When do I need to change the carbon filter in an extractor hood?

Extractor hoods have two completely different filters and people conflate them constantly. The metal mesh catches grease, is washable, and lasts the life of the hood. The carbon filter adsorbs smell, is a consumable, and only exists on hoods that return air to the room. Knowing which one you are dealing with is most of this job.

Step by step

1. Find out whether your hood has a carbon filter at all.Only recirculating hoods do. If the hood ducts to outside, there is no carbon filter and nothing to change. Many convertible hoods are sold ducted-ready and the carbon filter is a separate purchase, so a recirculating hood may never have had one fitted.
2. Learn the difference between the two symptoms.Weak airflow with the paper test failing means the metal grease filter or the duct. Normal airflow but smells lingering in the kitchen means the carbon filter. The two faults feel similar in the room and have completely separate fixes.
3. Use three to six months as the working interval.Manufacturers usually state four to six months for average use. Heavy frying, spice-heavy cooking and running the hood constantly all shorten it. Light use in a well-ventilated kitchen can stretch it, but the carbon is being consumed whether or not you notice.
4. Locate the filter behind the metal mesh.Drop the metal grease filters out first. The carbon filter is usually a round cassette that twists onto the fan housing, or a rectangular pad clipped over it. Some hoods take one, some take two.
5. Buy by hood model, not by shape.Carbon filters are model-specific and the fitting is often a proprietary bayonet. Take the model number from the rating plate inside the canopy. A filter that nearly fits will leak air past itself and do nothing.
6. Fit it dry and record the date.Twist or clip it home firmly so all the air goes through it rather than round it. Write the date on the filter edge or on a note inside the canopy — this is a maintenance item nobody remembers and the date is the only reliable prompt.
7. Wash the metal filters at the same time.A new carbon filter behind a loaded grease filter gets far less airflow and its life is wasted. Do the pair together and the interval takes care of itself.
8. Do not attempt to regenerate or wash a carbon filter.Some are marketed as regenerable in an oven and the results are marginal. A standard carbon filter that has been washed is destroyed: the carbon compacts, the airflow drops, and the adsorption does not return.

Tips

• If you are on a ducted hood and someone has sold you carbon filters, you do not need them. Check where the air actually goes before buying consumables.
• Third-party carbon filters are usually much cheaper than branded ones and are generally fine, provided the fitting matches exactly.
• Set a recurring reminder rather than relying on noticing the smell. By the time it is noticeable, the filter has been doing nothing for weeks.
• If you fry two or three times a week, buy filters in pairs. The interval is short enough that a spare in the cupboard saves a month of a hood doing nothing about smell.

Common mistakes

• Washing the carbon filter with the metal ones — Water destroys it. The carbon compacts and loses its surface area, and the filter blocks airflow while adsorbing nothing.
• Running a recirculating hood with no carbon filter — Grease is still caught but nothing removes smell, and the hood becomes a noisy fan that moves cooking odour around the kitchen.

Keeping on top of it

• Wash the metal grease filters in hot water with soda crystals (Monthly if you fry, quarterly otherwise)
• Replace the carbon filter on a recirculating hood (Every 3 to 6 months)
• Check the external grille and backdraught flap on a ducted hood (Twice a year)
• Degrease the canopy and the fan housing (Every 6 months)

Questions people ask