Phoenix Journal · Ductwork
A grease loading survey turns something invisible - the film of fat lining your extract ductwork - into a measured figure you can act on. Here is exactly what it records, and what those numbers go on to decide.
The engineer arrives with a torch, a clipboard and a small pot of white paste, and within twenty minutes has told you more about your extract system than a year of glancing up at the canopy ever could.
It looks unremarkable from the kitchen floor. Someone opens an access panel, wipes a measured film onto the inside of the duct, waits, and reads a number off a gauge. But that number - taken again and again along the run, from the canopy right through to the fan - is the whole point. A grease loading survey turns something invisible, sitting several metres above the pass and hidden inside sealed ductwork, into a figure you can act on. It tells you how much grease has built up, where it is worst, and how close the system is sitting to the line that separates a maintained extract from a genuine fire risk.
If you have ever wondered what your cleaning contractor is really checking, or why one kitchen gets signed off for another six months while a near-identical one is booked straight back in, the answer lives in that survey. Here is what it measures, and what those measurements go on to decide.
A grease loading survey exists to answer one deceptively simple question - how thick is the layer of grease coating the inside of your ductwork? Everything else follows from that. The relevant benchmark comes from BESA, the Building Engineering Services Association, whose specification TR19® Grease has been the reference standard for kitchen extract fire safety in the UK since it was published in 2019. It sets the thresholds, defines the test methods, and describes how the result should be recorded.
The measurement is taken in microns - a micron being one thousandth of a millimetre, so we are dealing with a film far too fine to judge by eye. Two test methods are used depending on what the engineer finds. Soft, wet, freshly deposited grease is measured with a Wet Film Thickness Test (WFTT), where a comb-like gauge is pressed into the deposit and the reading taken from the teeth that come up coated. Harder, older, carbonised deposits - the baked-on layer that builds up nearer the fan and in the hottest sections - are measured with a Deposit Thickness Test (DTT) instead. Both give a figure in microns, and both are recorded rather than estimated.
The number that matters is the mean average across the system. TR19® Grease asks that grease is controlled so it does not exceed an average of 200 microns between scheduled cleans - and to put that in human terms, 200 microns is roughly half the thickness of a business card. It sounds like nothing. But a continuous film of hot, combustible fat lining every internal surface of a duct is exactly the fuel that turns a small flare-up on the range into a fire that travels the length of the system.
A single measurement would be close to meaningless, because grease does not settle evenly. It is always heaviest close to the cooking - just above the canopy and in the first stretch of horizontal duct - and it changes again around bends and at the fan. So the engineer takes readings at defined points along the run rather than picking a spot at random. Where your survey looks, and what a thorough one records, typically includes the following.
Heavy, uneven loading in that first stretch is also what you tend to see behind the more visible symptoms in a busy kitchen. If you have noticed grease dripping from the canopy during service, or grease marks appearing on the ceiling around the extract, the survey is the tool that quantifies what those signs are hinting at from the outside.
A survey is not an end in itself. The figures it produces feed straight into three practical decisions - whether the system needs cleaning now, how often it should be cleaned going forward, and whether a completed clean has actually worked. This is where the numbers earn their keep.
The readings sort your system into broad bands. A mean average below 200 microns is generally regarded as an acceptable, well-maintained level - the system is doing its job and presents a lower fire risk. Once the average climbs past 200 microns, cleaning is due, because grease has built up beyond the recommended benchmark. And where individual points show heavy localised deposits - in the region of 500 microns or more - those areas warrant prompt, targeted attention rather than waiting for the next scheduled visit. A good survey does not just hand you an average; it flags the hotspots, because a single heavily loaded bend can matter more than a tidy overall figure.
The same survey data sets your cleaning interval. TR19® Grease links frequency to how hard the kitchen works, measured in daily hours of use, and the survey confirms whether the assumed schedule is actually keeping you under the threshold. As a rough guide, the specification points to the following pattern - though your own loading readings are what confirm or adjust it.
That last point is where the survey stops being a technical exercise and becomes a business one. Your insurance and your responsibilities under the Regulatory Reform (Fire Safety) Order rest on being able to show your extract system is being managed, not merely that someone came and cleaned it. Verified before-and-after readings, taken to a recognised method and recorded properly, are the evidence. A clean without measurement is a hopeful gesture. A clean bracketed by a survey is proof - and if the worst ever happened, proof is exactly what you would want in your file.
The work should be carried out by a competent technician too. BESA sets out a Grease Hygiene Technician qualification for exactly this reason, so the person taking your readings and interpreting them knows how the methods and thresholds are meant to be applied. When you commission a survey, it is fair to ask who is doing it and what they hold.
Questions
Both measure the thickness of grease inside your ductwork in microns, but they suit different deposits. The Wet Film Thickness Test (WFTT) is used on soft, wet, freshly laid grease, typically nearer the canopy. The Deposit Thickness Test (DTT) is used on harder, carbonised deposits that build up in the hotter sections and around the fan. A good survey uses whichever method matches what the engineer actually finds at each point.
Under the BESA TR19 Grease specification, grease should be controlled so the mean average stays below 200 microns between scheduled cleans. Once the average climbs past 200 microns, cleaning is due, and any localised readings around 500 microns or more warrant prompt targeted attention. After a clean, fresh readings should confirm the average has been brought back below 50 microns across the system.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.