Cleaning a Bosch Range Hood Filter, and the One Most Owners Do Not Know About

There are two filters and they are nothing alike

Nearly every hood problem we are called out to comes down to which of these two the owner has, and whether it has been dealt with.

The grease filter is the metal mesh panel you can see from underneath. It catches the fat that rises off the hob. It washes, it lasts for years, and it needs doing monthly.

The charcoal filter is a solid cassette hidden above or behind it, and it exists only on recirculating hoods. It removes odor from air that is about to be blown back into the kitchen. It does not wash. It is a consumable, like a water filter, and it needs replacing every six to twelve months.

Almost nobody knows about the second one.

Washing the grease filters

1. Unclip them. Reach under the hood; each filter has a sprung catch you pull toward you, and the panel drops down and out.
2. Soak in hot water with washing-up liquid, and a tablespoon of baking soda if they are badly loaded. Twenty minutes of soaking beats twenty minutes of scrubbing.
3. Brush along the mesh with a soft brush, then rinse thoroughly.
4. Hold one up to a light. You should see light through the whole panel. Dark patches are sections still blocked, and those are the sections not passing air.
5. Dry them completely before refitting. In this humidity, putting a wet filter back against a steel housing is asking for corrosion at the contact points.

In the dishwasher? Most models allow it and the manual usually says so. The one thing to know is that aluminum mesh comes out dull and chalky — permanently. On a visible chimney hood that matters; on an under-cabinet one nobody sees, it does not.

The charcoal filter

Find out first whether you have one. Look at the top of the hood: a duct going up or back means ducted, and there is no charcoal filter. A closed top with vents at the front means recirculating, and there is.

If you have one, its part number is model-specific. Note the model number off the plate — usually inside the hood behind the filters — and order against that rather than a description.

Six to twelve months. Heavy cooking, six.

The part that is not a filter at all

If the filters are clean and the hood still moves nothing, the problem is at the far end.

Near the water, the outside cap is the part that ages first. A wall or roof cap has a damper flap that opens when the fan runs and closes when it stops, and salt corrodes the hinge until it stops opening. The hood then sounds exactly as it always did and moves no air whatsoever.

It is worth looking at from outside, from a balcony or the garden, before anybody opens the hood itself. It is the single most missed cause of a "weak" hood on the coast.

What else is in there

While the filters are out, look up into the housing. Grease collects on the fan blades and inside the plenum, and enough of it changes the balance of the blade and the sound of the hood. That is not an owner job — it comes off with degreaser and time — but noticing it is worth doing, because a hood that has been running with blocked filters for years usually has a decade of it up there.

How to tell whether a hood is actually working

A hood is the one appliance in a kitchen with no display, no code and no obvious feedback, which is why so many of them are quietly doing nothing. Three tests, none of which needs a tool:

The tissue test. Hold a sheet of tissue paper against the underside with the fan on full. It should stay there. If it falls, nothing is being pulled and the question is filters, duct or cap.

The steam test. Boil a kettle under it. The plume should be drawn up and away rather than rolling out sideways along the underside of the cabinets.

The smell test, an hour later. If the kitchen still smells of dinner an hour after dinner, the hood moved air around the room rather than out of it. On a recirculating hood that is a saturated charcoal filter almost every time.

What a hood is up against in this market

Two things, and they are both about where the air is going.

In a high-rise, usually nowhere. Recirculating is the norm because there is no exterior wall to duct through, and a recirculating hood's entire capability is its charcoal cassette. Once saturated, it is a fan.

Near the water, the far end corrodes. A ducted hood's wall or roof cap has a damper that has to open every time the fan runs, and salt works on that hinge. It fails closed and it fails silently — the fan sounds exactly as it always did.

That second one is worth checking from outside once a year in a coastal house. It takes a minute from a balcony or a garden and it explains more "weak hoods" on the coast than anything inside the kitchen.

While you are up there

Two things to look at with the filters out:

• The fan blades. Grease builds on them and changes the balance, which shows up as a hood that has become noisier without becoming stronger. Cleaning it is a job rather than a wipe.
• The light fittings. Halogen and LED units in hoods run hot and in grease, and a flickering lamp is usually the lamp rather than the wiring — but a lamp that has cooked its holder is worth looking at properly.

Neither is urgent. Both are the sort of thing that is free to notice while the filters are already in the sink.