You looked up. You shouldn’t have looked up. But you did, and now you can’t un-see it — that extractor fan filter, squatting above your hob like something from a nature documentary, coated in a substance that defies easy categorisation. Is it grease? Is it varnish? Is it a portal to another dimension? The answer, reassuringly, is just grease. Very old grease, granted, but grease nonetheless — and however prehistoric it looks, the overwhelming likelihood is that it can be saved. Put the “new filter” tab in your browser on hold for ten minutes and read this first.
How Did It Get This Bad? (No Judgement, Mostly Science)
Here’s the thing about extractor fan filters: they are specifically designed to be out of sight, which means they are almost always out of mind. Every time you fry an egg, sear a steak, attempt a stir-fry at something approaching actual wok temperature, or even just make toast with ambition, a fine mist of grease particles, steam, and smoke rises from your hob and gets drawn upward. The filter catches it. Faithfully, silently, without complaint — and without anyone noticing until the day you happen to glance up with the light at just the right angle.
What makes neglected filters particularly challenging is the process of polymerisation. Fresh grease is oily, relatively fluid, and responds well to heat and detergent. But grease that’s been slowly baking under a combination of heat, oxygen, and time doesn’t just sit there — it chemically transforms, cross-linking into a hardened, varnish-like coating that has far more in common with a lacquer than with the stuff that just splashed out of your frying pan. This is why you can’t simply spray it with kitchen cleaner and wipe it away. You’re not dealing with a mess; you’re dealing with a material.
Why Old Grease Is a Different Beast Entirely
This distinction — fresh grease versus aged, polymerised grease — is the single most important thing to understand before you start cleaning, because it determines which method will actually work and which will just exhaust you. A filter that’s a few weeks overdue responds very well to hot soapy water and a bit of elbow grease (the metaphorical kind). A filter that’s been quietly carbonising since last autumn requires a fundamentally different chemical approach. Reaching for the washing-up liquid when you’re dealing with the latter isn’t just ineffective — it’s demoralising, and you deserve better than that.
Know Your Filter: One Size Does Not Fit All
Before you do anything else, identify what type of filter you’re actually dealing with. The most common type found across London’s flats, terraced houses, and rental properties is the aluminium mesh filter — a series of layered metal mesh panels, usually silver, that slide or clip out from the underside of the extractor hood. These are the workhorses of the domestic kitchen and, crucially, they are reusable. With the right method, they clean up beautifully.
Charcoal or carbon filters are a different matter entirely. These are typically circular or rectangular pads found in recirculating extractor fans (the kind that don’t vent outside). They absorb grease and odours, and once they’re saturated, that’s it — they cannot be cleaned and must be replaced. If yours is charcoal, skip straight to the supplier’s website.
Baffle filters are the third type, generally found in higher-end range hoods. They’re made from stainless steel with a series of curved channels, extremely durable, and the easiest of the three to maintain. If you’re lucky enough to have baffles, they respond exceptionally well to the methods below.
The Cleaning Arsenal: Methods Ranked by Severity
Think of what follows less as a single prescription and more as a diagnostic toolkit. Assess your filter honestly — light build-up, moderate neglect, or full crime scene — and select your method accordingly. Starting too gentle wastes your time; starting with the nuclear option when you don’t need it wastes your nerves.
Method One – Hot Water and Washing-Up Liquid (Light to Moderate Build-Up)
For filters that have been overlooked for a couple of months rather than a couple of years, this is your starting point. Fill your sink — or a large washing-up bowl — with the hottest water your tap will produce, and add a very generous squeeze of quality washing-up liquid. Fairy original remains the gold standard here, and that’s not a sponsored opinion, it’s thirty years of collective professional experience talking.
Submerge the filter completely and leave it for a minimum of 30 minutes, ideally an hour. The combination of heat and surfactant will begin to emulsify the grease and loosen it from the mesh. Follow up with a soft-bristled brush (an old washing-up brush or a dedicated cleaning brush, never anything that will damage the mesh), working in circular motions, then rinse thoroughly under hot running water. Repeat the soak if needed.
The critical mistake most people make here is using warm rather than hot water. Grease is hydrophobic and waxy — it needs heat to soften before detergent can do its job. Lukewarm water is essentially useless, and cold water is an insult to everyone involved.
Method Two – Bicarbonate of Soda and Boiling Water (Moderate to Heavy Build-Up)
Step up to this method when your filter has moved beyond “a bit neglected” into “I am genuinely concerned about what I’m looking at” territory. You’ll need a large roasting tin or a lined sink, a full kettle of boiling water, and a generous quantity of bicarbonate of soda — four to six tablespoons for a standard filter.
Place the filter in the tin, scatter the bicarb directly over it, and pour the boiling water over the top. What happens next is either deeply satisfying or mildly alarming, depending on your disposition: the alkaline solution reacts with the polymerised grease, breaking it down and releasing it into the water in brown, oily clouds. The water will turn a colour that, frankly, would concern you in any other context. This is normal, and it means the method is working.
Leave it to soak for 15 to 30 minutes, then scrub with a brush, rinse, and follow up with a quick washing-up liquid soak to lift any remaining residue. The difference between before and after, when this method works as it should, is the sort of transformation that makes you want to send before-and-after photos to people who definitely don’t want to receive them.
Method Three – Caustic Degreaser (Heavy to Severe Build-Up)
For filters so comprehensively neglected that they appear to have developed their own microclimate, it’s time to bring in the professional-grade option. Caustic degreasers — look for products containing sodium hydroxide or potassium hydroxide as active ingredients — are what commercial kitchens use for a reason. They cut through polymerised, carbonised grease where everything else has given up.
This is not a bare-hands, no-ventilation operation. Gloves, eye protection, and an open window are non-negotiable. Spray or apply the degreaser according to the product instructions, allow the full recommended dwell time (do not rush this step — the chemistry needs time to work), then rinse exhaustively with hot water. Multiple rinse cycles. You want no chemical residue left on a filter that sits above a heat source.
Applied correctly, this method can resurrect filters that looked genuinely terminal. It is the last step before replacement, and it is worth trying.
The Dishwasher Gambit: Shortcut or Stroke of Genius?
Running your aluminium mesh filter through the dishwasher is one of those cleaning tips that circulates endlessly online, and it deserves an honest verdict rather than either uncritical enthusiasm or reflexive dismissal. For lightly soiled filters, it works well enough — a hot cycle with a good quality tablet will shift surface grease and keep on top of things between deeper cleans. As a maintenance tool, it’s entirely valid.
For heavily greased filters, however, it is not sufficient on its own. A dishwasher cycle cannot replicate the prolonged alkaline soak that polymerised grease requires, and you’ll likely find yourself unloading a filter that’s improved but far from clean. The play, if you want to use the dishwasher, is to do a bicarb pre-soak first and then run it through — at which point the dishwasher handles the final polish. Just ensure your specific filter is manufacturer-rated as dishwasher-safe, and always run the hottest cycle available.
How Often Should You Actually Clean These Things?
Realistic schedules, calibrated to actual behaviour rather than aspirational lifestyle:
- Light cooks (weeknight pasta, occasional fry-up, more Deliveroo than hob): every two to three months
- Regular cooks (daily meals, frying a few times a week): monthly
- Serious cooks, wok users, anyone who considers a smoke alarm a doneness indicator: every three to four weeks
The simplest system is to tie filter checks to an existing habit — the first of the month, when you pay rent, or when you notice your extractor fan working harder and louder than usual (a reliable sign of a partially blocked filter, incidentally). Remove it, hold it up to the light, and make a call. Thirty seconds of inspection, once a month, is infinitely better than discovering the situation in the way you discovered it today.
When the Filter Is Beyond Saving — and When the Whole Fan Is
Some filters are past the point of redemption, and it’s worth knowing the signs. Structural deformation — warped, bent, or punctured mesh — means replacement, not cleaning. Charcoal filters, as already noted, are a write-off once saturated. And grease so deeply carbonised that it has essentially become part of the filter’s structure will not shift without damaging the mesh itself.
More seriously: if your filter has been neglected long enough that grease has migrated past it and into the fan housing, the ducting, or — worst case — the motor itself, you’re looking at a different category of problem. A grease-saturated duct isn’t merely inefficient; it’s a fire risk, and it’s not something that a soak in bicarb will address. At that point, the conversation moves from cleaning to professional inspection.
Most London kitchens we work in never get to that stage — but a surprising number do, particularly in rental properties where the extractor fan occupies the same mental real estate as the boiler: it exists, it probably does something, and no one has thought about it in years. If your filter is just the beginning of a longer list, you don’t have to work through that list alone.