Phoenix Journal · Ductwork
A wall-mounted air quality monitor can read reassuringly green while grease quietly builds inside your extract duct. Here's the gap between what a sensor sees and what actually keeps a commercial kitchen safe.
From the field
A busy restaurant kitchen had done everything a modern operator is told to do - including mounting a smart air quality monitor on the wall where the whole team could see it.
The little screen glowed green through service after service. Carbon dioxide sat comfortably below the level that flags stuffy, under-ventilated air. Particulate readings barely twitched. The head chef took it as a daily all-clear: if the air the team was breathing looked clean, surely the extraction was doing its job.
Then the insurer's risk surveyor came round. A quick inspection hatch on the horizontal run of the extract duct told a very different story - a soft, tacky film of grease coating the ductwork, well beyond the point at which it should have been cleaned. The system had not been professionally cleaned in over two years. The sensor had said nothing was wrong because, in truth, the sensor had never been looking at the duct at all. The cover was flagged, a clean was ordered under pressure, and the kitchen came close to a grease fire it never saw coming.
The mistake was not buying the sensor. It was trusting it to answer a question it was never built to answer. An air quality monitor tells you about the air in the room. Grease fire risk lives inside the duct - a different place, measured a different way.
Here is where the gap opens up:
In short, the kitchen had a working instrument giving accurate answers to the wrong question. If you want the full picture of why room air still matters in its own right, it is worth reading why indoor air quality matters in commercial buildings alongside this - the two jobs are complementary, not interchangeable.
None of this makes air quality monitors useless. It makes them a tool with a lane. Kept in that lane, they genuinely earn their place.
A good monitor is a strong early warning for ventilation and comfort. If carbon dioxide creeps up towards and past roughly 1,000 parts per million, that is a real signal that fresh-air supply is not keeping pace with the kitchen and the covers - the sort of stale, sluggish air that dulls a team over a long shift. The link between that and how sharply people think is more direct than most operators expect; it is spelled out in our piece on how poor air quality affects cognitive performance. Monitors are also good at catching sudden particulate spikes from a failed filter or a propped-open door, and at giving you a trend line over weeks rather than a single guess.
Fire risk from grease is assessed by measuring the grease itself, on the metal, not by sniffing the room. The UK benchmark is BESA's TR19® Grease specification, which sets a clear, testable limit - and a sensor plays no part in it.
Grease is checked with a wet film thickness test for soft deposits and a deposit thickness test for harder, carbonised build-up, carried out at set points along the system by a technician holding the BESA Grease Hygiene Technician qualification. That is a physical measurement of the risk that actually starts fires - something no room-mounted dashboard, however smart, will ever give you.
The restaurant did not throw the sensor away - it put it back in its lane and added the checks it had been quietly standing in for. If you have leaned on a monitor as a stand-in for extract safety, this is the sequence that closes the gap:
Do those five things and the sensor becomes what it should always have been - a helpful extra pair of eyes on the room, sitting alongside proper, measured attention to the duct, rather than quietly excusing you from it.
Questions
No. A room sensor measures the air people breathe, not the grease deposited inside the ductwork, which is where fire risk builds up. Grease condenses onto cool duct walls and does not stay airborne to be counted by an optical particle sensor. The only reliable check is a physical inspection with wet film and deposit thickness testing against the TR19 Grease limit.
They are useful indicators rather than precise measurements. Low-cost particulate sensors use light scattering, which is cross-sensitive to humidity and steam and drifts over time as dust settles on the optics. The VOC channel usually returns a single total figure and cannot tell cooking grease from cleaning spray. The carbon dioxide channel, which uses infra-red sensing, is generally the most trustworthy reading.
Yes, kept to what it does well. A good monitor gives early warning of poor ventilation - for example carbon dioxide climbing past around 1,000 parts per million - and flags sudden spikes from a failed filter or propped door. That protects comfort and staff alertness. It simply should not be treated as a substitute for inspecting and cleaning the extract 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.