Why is my extractor hood not clearing steam and smells?

A hood that runs but does not clear the kitchen is rarely a failed motor. It is usually resistance somewhere between the pan and the outside air, or a hood that never had the capacity for the room. Working from the pan outwards finds it quickly, and most of the causes are free to fix.

What you'll need

• A sheet of kitchen paper for the airflow test
• Soda crystals or biological washing powder and a sink of very hot water (optional)
• A replacement carbon filter, if recirculating (optional)

Step by step

1. Establish whether the hood is ducted or recirculating.Look on top of the hood or behind the chimney. A duct going into the wall or ceiling means ducted; a vent blowing back into the room, usually at the top of the chimney or the front of the canopy, means recirculating. A recirculating hood cannot clear steam because it never removes moisture, and if that is what you have, that is the answer.
2. Do the paper test at the filter.Hold a sheet of kitchen paper flat against the underside of the hood with it running on full. It should be held in place. If it drops immediately, air is barely moving, and the cause is resistance somewhere in the path rather than a hood that is merely undersized.
3. Pull the metal grease filters out and look at them against the light.A loaded aluminium mesh filter can halve the airflow on its own, and it is the most common single cause. If light barely passes through, that is your problem. Soak in very hot water with soda crystals or biological washing powder for half an hour, rinse and dry completely.
4. If it recirculates, find the carbon filter and check its age.Carbon filters adsorb smell until they are saturated, then do nothing. They cannot be washed. Three to six months of regular cooking is the usual life. Many hoods are sold without one fitted at all, which people discover years later.
5. Check the external termination.Go outside with the hood running. You should feel air. A backdraught flap stuck shut with grease, a bird's nest, a fine insect mesh loaded with fluff, or a grille painted over during decorating are all common, and all of them choke the hood completely.
6. Trace the duct run for length, bends and crushing.Flexible duct compressed behind a cabinet, an unnecessary bend, a reducer from 150mm down to 120mm, or a run of five metres with four bends. Each bend costs the equivalent of three to six metres of straight duct, so the bend count usually matters more than the length.
7. Check the hood was never venting into a void.A duct that terminates in a ceiling void, a loft or a cupboard puts litres of water into the fabric of the house every week. It also does nothing for the kitchen. If you find this, it needs correcting for the building's sake, not just the hood's.
8. Finally, check size and height against the requirement.Work out the kitchen's volume and compare it with what the hood is rated for after derating. Then check the hood is at least as wide as the hob and mounted at the maker's stated height. A correctly working hood that is simply too small has a different remedy from a blocked one.

Tips

• Wash metal grease filters monthly if you fry, quarterly if you do not. Most dishwashers will take them, though repeated hot cycles can dull anodised aluminium.
• Switch the hood on before you start cooking, not when the pan is already steaming. A hood is far better at catching a plume than at clearing a room.
• Give the kitchen a source of replacement air. A tightly sealed kitchen with a powerful hood and no trickle vent or open door starves the hood, and in a home with an open-flued gas appliance that is a genuine safety issue.
• Feel the difference between settings. A hood with no perceptible change between low and boost has a blockage, not a control fault.

Common mistakes

• Buying a more powerful hood to fix a blocked one — The new hood pushes against the same restricted duct and delivers almost as little. Fix the resistance first; it is nearly always cheaper and it is often free.
• Cleaning the visible canopy and not the filters — The canopy is cosmetic. The filters are the airflow, and they are the part that actually degrades performance as they load.
• Assuming a recirculating hood is faulty — It is working as designed. Returning heat and moisture to the room is what recirculating means, and no amount of servicing will change it.

If it doesn't work