Kitchen Extraction Calculator — Part F Ventilation Rates

Approved Document F Volume 1 (2021) Table 1.1 sets the minimum intermittent kitchen extract rate at 30 l/s where a cooker hood over the hob extracts to outside, or 60 l/s where there is no cooker hood, the hood does not extract to outside (e.g. recirculating), or the extract is sited elsewhere in the kitchen. For continuous mechanical extract the minimum is 13 l/s (Table 1.2). A recirculating hood on its own does not satisfy Part F (note to Diagram 1.2) — a ducted extract to outside is still required, at the 60 l/s rate. These rates apply to new builds and extensions in England; Scotland (Section 3 Technical Handbook) and Wales have similar but not identical requirements.

A 150mm (6 inch) rigid duct is recommended for most domestic cooker hood installations. This minimises pressure drop and noise. A 125mm duct is only acceptable for very short, near-straight runs — keep the effective duct length (the straight run plus roughly 1.5m for each 90-degree bend) to about 3m or less, which in practice means a short run with at most one or two bends, not both. Avoid 100mm ducting for cooker hoods as it restricts airflow significantly. Always check the hood manufacturer's instructions (many require 150mm and reducing the diameter can invalidate the warranty), and use rigid or semi-rigid ducting rather than flexible aluminium hose, which causes turbulence and grease build-up.

Each 90-degree bend in the ducting adds approximately 1.5 metres of equivalent duct length in terms of air resistance. A 45-degree bend adds roughly 0.75m equivalent. For optimal performance, minimise the number of bends and keep the total effective duct length (actual length plus bend equivalents) as short as possible. Most cooker hood manufacturers recommend a maximum effective duct length of 5-6m.

Yes, recirculating hoods require activated carbon filters to remove cooking odours before returning air to the kitchen. These filters need replacing every 3-6 months depending on usage. Note that a recirculating hood does not meet Part F kitchen extract requirements on its own (note to Diagram 1.2, ADF 2021) — it does not remove cooking moisture or combustion products from the dwelling. You still need a ducted extract to outside (60 l/s intermittent, or 13 l/s continuous) plus background ventilation.

As an indicative screen, a commercial kitchen with cooking needs a minimum of about 30 air changes per hour (CIBSE Guide B); roughly 20 ACH suits a non-cooking preparation area. Gas-fired equipment needs higher rates due to combustion products. However, air changes alone are not the design method — commercial kitchens should be sized by the thermal/canopy approach in DW/172 (BESA) and BS EN 16282-1, which determines the extract from the heat and effluent released by the appliances. Under DW/172 (Canopy Dimensions section) the canopy plan dimensions must exceed the cooking equipment by a minimum of 250mm on each free side (measured to the inside of the condensation channel), increasing to 600mm at the front for combination ovens and 300mm for solid-fuel appliances.