Phoenix Journal · Ductwork
A grease loading survey turns a vague worry into hard, measured numbers - and what one recently uncovered behind the access panels is rarely what the kitchen team expects.
Kitchen extract · Fire risk
A grease loading survey turns a vague worry - is our ductwork clean enough? - into a set of hard numbers you can act on, and what those numbers reveal is rarely what the kitchen team expects.
Most extract systems look fine from the floor. The canopy wipes down, the filters go through the dishwasher, and the fan hums away on the roof. But the risk in a commercial kitchen does not sit where you can see it - it builds inside the horizontal duct runs, around bends, and at the fan, where warm grease-laden air slows down and drops its load onto cold metal. A survey is simply the process of opening that hidden system up, measuring what has settled there, and writing it down against a recognised standard.
Below we walk through what a grease loading survey measures, what one recently uncovered inside an otherwise tidy-looking system, and what the findings mean for your fire risk, your insurance position, and your cleaning schedule.
A grease loading survey is a structured inspection of the whole extract route - from canopy and filters, through every duct run, to the fan and the external discharge point. Rather than a subjective look, it produces a quantified reading of how much combustible grease has accumulated on the internal surfaces.
The measurement itself is taken with a Deposit Thickness Test (DTT) or a Wet Film Thickness Test (WFTT). A calibrated gauge is pressed against the internal surface at set points along the system, and the thickness of the soft deposit is recorded in microns. Because the readings are objective and repeatable, two surveyors measuring the same duct should arrive at the same figure - which is exactly what you want when the result feeds into a fire risk assessment or an insurance file.
The benchmark is the BESA specification TR19® Grease, the accepted UK standard for fire risk management of grease accumulation in kitchen extract systems. It sets three numbers worth remembering:
A good survey does not stop at thickness. It records access - or the lack of it. TR19® Grease expects enough access panels to reach and clean the full internal surface, and a surveyor will note every stretch of duct that cannot currently be reached, because an unmeasurable run is an unmanaged run. If you want the full detail on the gauges, sample points and grading behind the figures, see our breakdown of what a grease loading survey measures.
Here is where surveys earn their keep. On the system in question, the canopy and filters read well - light, easily within tolerance, exactly what the daily routine would suggest. The story changed the moment the horizontal duct run above the false ceiling was opened.
The first access panel showed a mean deposit comfortably over 200 microns. A metre further along, at a bend where airflow slows, a single spot reading passed the 500 micron threshold - the point at which TR19® Grease calls for immediate localised cleaning. The deposit there was no longer a thin film; it was a soft, tacky layer thick enough to hold a fingerprint. Warm grease vapour had been condensing at that turn for months, and gravity had done the rest.
Two further findings mattered as much as the thickness itself. First, a long vertical riser had no access panels at all, so a stretch of the highest-risk ductwork could not be measured or cleaned - it simply had to be recorded as unknown. Second, the deposit near the canopy plenum had begun to weep, the early sign that untreated build-up eventually shows up as grease dripping back down onto the cooking line. If that symptom sounds familiar, our note on why grease drips from your canopy explains what it is telling you.
The lesson from the panels is consistent: the parts of the system you interact with daily are usually the cleanest, and the parts you never see are usually the ones carrying the fire load.
Grease is fuel. When a flare-up on the hob draws flame into the extract, a duct coated in soft grease gives fire a continuous path away from the kitchen and into the fabric of the building. The Association of British Insurers has attributed a large share of commercial kitchen fires each year to the grease extract system - which is why the deposit figures are treated as a fire safety measure, not a housekeeping one.
That is also why the survey has legal and contractual weight. Under the Regulatory Reform (Fire Safety) Order 2005, the responsible person must assess and control fire risk, and a documented grease loading survey is direct evidence that the extract system has been assessed against a recognised standard. Alongside it, food hygiene regulation expects equipment - ductwork included - to be kept clean.
Insurers lean on the same numbers. Many policies now carry a specific condition requiring the extract system to be cleaned to TR19® Grease at an appropriate frequency, with certification retained. If a fire is later linked to neglected ductwork and you cannot produce survey and cleaning records, your right to indemnity can be challenged. A clean survey report, with before-and-after thickness readings and dated photographs from fixed positions, is precisely the paperwork that protects the claim.
The frequency you should be working to follows the system's usage. TR19® Grease points to heavy use of roughly 12 to 16 hours a day cleaned quarterly, moderate use of 6 to 12 hours cleaned every six months, and light use of 2 to 6 hours cleaned annually - with the survey results, cooking type and your fire risk assessment able to shorten those intervals where the readings demand it.
A survey is only useful if it drives action. Once the readings are in, the sensible order of work is clear: clean the areas already over threshold, fit access panels wherever the duct could not be reached, then set a cleaning frequency matched to how hard the kitchen runs. From that point the system moves from guesswork to a managed cycle - measured, cleaned, verified, and recorded each time.
Verification is the step people skip and later regret. A post-clean reading below 50 microns, captured on the report next to the pre-clean figure, is what turns a day's work into defensible evidence. Fixed-position photographs and a clear line diagram showing every access point and sample location complete the file your insurer and fire risk assessor will want to see.
Staying ahead of it is mostly about rhythm. Book the survey and clean together so measurement and remediation happen in one visit, keep the certificates where you can find them at renewal, and treat any early symptom - a whiff of stale grease, a drip on the range, a fan running slower - as a prompt to look inside rather than wait for the calendar. The kitchens that never have an awkward conversation with their insurer are simply the ones that measure, clean and file on schedule.
Questions
A visual inspection tells you whether a surface looks dirty; a grease loading survey measures how thick the deposit actually is, in microns, using a Deposit Thickness Test or Wet Film Thickness Test against the TR19 Grease standard. The readings are objective and repeatable, so they stand up in a fire risk assessment or an insurance file. It also records where access panels are missing, flagging duct runs that cannot currently be cleaned at all.
Under TR19 Grease, the mean deposit across the system should stay below 200 microns between cleans. Any single reading of 500 microns or more marks an area needing immediate localised cleaning, because that build-up is a genuine fire load. After cleaning, a verification reading below 50 microns confirms the work was effective and gives you the evidence your insurer will look for.
Phoenix Duct Clean · by the numbers
Phoenix surveys and cleans kitchen and building ductwork to the TR19 standard - measured, cleaned and certificated, UK-wide.