Every commercial kitchen produces grease. Most of it ends up on the pans and the plates. Some is carried up into the canopy as vapour, where it cools and settles on the filters, the plenum and the inside of the ductwork. Left alone, that layer keeps growing.
A fire inside an extract system is hard to see, hard to reach and can travel well away from the kitchen before anyone knows about it. This guide explains how those fires start, why they spread, the habits that keep grease down and how professional cleaning fits in.
How grease gets into the duct
When food is fried, grilled or roasted, fat turns to vapour and tiny droplets. The extract draws that warm, greasy air into the canopy. Filters catch some of it. The rest passes into the plenum and the ductwork, where it cools and condenses on the metal.
Over weeks and months, the layer thickens. It also gathers dust and, with solid fuel cooking, soot. Grease collects most at bends, horizontal runs and around the fan, where the airflow slows or changes direction.
Grease also builds up faster when filters are dirty, damaged or the wrong size for the canopy. Gaps around filters let fumes straight past them into the plenum.
How an extract fire starts
Most extract fires start at the cooking range. A pan catches fire, a grill flares up or a fryer overheats. Flames and hot gases are drawn straight up into the canopy by the extract fan.
If the filters are clogged or missing, or the plenum is coated in grease, the fire has fuel right there. Once grease inside the duct ignites, it can keep burning on its own, well away from the original flame.
Why fires spread through ductwork
Ductwork runs through ceilings, walls, risers and roof spaces, often through areas nobody visits. A fire inside it can:
- travel along the duct, feeding on the grease layer;
- heat the duct walls enough to ignite nearby materials;
- pass from one part of the building to another;
- break out at the fan or discharge, sometimes onto the roof.
Because it’s hidden, it can be hard for anyone in the kitchen to tell how far it has gone. That’s why the fire and rescue service may need to check ducts, ceilings and roof spaces well away from the kitchen, and why prevention matters so much more than reaction.
Daily habits in the kitchen
- Clean or swap grease filters as often as your cooking demands; for busy kitchens that can be daily.
- Never cook without filters in place.
- Wipe down canopy surfaces and empty grease channels and drip trays.
- Keep fryer oil at the right temperature and change it regularly.
- Don’t leave cooking unattended, especially at high heat.
- Make sure staff know how to isolate the gas and use the kitchen’s fire suppression or extinguishers.
Our grease filter cleaning guide has more.
Need it done? Duct Doctor cleans and tests kitchen extract and ventilation systems across London and the South East, with a certificate and photo report after every clean. Get a fixed quote.
Weekly and monthly checks
- Look behind the filters at the plenum for grease build-up.
- Listen to the extract fan; new noises or vibration can mean grease or wear.
- Check for grease dripping from the canopy or joints.
- Check the area around the discharge outside for grease staining.
- Check that fire suppression systems and extinguishers have been serviced on schedule.
Keep a simple log of filter cleaning and these checks. It helps staff stay on top of them and shows an inspector that the kitchen takes grease seriously.
Anything that looks wrong is worth reporting straight away. Our list of signs your extract needs cleaning helps.
Professional cleaning on the right cycle
Kitchen staff can look after the filters and canopy. The ductwork, risers and fan need a specialist. BESA’s guidance on kitchen extract cleaning sets minimum intervals:
| Kitchen use | Typical daily cooking | Minimum cleaning interval |
|---|---|---|
| Heavy | 12–16 hours a day | Every 3 months |
| Moderate | 6–12 hours a day | Every 6 months |
| Light | 2–6 hours a day | Every 12 months |
A risk assessment and grease readings set the real interval. Grease should not exceed a mean of 200 microns between cleans, and 500 microns or more at any point needs immediate spot cleaning. See how often a kitchen extract should be cleaned.
Fire dampers and access panels
Where extract ductwork passes through fire-resisting walls or floors, fire safety measures in the duct help stop a fire spreading. Any damper fitted must be able to close, and BESA says dampers should be tested at least annually. Grease can stop them working.
Access panels matter too. A section of duct that can’t be reached can’t be cleaned, and that’s where grease builds unchecked. See our fire damper testing guide and access panel installation.
Your legal duty
Under the Regulatory Reform (Fire Safety) Order 2005, the responsible person must assess fire risks and take reasonable steps to reduce them. In a commercial kitchen, grease in the extract is one of the most obvious risks. GOV.UK explains the duties in its guide to workplace fire safety.
TR19® Grease is not itself law, but it’s the recognised way to show the extract is under control, and it’s what many insurers ask for.
Getting your system checked
If you don’t know when your extract was last cleaned, or you’ve never seen a report with photos of the ductwork, start with a survey. It will show how much grease is in the system and whether any parts can’t be reached. Duct Doctor diagnoses the system first, then cleans it end to end out of hours and leaves you with a TR19® certificate and photo report. We also offer a 24/7 emergency callout if something goes wrong.
Get in touch for a quick quote.
Quick answers
How do kitchen extract fires start?
Usually at the range. A flare-up or pan fire is drawn into the canopy, and grease on the filters, plenum or duct catches light.
Why are duct fires so dangerous?
They’re hidden inside the building, can keep burning on the grease layer and can spread along ductwork to other areas.
How can we prevent an extract fire?
Keep filters clean and in place, never leave high-heat cooking unattended, and have the whole system cleaned to TR19 on the right cycle.
Does cleaning the filters stop grease reaching the duct?
It reduces it, but some grease always gets past. The plenum, ductwork and fan still need professional cleaning.
Who is responsible for extract fire safety?
The responsible person under the Regulatory Reform (Fire Safety) Order 2005, usually the employer, owner or person in control of the premises.
Related services
- Kitchen extract cleaning
- Ductwork cleaning
- Grease filter exchange
- Fire damper testing
- Extract fan cleaning
More advice
- How Often Should a Kitchen Extract Be Cleaned?
- Signs Your Kitchen Extract Needs Cleaning
- Grease Filter Cleaning: How Often and How
- The Responsible Person Under the Fire Safety Order
- Fire Damper Testing: A Guide for Building Owners
- All advice
This guide is general information. It isn’t legal or insurance advice: check your own policy wording and lease, and speak to a qualified adviser about your fire risk assessment.