Cooker Hood Extraction Calculator — Kitchen Ventilation

Approved Document F 2021 Table 1.1 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. For a continuous (background) mechanical extract system instead, the minimum high rate is 13 l/s (ADF 2021 Table 1.2, a separate table). 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.

Gas hobs produce combustion by-products — CO₂, water vapour, and small amounts of NOx — in addition to cooking fumes, so as a design rule of thumb many installers allow roughly 20% more extraction than for electric or induction. (This uplift is a margin, not an ADF requirement: ADF sets a fixed minimum rate, not a hob-type multiplier.) More importantly, the kitchen must have adequate replacement/combustion air so extraction does not create negative pressure. Note the BS 5440-2 caveat: where an OPEN-FLUED gas appliance (such as an open-flued boiler or gas fire) shares the kitchen, extract is limited to 20 l/s in that room and a spillage test to BS 5440-1 is required, because over-pumping can pull flue gases back into the room. Room-sealed (balanced-flue) appliances are not affected.

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 1.1, 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.

Most domestic cooker hoods have a 150mm (6-inch) round outlet (or a 220×90mm flat rectangular spigot of equivalent area), and the duct should match it. This calculator sizes the duct for a conservative 4 m/s target velocity and never recommends below 150mm, because the duct must never be smaller than the hood outlet — reducing it increases back-pressure and noise and chokes the airflow. Larger sizes (200mm+) only apply to high-flow boosted or open-plan setups. Use rigid metal or semi-rigid aluminium duct rather than flexible duct wherever possible — flexible duct has roughly 3× the friction loss and should be kept under 1m in length.

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 where gas appliances are present — can disturb combustion. Make-up air is provided through trickle vents, background ventilators, or dedicated air inlets. For gas installations, BS 5440-2 requires adequate combustion/replacement air, and where an open-flued appliance shares the space the extract is limited (20 l/s in the same room) with a spillage test to BS 5440-1, so that extraction never starves the appliance of air or causes flue-gas spillage.