Phoenix Journal · Ductwork
A cleaning schedule pulled from a generic table is a guess dressed up as a plan. Grease deposit data lets you set a frequency that matches how your kitchen actually cooks - no more, no less.
Frequency, done properly
Ask three cleaning contractors how often your kitchen extract system needs cleaning and you may get three different answers - most of them plucked from a usage table rather than from your ductwork. The honest answer is that the interval should be set by what is actually accumulating inside the system, and that is something you can measure.
The standard that governs this in the UK is BESA's TR19® Grease, the guide to good practice for the fire risk management of grease-laden extract systems. Its central principle is simple - cleaning should be frequent enough that grease deposits never build past a safe limit. Everything else, including the familiar three, six and twelve month intervals, is a way of estimating that limit when you have no data. Once you do have data, you can stop estimating.
Two kitchens can run the same hours and end up needing wildly different cleaning intervals. Grease accumulation is driven by the cooking, not the clock - a wok range searing at high heat throws far more airborne fat into the ductwork than a kitchen reheating pre-prepared dishes. Charcoal grills, deep fryers and heavy sauté work are the biggest contributors, and a menu that leans on any of them will load a system quickly.
The layout matters too. Long horizontal duct runs, tight bends and low air velocities all give grease more chance to settle and cling rather than pass through. Older or undersized fans that struggle to shift air let deposits form on surfaces that a well-balanced system would keep clearer. This is exactly why a time-and-usage table can only ever be a starting point - it knows your opening hours but nothing about your grills, your ductwork or your airflow.
TR19® Grease reflects this. It sets out the well-known baseline intervals for use where no measurements exist - heavy use of twelve to sixteen hours a day suggesting cleaning every three months, moderate use of six to twelve hours every six months, and light use of two to six hours every twelve months. But it is explicit that these are estimates to fall back on in the absence of data on measured levels of cleanliness. The moment you can measure, the measurement takes priority.
The headline figure is grease deposit thickness - how much fat, in microns, has settled on the internal surfaces of the system. TR19® Grease sets clear thresholds. A mean grease deposit thickness below 200 microns across the system is regarded as an acceptable, lower-risk level. Once the mean climbs beyond that, cleaning is due, and any single localised reading of 500 microns or more calls for immediate spot cleaning regardless of the average. A properly cleaned system should be brought back below 50 microns - roughly half the thickness of a human hair.
Those numbers are gathered with two straightforward tests. The Wet Film Thickness Test measures soft, greasy deposits, while the Deposit Thickness Test is used for the harder, carbonised layers that build up nearer the cooking. Readings are taken at multiple access points along the system, because grease is never spread evenly - the run directly above the fryer will always read heavier than a distant vertical riser. What you want is a picture of the whole system, not one convenient panel.
Alongside the raw readings, keep the context that explains them - the date, the access points used, the cooking that has happened since the last clean, and photographs at each point. Over a few visits this turns a single snapshot into a trend, and a trend is what lets you predict when the next 200 microns will arrive rather than react after the fact.
The method is to work backwards from the limit. If your system reads a mean of 150 microns four months after its last clean, you know roughly how fast grease is arriving and you can set the next interval so that you clean well before the 200 micron ceiling is reached - not months after you have quietly sailed past it. Measure again at that next clean, adjust, and the schedule tightens around reality visit by visit.
In practice this usually means one of three outcomes. A kitchen that a generic table would have put on a six-month cycle may turn out to be reading comfortably low, and the evidence supports stretching the interval - saving money without adding risk. Another kitchen on the same nominal cycle may be pushing past 200 microns in four months, and the data makes the case for cleaning more often before an insurer or fire risk assessor does it for you. The third outcome is that the table happened to be about right, and now you can prove it.
Data also protects you in the other direction. A frequency backed by deposit readings is a frequency you can stand behind, which makes it much harder to be pushed into cleans you do not need. If you have ever suspected a quote was heavier than your kitchen warranted, it is worth understanding how to avoid being oversold on cleaning frequency - the readings are your defence.
A data-led frequency is not something you set once. Menus change, trade grows, a new grill goes in, or a fan starts to tire - and any of these shifts how quickly grease arrives. The point of measuring at every clean is that the schedule keeps correcting itself. TR19® Grease also expects the system to be inspected at intervals not exceeding twelve months regardless of how light the use looks on paper, so even a quiet kitchen gets a fresh reading at least once a year.
Each clean should leave you with a post-clean verification report showing the system brought back below the 50 micron mark, with before-and-after readings and photographs at the access points. That report is what your insurer and your fire risk assessor will want to see, and it is what carries your schedule forward - the closing readings of one visit become the baseline for planning the next. Keep the chain unbroken and your frequency stays anchored to evidence rather than drifting back toward guesswork.
Questions
Yes, and this is one of the main benefits of measuring. If your grease deposit readings stay comfortably below the 200 micron limit over a couple of cleaning cycles, the evidence supports extending the interval rather than cleaning to a calendar. The usage table is only a fallback for systems with no measured history, so once you have reliable readings they take priority. You still need an inspection at least every twelve months to confirm nothing has changed.
Your insurer and your fire risk assessor are the two parties most likely to ask, since grease build-up is a recognised fire risk and cover can depend on demonstrating control of it. Environmental health officers and, after any incident, a fire investigator may also want the evidence. Keeping dated before-and-after readings, access points and photographs from every clean means you can show a defensible, data-led schedule rather than an unsupported claim about frequency.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.