The most common type of cooker hood is a chimney style (although it doesn't have to be stainless steel)

After looking at the new cooker hood designs that caught my eye, at the recent trade show in Birmingham – and then (in my last post) looking at the types of cooker hood that are available – it’s time to get down to the nitty gritty and decide how powerful your cooker hood needs to be and where you can fit it.

A cooker hood needs to remove steam, vapourised grease and oil, and smells from the kitchen. In order to do this efficiently, it needs to be able to move ten times the volume of air in the room, in one hour.

If your kitchen is, for example, 4 metres by 3 metres by 2.3 metres high, then it has a volume of 4 x 3 x 2.3 = 27.6 cubic metres. So your cooker hood needs to have an extraction rate of 276 m³ per hour. The vast majority of canopy and chimney cooker hoods could manage this … but quite a lot of conventional and a few integrated hoods couldn’t.

When you get to much larger rooms, with high ceilings … 6m x 5m x 2.5m, say … the room volume is 75 m³ and the cooker hood needs to be able to move 750 m³ per hour. A lot of canopy and chimney hoods couldn’t achieve a rate that high, so you need to choose more carefully.

You also need to consider what types of cooking you do. Using a big wok, over a very powerful wok burner would benefit from more powerful extraction.

Of course to extract efficiently, a cooker hood needs to be vented to the outside. That way the smells and steam are taken out of the room altogether. In order to do that, it’s best to site your hob and cooker hood on (or near) an outside wall.

A cooker hood is best situated on an outside wall

When fitted onto an outside wall, a cooker hood can be fitted in isolation and vented straight out at the back (as shown above). If you need to fit the hob on an internal wall, then the hood is best fitted between wall units – with a “flyover” shelf running across the space (see below). The vent tube can then run along the top of the shelf and units, to the nearest outside wall and you won’t be able to see it.

Cooker hoods can be vented out sideways, above the wall units

The longer the vent tube has to be, the less efficient the extraction rate will be … and bends in the tube will reduce the efficiency even further. Try to keep the distance straight and less than a couple of metres. It’s also important to use the type of vent kit specified for your cooker hood. If it has a circular 150mm connection, don’t reduce that down and use some 100mm vent tubing you’ve bought from the local DIY store. Don’t use that concertina type, expanding duct tube (that you use for tumble dryers) either!  Buy the 150mm venting designed to go with the hood; circular, if possible, although equivalent sized rectangular tubing can also be used. If you don’t, you risk … decreasing the efficiency of extraction;  making operation of the hood noisy … and invalidating the guarantee.

Of course, a cooker hood doesn’t have to be vented to the outside at all. Most have a re-circulating mode … requiring the addition of a carbon/charcoal filter … which will remove smells fairly efficiently but will not be so good with steam. If you need to site your hob well away from an outside wall, then choose a cooker hood with a fairly powerful extraction rate because the carbon filter will produce some extra resistance to the air flow … and change the carbon filter regularly (every two to three months, if you cook regularly). Alternatively some cooker hoods from Elica have a “long life” carbon filter which can be washed and re-generated (by heating in the oven).

You will also need to have a re-circulating cooker hood if you have any type of fire or boiler in the room, that burns fossil fuel and has an open flue … to avoid the possibility of sucking noxious fumes back into the room. That doesn’t apply to appliances with balanced or sealed flues … they’re fine with vented out extractors … although it’s advisable (and essential with very powerful hoods) to have ventilation in the room, allowing air back in from outside.

One other option, well worth considering if you have a very large room, or need to use a long vent run … is to use an external motor. This can both increase the extraction rate and reduce the noise in the kitchen. You can get external motors which fit on the outside of the kitchen wall … or you can get them to fit on the roof (especially useful for large kitchens in single storey extensions – with a hob on an island).

Noise is measured in dBA (basically a measure of sound in the human hearing range) and it’s a logarithmic scale (thanks to Richard Cooley of Westin Cooker Hoods, for clarifying this for me).  I know, for a fact, that some of you don’t use your cooker hoods because of the noise they make and, a cooker hood that is 4 or 5 decibels louder than another, can be ten times as noisy!

Finally, let me answer the often asked question … how high should the cooker hood be fitted above the hob?  First – check the hob installation instructions – if they specify a height, use that. If they don’t, then check the cooker hood instructions. If they don’t specify a height either, then use a minimum of 760mm (from the top of the hob) for gas and 650mm for electric. It’s also best practice to leave a gap of 50mm either side of a gas hob before fitting any wall units (or a larger gap, if specified by the cooker/hob manufacturer). Some gas fitters will insist on this gap.

So – to sum up. Cooker hoods play an important role in the kitchen – and your decisions as to which hood to buy and where to site it, are worth giving careful consideration. Choose one with good looks, of course … but make sure it’ll do the job properly too.

Tags: cooker hood, kitchen design, Kitchen Equipment, Kitchen Gadgets and Appliances, Kitchen Planning & Installation, siting a cooker hood