Two kitchens on the same street can be cleaned on the same day and be in very different states three months later. That is not bad luck. The rate grease settles in an extract system follows a handful of things you can see on the survey - and the published guidance from insurers and fire services says the cleaning interval should follow them too.
The time-based starting point
Where nobody has measured the grease, both Aviva's loss prevention standard for commercial kitchens and Norfolk Fire and Rescue Service's business guidance give the same rule of thumb for the ductwork, fans and associated parts, based on daily cooking hours:
| Cooking per day | Suggested interval for ductwork and fans |
|---|---|
| 12 to 16 hours (heavy) | Every 3 months |
| 6 to 12 hours (moderate) | Every 6 months |
| Up to 6 hours (light) | Every 12 months |
Aviva separately suggests hoods, filters and canopies are cleaned weekly; Norfolk FRS says daily or as the manufacturer advises. Both are describing a starting point, not a fixed answer. (Sources below.)
What pushes a kitchen faster than its hours suggest
1. What is on the menu. Aviva names the type and nature of cooking as a driver of build-up. Norfolk FRS goes further: frying, and cooking over wood or charcoal, may call for more frequent cleaning than the hours alone would give. A ten-hour grill house is not a ten-hour café.
2. Filters - and whether they are in. Filters never catch everything, which is why ducts grease up at all. Norfolk FRS is blunt that cooking should never happen with the filters out. A missing or overloaded filter moves the problem straight into the duct, where it is harder and dearer to reach.
3. The duct route. Aviva notes that long runs with many bends give grease droplets more chances to settle on the walls, and prefers the most direct route. Two kitchens with the same menu and hours can diverge simply because one duct goes straight up and the other snakes across a ceiling void.
4. Pressure in the duct. Aviva also points out that ductwork under positive pressure tends to push grease onto the surfaces, while negative pressure leaves fewer deposits. That is a design point, but it shows up in the readings.
5. Grease reduction kit. Aviva says UV systems can reduce deposits and, subject to approval, may allow less frequent deep cleans. "Subject to approval" matters: the insurer, not the installer, has the final word.
Measure, then set the interval
The hours table is where you start when there is no data. Once there is data, the readings should lead. Aviva's standard describes measuring grease thickness on the duct surfaces and setting the cleaning frequency to keep the average below 200 microns across the system, and expects each report to recommend a frequency based on the readings. It also says reports should be compared with earlier ones to spot trends.
That last point is where many firms lose the thread. A single report shows a kitchen's state on one day. A run of reports for the same system shows whether the interval is right - and gives the firm the evidence to tell a client, in writing, that they need to come more often.
Why it matters beyond the clean
- Fire risk assessment. Norfolk FRS reminds businesses that the extract system belongs in the fire risk assessment under the Regulatory Reform (Fire Safety) Order 2005.
- Insurance. Both sources warn that poor cleaning can affect cover, and Aviva says cleaning contracts should meet any interval the policy sets. The policy can override the table above.
- The fire record. Of the seven London fires our tracker has logged in 2026 where the fire service said the ducting was involved, one was put down to grease build-up in the duct (Lee Green, May) and another to unsafe disposal of grill charcoal (Ilford, April). The tracker reflects London far more than the rest of the UK, because the London Fire Brigade publishes incident reports in a way most services do not.
How SMART handles it
SMART - TR19 Grease reporting is built for firms working to TR19 Grease. Each extract system carries its own cleaning frequency, so a busy fryer line and a quiet staff kitchen on the same site keep their own due dates. The engineer records readings and photographs on the phone, the report recommends the next interval, and the planner counts forward to the next clean. Earlier reports for the same system sit alongside, so a change in readings is easy to see and explain to the client.
Sources (read 13 October 2026)
- Aviva Risk Solutions, Loss Prevention Standard: Commercial Kitchens - Extract Systems and Cooking Ranges, version 1.4, 10 October 2024
- Norfolk Fire and Rescue Service, Kitchen extraction systems (no page date shown)
- London Fire Brigade, Fire at a restaurant - Lee Green, 10 May 2026
- London Fire Brigade, Fire at restaurant with flats above - Ilford, 6 April 2026
- Smart Solutions, UK kitchen extract fire tracker 2026 (London-only limit applies)
TR19® is a registered trade mark of the Building Engineering Services Association (BESA). Smart Solutions Bristol Ltd is not affiliated with or endorsed by BESA.
Written by Edward Bristow, an engineer of more than 15 years. Published 13 October 2026.