Phoenix Journal · Extraction
The fan is part of the greasy extract path, yet it is the component most often left unreachable. Choosing between a roof-mounted and an in-line fan is really a decision about how your system will be cleaned, verified and kept insurable.
Case postmortem
The kitchen passed every deep clean for two years, and then a single fan failed the whole system.
A busy production kitchen had a roof-mounted extract fan fitted at the original build. Plant space inside was tight, the duct wanted to rise and discharge at roof level anyway, and putting the fan up top kept the noise off the pass. On paper it was the neat answer. The canopy stayed spotless, the accessible duct runs came back clean visit after visit, and the paperwork looked healthy.
Then the insurer commissioned a full post-clean verification ahead of renewal. The technician worked the horizontal runs, took readings, and reached the base of the riser - and stopped. The fan sat on a plinth on a flat roof with no fixed guard rail, no fitted access hatch beside it, and no safe way to open the housing at height. Inside that housing, two years of grease-laden air had baked onto the impeller and the scroll. Nobody had touched it since commissioning, because nobody had ever been able to.
The reading that mattered could not be taken. Without a verified figure at the fan, the hygiene certificate was withheld, and a withheld certificate is exactly the sort of gap that lets an insurer question cover after a fire. The fan that had been chosen to save space had quietly become the one part of the system that could not be proven clean.
Nothing failed mechanically. The fan pulled air the whole time. The failure was that it had been selected for structural convenience and never for cleanability, and in a grease extract system those are not the same decision.
Under TR19 Grease, the BESA specification insurers routinely reference, the whole extract path has to be brought below a mean average of 200 microns of grease deposit and then verified, usually with a Deposit Thickness Test or a Wet Film Thickness Test and matched before-and-after photographs. The fan is not exempt from that path. It sits directly in the greasy airstream, so its impeller and housing collect deposit like any duct wall does - often faster, because that is where the air is compressed and turned.
Three things stacked up on this job:
Grease of 500 microns or more needs spot cleaning wherever it is found, and a fan housing left untouched for two years is well past that. The deposit was not just a compliance gap - baked grease on a hot, spinning impeller near an ignition-friendly airstream is precisely the fire risk TR19 Grease exists to control.
Roof-mounted and in-line fans both move the same grease-laden air. Where they part company is location, and location decides who can reach the fan, how safely, and how often. That is the lens to choose through.
A roof fan sits at the top of a vertical riser and discharges at roof level. It is the cleanest routing when the duct wants to go up and out anyway, it frees internal plant space, and it keeps fan noise well away from the kitchen. The trade-off is access. Every clean and every verification now involves working at height, and unless the roof has a fixed platform, guard rail and a housing you can actually open, the fan tends to be the corner that gets cut. If you go this way, the safe-access provision is not an optional extra - it is the thing that makes the fan cleanable at all.
An in-line fan sits within the duct run itself, commonly in a ceiling void or a plant area along the horizontal path. It handles longer runs and higher static pressure well, keeps noise contained, and - the point that matters here - it can be positioned next to a proper access panel at working height. That makes routine cleaning and post-clean verification straightforward, because a technician can open the housing, clean the impeller and scroll, and take a reading without a work-at-height plan every time. The trade-off is that it needs the void or plant space to live in, and the duct routing has to suit it.
Neither is the right answer in the abstract. A roof fan on a building with safe roof access and a fitted hatch can be perfectly maintainable, and an in-line fan buried in a sealed void with no panel is just as much of a trap as the roof fan in this story. The question is never only which fan moves the air - it is which fan your team can reach, open and prove clean on the schedule your insurer expects.
The kitchen in the story kept its roof fan but had a safe-access solution retro-fitted so it could finally be opened, cleaned and verified. That works, but it is far cheaper to design the decision in from the start. Whether you are specifying a new system or reviewing an existing one, work through it in this order:
Done in that order, the fan stops being the weak point. You end up with a system where every part of the greasy path - canopy, duct and fan - can be reached, cleaned to below the 200 micron threshold, and shown to be there in writing. That is what keeps the certificate valid and the cover intact.
Questions
Yes. The fan sits directly in the grease-laden airstream, so its impeller and housing collect deposit just as the duct walls do, often faster because that is where the air is turned and compressed. TR19 Grease treats the whole extract path as one system that must be brought below a mean average of 200 microns and then verified. A fan left out of the clean is both a compliance gap and a genuine fire risk.
Not at all, provided it can be reached and cleaned safely. A roof fan is often the cleanest routing when the duct rises and discharges at roof level, and it frees internal plant space. The problem only appears when there is no fitted access hatch and no safe work-at-height provision, so the fan gets skipped at every clean. If you fit a fixed platform, guard rail and an openable housing, a roof fan can be perfectly maintainable.
Ask a simple question: can a technician open the fan housing to clean the impeller and scroll, and take a Deposit Thickness Test or Wet Film Thickness Test reading at that point, without an unplanned work-at-height exercise every visit? If the answer is no, the fan is likely being skipped and your reports may look clean while hiding a gap. A proper post-clean verification should include a measured reading at the fan itself, with matched before-and-after photographs.
Phoenix Duct Clean · by the numbers
The right kit only helps if the system stays clean. Phoenix degreases canopies, filters and ductwork to TR19 Grease - UK-wide, overnight.