The Slow Decline of a Neglected Extraction System

How a healthy system slips into failure

The pattern below is remarkably consistent across sites. The timescale differs with how hard you cook - a heavy chargrill kitchen runs through these stages far faster than a light-use servery - but the order rarely changes.

1. The first film settles. Within days of a proper clean, a fine layer of airborne grease and carbon begins to condense on filter faces, canopy plenums and the first metre of duct. At this stage it wipes off with a cloth, and nobody would call the system dirty - but the process of accumulation has already started.
2. The filters stop coping. Baffle and mesh filters are your first line of defence, and they saturate fastest. Once they are loaded, they capture far less grease and pass the rest downstream into the ductwork you cannot see or easily reach. A filter that used to feel tacky now feels caked, and if it is not being washed on a daily or weekly cycle, the system is already sending its grease deeper.
3. The ductwork silts up. Beyond the canopy, grease deposits build on the internal walls of the duct, particularly at bends, transitions and horizontal runs where velocity drops. This is the hidden reservoir that matters most, because it is combustible, out of sight, and only reachable through access panels. Under BESA’s TR19® Grease specification, a deposit averaging 200 microns - roughly half the thickness of a business card - is the point at which cleaning is required.
4. Airflow falls away. As filters load and ducts narrow, the system moves less air for the same fan speed. The extract that once pulled smoke and steam cleanly off the pass now leaves a haze hanging over the range. Heat and humidity climb, condensation appears on ceilings and walls, and the kitchen becomes noticeably harder to work in.
5. The fan labours - and your bills climb. To shift air through a restricted system, the extract fan runs harder and longer, drawing more power for less result. A struggling, grease-laden system is quietly one of the most expensive things in the building to run, which is exactly why a well-maintained extract pays you back - the quiet savings of a properly maintained system come from a fan that is not fighting its own dirt. If the extract was borderline to begin with, correct sizing of the system matters even more, because there is no spare capacity to lose.
6. The system becomes a fire load. Grease is fuel. As the internal coating thickens past 200 microns and towards the 1,000-micron mark, TR19® Grease classes the system as a high fire risk - a continuous combustible lining running from the range, through the ceiling void, and often across the building. A single flare-up at the hob no longer stays at the hob; it has a path to travel.
7. The compliance gap opens. Somewhere in this slide, the paperwork stops matching reality. Under the Regulatory Reform (Fire Safety) Order 2005, the responsible person must keep the risk from grease accumulation under control, and most commercial insurers now expect TR19®-standard cleaning evidenced by a post-clean report. Without a current certificate and photographs, a fire claim can be reduced or refused outright.
8. The forced intervention. Eventually something forces the issue - a failed inspection, an insurance renewal, a small fire, or a fan that finally trips. Now the clean is not routine maintenance carried out overnight, but a heavy remedial job, sometimes with parts of the kitchen out of use. The cheapest stage to act was the first; the most expensive is this one.

Why the slow version is the dangerous one

A sudden failure gets attention. A slow one gets absorbed into the routine, and that is precisely what makes it costly. Nobody signs off on running a high fire-risk system through a busy service - they simply never notice the moment it became one, because every day looked almost identical to the last. The graduation from “a bit greasy” to “a documented hazard” happens without a single alarming day to mark it.

The frequency guidance in TR19® Grease exists to break that pattern by putting the decision on a calendar rather than on a hunch. As a rule of thumb, heavy-use kitchens cooking twelve to sixteen hours a day are cleaned roughly every three months; moderate-use sites running six to twelve hours every six months; and light-use kitchens of two to six hours a day annually. These are starting points, not guarantees - a wok or chargrill operation produces far more airborne grease than the same hours spent on a griddle - so the deposit measurement, not just the date, decides whether a system is actually compliant.

Crucially, none of these stages is visible from the kitchen floor. You can see a loaded filter, but you cannot see the horizontal duct run above the ceiling tiles, and that is where the risk lives. This is why a proper clean is measured and photographed at multiple access points along the full length of the system, not judged by how the canopy looks. A report that only covers the visible bits tells you nothing about the reservoir upstream.

The reassuring part is that decline is completely reversible, and the earlier you intervene the less it costs. Washing filters on a tight daily or weekly cycle slows stage two dramatically. Keeping adequate access panels along the ductwork means the whole system can be reached, measured and cleaned back to bare metal rather than just wiped where hands can fit. And booking cleans on a schedule that reflects how you actually cook - then holding a current, photographed TR19® Grease certificate - keeps the fire risk low and the insurance position clean at the same time. The system never has to reach the last stage if someone is watching the middle ones.