How do you choose the right cooker hood extraction rate?
By calculating the air volume of the kitchen and how many times an hour it needs changing — and most hoods are undersized because the number on the box is measured in conditions that do not resemble a real kitchen.
The calculation. Extraction rate is quoted in cubic metres per hour (m³/h).
Work out the room volume — length × width × height, in metres.
Multiply by the air changes per hour required. For kitchens, ten to twelve is the conventional figure, with more for heavy cooking.
A kitchen 4m × 3m × 2.4m is 28.8m³; at ten changes an hour that is 288m³/h as a minimum.
Why the quoted figure overstates real performance. Manufacturers' headline figures are typically measured at maximum setting, with no ducting attached. In reality:
Every metre of duct adds resistance.
Every bend adds considerably more — a right-angle bend can cost a substantial proportion of the flow, and two or three bends together are worse than the duct length.
The grille or cowl at the outside wall restricts flow, and gravity flaps more so.
Duct diameter matters enormously. Reducing from 150mm to 125mm or 100mm increases resistance sharply. Fitting a large hood to a small duct wastes the hood, and it is an extremely common installation error.
Flexible ducting is far worse than rigid, because the corrugations create turbulence — and it is what most hoods are installed with.
The practical rules:
Use rigid ducting of the largest diameter the hood supports, as short and straight as possible.
Size up the hood rather than down, and run it on a lower setting — quieter and more effective than a small hood at maximum.
The hood should be wider than the hob, ideally, since plumes spread.
Mount at the manufacturer's stated height — too high and capture fails, too low and it is impractical.
Run it before and after cooking, since capturing early is far easier than clearing afterwards.
Recirculating hoods filter and return air, removing grease and some odour but no heat or moisture — considerably less effective, and used where ducting is impossible.
Noise is rated in decibels, and a quiet hood gets used.