Carbon filters are the most neglected consumable in a British kitchen. Grease filters get noticed because they go visibly sticky and most people put them through the dishwasher eventually. Carbon filters sit hidden behind them, do their job silently, then quietly stop working — no warning light on most hoods, no change in noise, and no obvious sign beyond a kitchen that smells of last night's dinner rather longer than it used to.
If your extractor recirculates rather than venting outside, this is the part that actually removes the smell. Everything else just catches fat.
The Short Answer
Replace a cooker hood carbon filter every three to six months for typical family cooking, or every nine to twelve months if you cook lightly and rarely fry. Manufacturers usually state six months, which is a reasonable default for a household cooking a couple of proper meals a day.
They cannot be washed or cleaned, whatever a YouTube video may claim. Washing rinses off surface grease and does nothing to the saturated carbon underneath, which has adsorbed odour molecules into its pore structure. Baking them in the oven — another popular suggestion — regenerates activated carbon only at industrial temperatures far above what a domestic oven reaches, and risks melting the plastic frame. When it's spent, it's spent.
And if your hood ducts outside through a wall or roof, you don't need carbon filters at all. Some hoods ship with them fitted regardless.
What Actually Matters
First establish which type of hood you have, because it changes the answer entirely.
A ducted hood pushes cooking air through a pipe and out of the building. Grease filters catch the fat; the smells go outside. No carbon filter required, and fitting one only restricts airflow and makes the hood noisier for no benefit.
A recirculating hood — sometimes labelled ductless — pulls air through the grease filters, then a carbon filter, then blows it back into the kitchen through a vent at the top of the chimney. The carbon is the only thing standing between you and the smell of yesterday's fish. This is the arrangement in most flats, most island installations, and a surprising number of new-build kitchens.
Activated carbon works by adsorption: its enormous internal surface area — a few grams can have the surface area of a tennis court — traps odour molecules in microscopic pores. Those pores fill up. Once full, air passes straight through and the filter becomes an expensive piece of airflow resistance.
Detailed Analysis
Realistic replacement intervals by how you cook
The manufacturer's six months assumes an average. Your kitchen probably isn't average.
| Cooking pattern | Realistic filter life |
|---|---|
| Heavy frying, curries, wok cooking, most days | 2–3 months |
| Family cooking, hob used twice daily | 4–6 months |
| Moderate cooking, mostly oven, some hob | 6–9 months |
| Light use, ready meals, occasional cooking | 9–12 months |
| Second home or rarely used kitchen | 12 months (they degrade with humidity even unused) |
That last row surprises people. Activated carbon adsorbs whatever's in the air, including moisture, whether or not the hood is switched on. A filter fitted three years ago in a lightly used kitchen is not still fresh — twelve months is the sensible outer limit regardless of use.
How to tell yours is finished
Very few hoods have a filter indicator, and the ones that do are just counting hours rather than measuring anything. So it comes down to observation:
- Smells linger. The clearest sign. If frying onions still hangs around an hour later when it used to clear in fifteen minutes, the carbon is done.
- Smells travel. Odours reaching the hallway, landing or soft furnishings mean the hood is recirculating unfiltered air.
- The filter looks and feels heavy. A spent one is visibly darker, slightly greasy at the edges and noticeably heavier than a new one.
- A musty smell when the hood first starts. Saturated carbon can release what it's holding, which is why an old filter sometimes makes things worse rather than better.
A habit that works better than any of the above: write the fitting date on the filter frame in marker pen the day you install it. It removes all guesswork for the price of nothing.
Genuine versus third-party filters
Genuine filters from the hood manufacturer typically cost £20–£45 a pair. Third-party equivalents from specialist spares suppliers run £10–£25. Universal carbon sheet — a roll you cut to size and fit behind the grease filters — costs around £8–£15 and covers several changes.
My honest view after using all three: reputable third-party filters are fine, and the premium is hard to justify for what is essentially a bag of carbon in a plastic frame. What matters is the quantity and quality of the carbon, not the badge. Very cheap unbranded filters are the ones to avoid — a thin scattering of carbon in a mostly-empty frame, exhausted within weeks. Universal cut-to-size sheet works but is weaker than a packed cassette: fine for a rarely used kitchen, not for a household that cooks seriously.
One practical point: the correct filter is identified by the hood's model number, on a rating plate inside the hood — you'll need to remove the grease filters to see it. Don't order by the hood's marketing name, because manufacturers reuse those across several incompatible chassis.
Long-life and washable carbon filters
Some newer hoods, particularly from the German brands, come with "long-life" or "regenerable" carbon filters. These are real, not marketing nonsense, but the detail matters. They use a carbon-impregnated matrix rather than loose granules, and the process is a dishwasher wash at 65°C or above followed by ten minutes in the oven at around 100°C to reactivate. That regenerates them partially, not fully — the stated life is usually about three years or ten regeneration cycles.
Crucially, this only applies to filters explicitly sold as regenerable. A standard carbon cassette put through a dishwasher is simply ruined: the granules clump, the frame can distort, and any residual function disappears.
Don't forget the grease filters
Carbon filters last longer when the grease filters ahead of them are doing their job. Fat that gets past a clogged grease filter lands on the carbon and blocks the pores that should be catching odour.
Metal mesh grease filters should be degreased monthly with heavy use, or every two to three months otherwise. A dishwasher on a hot cycle works, though it dulls aluminium over time; a soak in hot water with soda crystals is just as effective and kinder to the finish. Let them dry fully before refitting.
Real-World Considerations
Recirculating hoods are always a compromise. They deal with smells reasonably and grease adequately, but return heat and moisture to the room — which matters more in a small kitchen or a flat. If you have any option to duct outside, ducting is meaningfully better and removes the filter cost permanently.
On annual cost, budget around £30–£60 a year for carbon filters in a normal family kitchen, less if you buy third-party or in pairs. It's a real cost that almost nobody factors in when comparing hood prices, and a cheap hood with expensive proprietary filters can cost more over five years than a better model with common ones. Running costs for the hood itself are trivial by comparison — a 150–250W motor on medium for an hour a day is around £8–£12 a year.
A note on effectiveness: no recirculating hood clears smells as well as a ducted extractor. A fresh carbon filter running for ten minutes before you start cooking and fifteen minutes after you finish will do more for your kitchen than any amount of running it flat out at the moment the pan starts smoking.
Common Mistakes Buyers Make
Washing a standard carbon filter. It does nothing useful and can make it worse. Only filters explicitly sold as regenerable can be cleaned, and only by the manufacturer's stated method.
Fitting carbon filters to a ducted hood. Pure loss — reduced extraction, more noise, no benefit. If air goes outside, take them out.
Buying by hood name rather than model number. Filters are chassis-specific. The rating plate inside the hood is the only reliable reference.
Ignoring the grease filters. Clogged grease filters shorten carbon life and are the main reason a filter fails well before its stated interval.
Assuming no smell means the filter is fine. You acclimatise to your own kitchen. Ask someone who doesn't live there, or judge by how long a strong smell takes to clear rather than whether you can smell it at all.
Alternatives Worth Considering
If you're at the point of replacing filters on an old recirculating hood and wondering whether it's worth it, price up ducting instead. Where an external wall is close to the hob, a core drill and a short run of rigid ducting is a half-day job for a competent installer and removes the consumable cost for good, while improving extraction substantially.
For rented flats where that isn't possible, a good recirculating hood with proper cassette filters, changed on schedule, plus an open window while cooking, is the realistic best outcome. It's less satisfying advice than "duct it outside", but it's the situation most people are actually in.
Frequently Asked Questions
Can you wash a cooker hood carbon filter?
No, not a standard one. Washing removes surface grease but does nothing to the saturated carbon inside, and can damage the filter. Only filters specifically sold as regenerable or long-life can be cleaned, following the manufacturer's dishwasher-and-oven method.
How do I know if my charcoal filter needs changing?
Smells lingering longer than they used to is the main sign, along with odours spreading to other rooms. A spent filter is often darker, heavier and slightly greasy. If you don't know when it was last changed, assume it needs replacing.
How much do cooker hood carbon filters cost?
Around £20–£45 a pair for genuine filters, £10–£25 for good third-party equivalents, and £8–£15 for a roll of universal cut-to-size sheet. Expect £30–£60 a year for a typical family kitchen.
Do I need carbon filters if my hood vents outside?
No. Ducted hoods send odours outdoors, so carbon filters serve no purpose and only reduce airflow. If yours came with them fitted, remove them.
Final Thoughts
Three to six months for most households, twelve months at the absolute outside, and never wash one unless the manual explicitly says you can. Write the date on the frame when you fit it, keep the grease filters clean so the carbon isn't doing their job as well as its own, and buy a decent third-party filter rather than paying a premium for the badge.
It's a small, dull piece of maintenance that makes a genuine difference to how a kitchen smells — and unlike most appliance jobs, it takes about two minutes once you know where the filter lives.
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