Question 1

What extraction rate does a cooker hood need in the UK?

Accepted Answer

Approved Document F 2021 Table 5.1a sets the minimum intermittent kitchen extract rate at 30 l/s (108 m³/h) when a ducted cooker hood is fitted directly above the hob (the "adjacent to hob" row), or 60 l/s (216 m³/h) where there is no hood or the hood recirculates (the "elsewhere in kitchen" row), since a recirculating hood does not extract to the outside. Continuous extract: 13 l/s minimum. Most quality ducted hoods operate at 400–700 m³/h on boost, comfortably exceeding these minimums, and industry practice often sizes the hood at 60+ l/s to allow for duct pressure losses.

Question 2

Do gas hobs need more extraction than electric?

Accepted Answer

Yes. Gas hobs produce combustion by-products including CO2, water vapour, and small amounts of NOx in addition to cooking fumes. This typically requires 20% more extraction than electric or induction hobs. More critically, gas hobs need adequate make-up air to prevent negative pressure, which could cause incomplete combustion. Gas Safe regulations require that replacement air can enter the kitchen at the same rate as the extract.

Question 3

Is a ducted or recirculating cooker hood better?

Accepted Answer

Ducted hoods are significantly more effective as they remove moisture, grease, and odours from the building entirely. Recirculating hoods filter grease and some odours but return the moisture and heat to the kitchen. For gas hobs, ducted extraction is strongly recommended to remove combustion products. Under ADF 2021 Table 5.1a, a ducted hood directly above the hob satisfies the "adjacent to hob" row at 30 l/s intermittent; a recirculating hood does not extract to the outside and therefore counts under the "elsewhere in kitchen" 60 l/s row, so a separate extract path is still required.

Question 4

What size duct do I need for a cooker hood?

Accepted Answer

Most domestic cooker hoods use 150mm (6-inch) diameter rigid duct for optimal performance. Some compact hoods use 125mm or 100mm duct, but this increases back-pressure and noise. Never reduce the duct diameter below the hood outlet size. Use rigid metal or semi-rigid aluminium duct rather than flexible duct wherever possible — flexible duct has 3x the friction loss and should be kept under 1m in length.

Question 5

What is make-up air and why does it matter?

Accepted Answer

Make-up air is the replacement air that enters the kitchen to balance the air being extracted. In modern airtight homes, a powerful cooker hood can create significant negative pressure, which makes doors hard to open, causes draughts through gaps, and — most critically for gas hobs — can affect the combustion of gas appliances. Make-up air is provided through trickle vents, background ventilators, or dedicated air inlets. For gas installations, Gas Safe requires adequate make-up air.

Cooker Hood Extraction Calculator — Kitchen Ventilation

How We Calculate This

Base extraction rates (ADF 2021 Table 5.1a)

Adjustment factors

Duct sizing

Frequently Asked Questions

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