How to Remove Grease from Extractor Hood Filters in Your Wimbledon Kitchen
There’s a particular moment I’ve had in a lot of kitchens where the client pulls the filter out of the hood for the first time since they moved in, and then holds it away from themselves at arm’s length like something that might bite. What comes out is rarely what people expect. It isn’t loose fat and it doesn’t wipe. It’s a brown-amber film with the consistency of dried treacle, welded into the mesh, and it has been quietly cutting the airflow through that hood by half for about two years. Getting it off is straightforward once you know what you’re dealing with, and the standard advice you’ll find online will wreck a certain kind of filter.
Why does hood grease get so hard to shift?
What comes off a frying pan is an aerosol – tiny droplets of oil carried up in hot air, along with steam and combustion products. The droplets hit the cool metal of the filter and condense there. If you cleaned the filter that evening it would come off with hot water and washing-up liquid in about four minutes.
Leave it, and the oil starts to oxidise. Unsaturated fats react with oxygen at the double bonds, cross-link, and build up into a polymer film. This is the same reaction that seasons a cast iron pan, which is a useful way to think about it. What’s on the filter has the character of a coating rather than a soiling, and it formed by accident over hundreds of dinners. Trade word for it is varnish, and that’s what I’ll call it here. Detergent alone won’t lift the varnish, because there’s nothing left for a surfactant to emulsify.
What does work is alkali. Raise the pH and hot alkaline water hydrolyses the ester bonds in the fat, splitting it into glycerol and a fatty acid salt. That salt is soap, which then dissolves in the water and takes itself away. You are converting the grease into the thing that removes it. Everything below is a variation on that one reaction, plus enough heat to make it run at a sensible speed.
Why bicarbonate of soda doesn’t do it
Bicarbonate of soda sits at about pH 8.3 in solution, which is barely alkaline. It works on hood filters in the way that a bread knife works on a hedge. What people are getting when they scrub with a bicarb paste is the abrasion, not the chemistry. Soda crystals – sodium carbonate, the ones in the green box – reach roughly pH 11.5 in hot water and are a different animal. Same shelf in the supermarket, about the same price, and one of them will do the job.
Which filter have you got, and can it go in the dishwasher?
This is where people cause damage, so it’s worth getting right before anything goes in a sink.
Most domestic hoods in London use aluminium mesh filters – layered expanded aluminium in a light frame, silver, flexes slightly if you squeeze the corner. Commercial-style and some higher-end hoods use stainless steel baffle filters, which are heavier, solid to look at, and made of angled channels rather than mesh. And recirculating hoods have a third thing entirely: a carbon filter, usually a black cassette or a disc that clips behind the mesh.
The internet’s favourite tip is to put the filter through the dishwasher on the hottest cycle. On stainless baffles, fine. On aluminium mesh, that advice will ruin it, and I’ll be blunt about how common the mistake is – I’d say a third of the aluminium filters I meet in flats around Wimbledon Park have already been through it at least once.
Aluminium is amphoteric. It reacts with strong alkali as readily as it reacts with acid, and dishwasher detergent is strongly alkaline by design. Sodium hydroxide attacks the aluminium itself, producing sodium aluminate and hydrogen, and the filter comes out grey-black, chalky and dull, with the anodised surface gone. It never comes back. Worse, the pitted surface it leaves behind holds grease far more stubbornly than the original finish did, so every clean from then on is harder than the last.
Carbon filters can’t be washed at all, in a dishwasher or a sink. The whole point of activated carbon is a colossal internal surface area doing physical adsorption. Once it’s loaded it’s finished, and soaking it in water just fills the pores. Replace them every three to six months depending on how much you fry. If your kitchen smells of last night’s dinner in the morning and the mesh is clean, that’s the carbon, and it’s been dead a while.
How to tell a baffle from a mesh in two seconds
Hold it up to a window. If you can see daylight through thousands of small holes, it’s mesh and it’s aluminium. If you can see almost nothing through it because the channels overlap, it’s a baffle, and a magnet will usually stick to at least part of the frame.
What’s the soak that shifts it?
Fill the washing-up bowl or a deep roasting tin with water straight off the boil, mixed to about as hot as the tap will run alongside it – you want it near sixty degrees when the filter goes in, because the reaction slows right down as it cools. Add a heaped tablespoon of soda crystals and a good squeeze of washing-up liquid. Lay the filter in flat and submerged. Twenty minutes for a routine clean, forty-five for one that hasn’t been done since the last tenant.
The varnish will start to come away in soft brown sheets, and at that point a washing-up brush is enough. Work along the grain of the mesh rather than across it, because a wire brush or a scourer across expanded aluminium tears the layers apart and the filter never sits flat in the runners again.
Rinse hot, then dry it completely. That last part gets skipped constantly. Water trapped in the mesh layers mixes with what’s left of the residue and produces a rancid smell within days, and you’ll be smelling it every time the hood runs. Shake it hard, stand it on a radiator or in the airing cupboard, and give it a couple of hours.
If you’d rather not handle soda crystals
Biological washing powder in the same hot water works nearly as well and is gentler on the hands. It brings mild alkalinity plus lipase, an enzyme that breaks down fats directly. It’s slower than soda crystals and better suited to a filter that’s cleaned regularly rather than one carrying six years of buildup. Either way, wear gloves. Hot alkaline water finds every cut on your fingers.
How often does this need doing, and what happens if you leave it?
Once a month if you fry or wok-cook several times a week. Every two to three months if most of your cooking is oven-based.
I did a first clean on a flat on Arthur Road where the filters hadn’t come out since the kitchen was fitted. The hood was running, the fan sounded normal, and the extraction was so far gone that grease had condensed on the ceiling in a visible oval above the hob and the wall cupboard doors either side had gone tacky to the touch. That’s what a blocked filter does – air stops going through the hood and starts going round it, and everything above head height in the kitchen becomes the filter instead. Two of them soaked out fine. One had been dishwashered at some point and stayed grey.
There’s also the fire question, which nobody enjoys but which is the reason the filter is there in the first place. A saturated grease filter over a gas hob is fuel sitting directly above an open flame. Domestic hood fires are rare. They are not rare in kitchens where nobody has ever taken the filter out.
The thing that dies first is the motor
Restricted airflow makes the fan work against pressure it wasn’t designed for, and hood motors in flats around Raynes Park and Merton Park tend to fail on the boost setting, because that’s when the load is highest. A replacement motor on a mid-range hood, fitted, comes to more than the filters cost several times over.
What about the hood itself, and the ducting behind it?
The inside of the canopy needs doing at the same time, or you’ll drip fresh grease onto a clean filter. Neutral degreaser and a cloth, and on brushed stainless steel work along the grain lines. Across the grain leaves marks that catch the light and don’t come out. Take care around the light housing and don’t flood it.
Ducting is a different matter, and in Wimbledon it’s often not there at all. A lot of the Victorian terraces up towards the Ridgway and around Wimbledon Village sit in a conservation area, and external vents on visible elevations are a planning conversation most people would rather not have. So the hoods recirculate, which means the carbon cassette is doing all the odour work and the mesh is doing all the grease work, and both matter more than they would in a ducted setup.
If you do have a duct, the run behind the hood will hold grease in the first metre or so. It’s worth a look with a torch when you’ve got the filter out.
What a recirculating hood can and can’t do for you
A recirculating hood removes grease and, while the carbon is fresh, a fair amount of odour. It removes no steam and no heat whatsoever, because the air goes back into the room. If your kitchen windows stream on a Sunday roast, that isn’t a fault and no filter will fix it. Open the window and accept the arrangement, or price up a core drill through the rear wall and a conversation with Merton’s planning department.
This whole job takes forty minutes and costs about a pound in soda crystals, and I’d tell you to keep me for the oven and do the filters yourself. It’s one of the few bits of kitchen cleaning where doing it more often makes it quicker each time rather than just tidier.
Mine goes in the bowl on the first Sunday of the month, while the kettle’s still hot from the coffee.
