks airflow and insulates heat, forcing the fans to run hotter and pushing the system toward ignition. Once alight, the greasy lining carries the fire unseen through the building.
Filters and airflow capture much of the grease but cannot remove it all. Over time the remainder condenses and builds up on the canopy, fan and internal duct surfaces, which is why the ductwork still needs periodic professional cleaning.
It can. Insurers commonly require evidence that the extract system is cleaned to the recognised standard, and a fire in an uncleaned or only partially cleaned system can lead to a claim being refused and the responsible person prosecuted.
Remove the fuel by cleaning the whole extract system, canopy, plenum, fan and full ductwork run, to a measurable standard at a frequency matched to how hard the kitchen cooks, and back it with a maintained fixed suppression system over the cookline.
Phoenix Duct Clean · by the numbers
The grease lining your extract is the fuel a fire needs to spread. Talk to Phoenix about removing it from the whole system to the recognised standard, with full documentation.