A head chef at a busy Indian restaurant in Brick Lane called us mid-service. Smoke from the tandoor was filling the kitchen instead of being pulled up through the canopy. Front-of-house staff were propping open the fire escape to clear the haze. Customers could smell it in the dining room. The extract system had been “a bit sluggish” for weeks — that evening it gave up entirely.
We found the grease filters so clogged they were effectively solid sheets of carbonised fat. Behind them, the ductwork was coated in a thick layer of grease buildup. The extract fan motor was still running, but it was trying to pull air through a system that had the resistance of a brick wall. The kitchen had become a pressure cooker with no exhaust.
Kitchen extraction failures are a health risk, a fire risk, and — if the Environmental Health Officer walks in — a business risk. Here’s what actually goes wrong and how to fix it.
Blocked Grease Filters — The One We See Every Time
Grease filters sit inside the canopy hood and catch airborne grease before it enters the ductwork. In a commercial kitchen producing any volume of fried or grilled food, these filters need cleaning weekly. Not monthly. Not “when they look bad.” Weekly.
Baffle-type stainless steel filters can go through a commercial dishwasher. Mesh filters need soaking in a degreasing solution. When this maintenance gets skipped — and in busy kitchens it always gets skipped eventually — the filters become a solid barrier. Airflow drops, the kitchen fills with smoke and steam, and the extract fan works overtime pulling against the blockage. That overwork kills the motor faster.
If your extraction feels weak, check the filters first. Nine times out of ten, that’s the problem.
Failed Extract Fan Motor
The extract fan is usually mounted on the roof or externally on the building wall. It runs for every hour the kitchen is operational — often 14-16 hours a day in a restaurant. Motors have a finite life, and in grease-laden environments that life is shorter than the manufacturer’s data sheet suggests.
Signs of a failing motor: reduced extraction even with clean filters, unusual humming or buzzing from the fan, the motor casing hot to the touch, or a tripped circuit breaker that keeps tripping when you reset it. A completely dead motor is obvious — no airflow at all and silence where there should be a constant hum from the roof.
Motor replacement is straightforward but needs to be correctly sized for the ductwork and kitchen volume. We carry common motor sizes and can usually replace same-day for standard installations.
Belt-Driven Fan With a Snapped or Slipped Belt
Larger kitchen extract systems often use belt-driven centrifugal fans rather than direct-drive motors. The fan impeller is connected to the motor via a V-belt. These belts wear, stretch, crack, and eventually snap. A slipping belt gives you reduced extraction with a squealing noise. A snapped belt gives you zero extraction with the motor still audibly running — confusing if you don’t know the system is belt-driven.
Belt condition should be checked at every service visit. A spare belt kept on site is cheap insurance — a few pounds for the belt versus lost revenue from a kitchen shutdown.
Make-Up Air Imbalance
This is the one most people miss. An extract system removes air from the kitchen. That air has to be replaced from somewhere. If the kitchen is sealed — doors closed, no supply air system, windows shut — the extract fan is trying to create a vacuum. It can’t. Extraction drops dramatically, and you get back-draughting through any available opening, including flues from gas appliances, which is genuinely dangerous.
Proper kitchen ventilation requires a balanced system: extract air out, supply air in. The supply air should be tempered (heated or cooled) so you’re not dumping freezing outside air into the kitchen in January. Many older kitchens were fitted with extract-only systems and never had adequate make-up air provision. If your extraction was always marginal, this is probably why.
Grease Buildup in Ductwork and Canopy
Even with properly maintained grease filters, some grease gets past and accumulates inside the ductwork over months and years. This narrows the effective duct diameter, increases resistance, and reduces airflow. It’s also a significant fire hazard — duct fires in commercial kitchens are more common than most operators realise.
Under DW/172 (the BESA specification for kitchen ventilation cleaning), extract ductwork should be cleaned at regular intervals depending on usage. Heavy-use kitchens — takeaways, restaurants cooking with solid fuel or high-volume frying — need cleaning every 3-6 months. Moderate-use kitchens every 6-12 months. The cleaning should be done by a specialist duct cleaning contractor, with before-and-after photographic evidence and a certificate of compliance.
Insurance companies are increasingly asking for DW/172 compliance documentation. If your ductwork hasn’t been cleaned and there’s a fire, your insurer has grounds to dispute the claim.
What to Check Before Calling
- Are the grease filters clean? Pull one out and hold it up to the light — if you can’t see through it, that’s your problem
- Is the extract fan running? Go outside or up to the roof and listen
- Is there a supply air system, and is it working?
- Has anything changed recently — new equipment, blocked vents, sealed-off openings?
- When was the ductwork last cleaned?
Kitchen Extraction Problems? Call ADK
A kitchen without proper extraction can’t operate. We diagnose and repair extract fan systems, replace motors, rebalance supply and extract airflow, and work with duct cleaning contractors to bring systems back to DW/172 compliance. Call 020 3974 1419 — we cover London and respond to urgent kitchen ventilation failures 24/7.