Re-circulating hoods draw in steam and odour saturated air through a filter, then returning the treated air into the kitchen. If ducting for extraction is not possible, then recirculation may be another option. In this case, the air is drawn through a charcoal filter that removes cooking odours and steam, and the clean air is recirculated back into the kitchen. Cooker hoods for recirculation are easy to install and are normally used in blocks of flats where installing ducts leading directly to the outside of the building can be difficult. Extraction hoods draw in cooking fumes and trap vapours in a filter while expelling odours to the outside through a ventilation duct. You can expect to have to change the charcoal filter once or twice a year, depending on how often the cooker hood is used.

What sort of capacity should a new cooker hood have? This depends largely on the size of the kitchen. A good rule of thumb is that the hood should be able to replace the air 10 to 20 times per hour. For instance, if a kitchen measures 4 x 4 m with a ceiling height of 2.5 m, the cooker hood should replace approximately 400-800/m3 of air per hour.

Thus, the necessary extraction capacity is determined by the volume of your kitchen.
Calculation of the hood capacity that you need is:
10 x volume of your kitchen (volume = width x height x depth).

There will always be some loss of pressure when installing a hood. That is why it is important to observe the following rules:

• Always observe the hood’s recommended venting diameter (O) specified on the venting tube or pipe.
• Where possible, use smooth pipes to minimise air resistance.
• Try to make the installation as short as possible and with as few bends in the pipes as possible.
• You want gentle curves in the pipe rather than sharp bends, so that the air can pass through unhindered, minimising pressure loss.