UK Kitchen Extraction Rules: Part F Ventilation Guide

Quick Answer: Building Regulations Approved Document F (England, 2021 edition) requires mechanical extraction in every kitchen with a cooking facility. The minimum intermittent extract rate is 30 L/s if directly above the hob, or 60 L/s elsewhere in the kitchen. Continuous extraction may be used at lower rates (13 L/s minimum). Ducting should be the minimum length possible, ideally <3m with no more than two bends, and discharge to outside air via a fixed external grille. Recirculating hoods do not meet Part F — there must be ducted extraction to outside.

Summary

Kitchen extraction does two jobs: removes cooking smells and moisture that would otherwise condense on walls and cause mould, and removes combustion products from gas hobs and any flueless gas appliances. Under Approved Document F (2021), all new and refurbished kitchens require mechanical extraction designed to specific rates.

Most domestic kitchens use an over-hob extractor hood (cooker hood). These come in two types: ducted (extracts to outside) and recirculating (filters through charcoal and returns to the room). Only ducted extraction meets Part F for new build and the extension of existing kitchens. Recirculating hoods are sometimes acceptable for replacement-like-for-like in existing kitchens but Building Control will normally require an additional wall fan.

The ducting matters as much as the hood. A "1000 m³/h" hood with cheap flexible 100mm corrugated duct and 4 bends might deliver 200 m³/h to the outside — failing the regs and leaving the customer with greasy ceilings. Specifying and installing the duct correctly is half the job.

Key Facts

• Approved Document F (2021) — Ventilation, England (Wales and Scotland have similar but separate documents)
• Volume 1 of ADF — covers dwellings
• Intermittent extract minimum — 30 L/s above hob, 60 L/s elsewhere in kitchen (109 m³/h / 216 m³/h)
• Continuous extract minimum — 13 L/s low rate (47 m³/h)
• BS 5440-2:2009 — Installation and maintenance of flues and ventilation for gas appliances
• BS EN 13141 — Ventilation for buildings — Performance testing of components
• Duct sizing rule — never duct smaller than the hood's spigot; ideally 150mm round or equivalent rectangular for 600mm+ wide hoods
• Flexible duct — semi-rigid aluminium preferred; corrugated flexible loses ~30% of flow over 3m
• Rigid duct — galvanised steel or rigid PVC; minimum 30% better airflow than flexible
• Bend allowance — each 90° bend adds equivalent of ~1m of straight duct length
• Maximum recommended duct length — 3m for typical 600mm hood; longer requires uprated motor
• External grille — fixed louvre or cowl, never against a wall or in a corner; weather hood preferred
• Make-up air — kitchens with cooking and extraction need adequate make-up air (trickle vents, opening windows, dedicated air inlet)
• Gas hob — requires its own combustion air provision separate from extraction

Quick Reference Table

Detailed Guidance

Selecting the cooker hood

Three primary hood types in UK kitchens:

• Chimney hood — wall-mounted, classic look, typically 600/700/900mm wide
• Integrated / canopy hood — sits inside a wall cabinet, lower profile
• Island hood / downdraft — for hobs on islands; downdraft hoods retract from worktop

The published "extraction rate" on the box (m³/h or L/s) is the maximum free-air flow with no duct attached. Real-world delivered flow is typically 50–70% of the rated figure after duct losses. Sizing rule of thumb: choose a hood rated at least 50% above the regulatory requirement.

For a 30 L/s (108 m³/h) requirement, specify a 200–250 m³/h+ hood; for 60 L/s (216 m³/h), specify 400 m³/h+. Larger and more powerful motors give margin for less-than-perfect duct runs.

Duct sizing

The first principle: never reduce the duct below the hood spigot diameter. A common error is fitting a 150mm spigot hood and ducting it to a 100mm wall vent — the wall vent becomes the limiting factor and flow drops dramatically.

Equivalent areas:

• 100mm round = 78 cm²
• 125mm round = 122 cm²
• 150mm round = 176 cm²
• 100×150mm rectangular = 150 cm²
• 110×229mm rectangular (low-profile) = 250 cm²

For 60 L/s and above, 150mm round (or equivalent rectangular) is the minimum. Many manufacturers now ship hoods with 150mm spigots specifically to meet ADF rates.

Duct routing

The shortest, straightest run to outside air. Critical rules:

1. Minimise bends — every 90° bend costs ~1m of equivalent length
2. Use rigid duct where possible — galvanised steel or rigid plastic
3. Flexible duct only at the hood connection (max 600mm) and at the external grille
4. No 180° bends or reverse turns
5. Duct sloped slightly down to outside (1:50) so condensation runs out
6. Insulate ducts running through cold roof spaces to prevent condensation drips
7. Seal all joints with aluminium tape or duct silicone — not standard duct tape (degrades in heat)

Discharge to outside

The duct must discharge to outside air via a fixed external grille or cowl. Options:

• Through-the-wall directly behind the hood — shortest path, ideal
• Up and out through soffit — common for kitchens on internal walls
• Out through roof — full vertical run; needs roof flashing kit (e.g. Klober GIF) and 1m above local flat roof level
• Side wall via ceiling void — through joists, around obstacles

External grille types:

• Fixed louvre — basic, weather-resistant
• Cowl with backdraft flap — prevents wind-blown reverse flow
• Standard wall cap — backdraft flap, integral mesh
• Vermin grille — required where rodents may enter

Never discharge to a roof void, ceiling void, eaves overhang or any internal space — moisture damage will follow.

Make-up air

When extraction is running, air must enter the building somewhere. In modern airtight homes (post-2010 build), there is rarely enough natural infiltration to make up 60 L/s. This causes:

• Backdraft from open-flue appliances (gas hob, fire) — CO risk
• Doors hard to open / close (pressure differential)
• Reduced extraction effectiveness

Provide make-up air via:

• Trickle vents in window frames (mandatory under ADF for replacement windows)
• Dedicated background ventilator (Vent-Axia, similar)
• Opening a window during cooking (rule-of-thumb adequate for occasional cooking)

For powerful extract rates (>60 L/s) in airtight homes, consider a balanced MVHR system as part of the kitchen design.

Gas hob requirements

BS 5440-2 requires gas hob installations to have a dedicated openable window or permanent ventilation in the room. For an open-plan kitchen, the volume calculation applies. The cooker hood does not replace this — even if the hood is extracting, the room still needs combustion air.

Continuous vs intermittent

Two strategies allowed under ADF:

• Intermittent — high-rate extract during cooking (typically the hood operating); user-controlled
• Continuous — low-rate background extraction at all times (typically via MVHR or a dedicated wall fan); higher rate when cooking is detected

Continuous gives better overall air quality and is required in many new builds for compliance with the overall whole-dwelling ventilation rate. Many new build kitchens have both: an MVHR taking continuous background extract, plus a separate cooker hood for high-rate cooking extract.

Common installation faults

Frequently Asked Questions

Is a recirculating hood compliant with Part F?

In general no, for new build and extensions. Recirculating hoods filter air through a charcoal cartridge and return it to the room — they do not extract moisture or combustion gases. For Part F compliance you need ducted extraction. The exception is a like-for-like replacement of an existing recirculating hood in a kitchen that already has alternative ventilation.

Can I duct through a cold loft space?

Yes, but insulate the duct to prevent condensation drips. Use pre-insulated flexible duct (e.g. Manrose ALD150) or wrap a rigid duct with 25mm mineral wool, foil-faced. The duct should be sloped slightly downward toward the outside grille so any condensate that does form drains out.

What's the maximum duct run for a 60 L/s hood?

3m total equivalent length is the practical maximum for a standard motor. Add bends as ~1m each. If your physical route is 4m straight with two bends (equivalent 6m), the hood rate will drop below 60 L/s. Either re-route (shorter), upsize the duct to 175mm or 200mm, or specify a higher-rated hood.

How do I test the actual extraction rate?

Use an anemometer at the grille. Multiply the average air velocity (m/s) by the grille free area (m²) to get flow in m³/s. Multiply by 1000 to get L/s. For commissioning, take readings at multiple points across the grille and average.

Does Part F apply if I'm just replacing the hood?

If you're replacing like-for-like (ducted to ducted), no notifiable work occurs and Part F doesn't trigger. If you're changing the type (e.g. installing ducted where there was no extraction), or if part of wider refurbishment, then Part F applies and the installation must meet the rates.

Regulations & Standards

• Approved Document F Volume 1: Dwellings (2021 edition) — Ventilation requirements for England

• Approved Document L Volume 1: Dwellings (2021 edition) — Conservation of fuel and power (interface with airtightness)

• BS 5440-2:2009 — Installation and maintenance of flues and ventilation for gas appliances

• BS EN 13141 parts 1–11 — Ventilation for buildings — Performance testing

• BS EN 13142 — Ventilation for buildings — Components/products for residential ventilation

• BS 5925:1991 — Ventilation principles and designing for natural ventilation

• BS EN 16798 parts 1–5 — Energy performance of buildings — Ventilation for buildings

• CIBSE Guide B2 — Ventilation and air conditioning

• HSE Gas Safe (Registered Engineer) — Required for any gas appliance work

• The Gas Safety (Installation and Use) Regulations 1998 — Gas hob and oven installation

• GOV.UK — Approved Document F (Ventilation) — Statutory ventilation requirements

• BEAMA Domestic Ventilation Compliance Guide — Industry guidance

• BSRIA — Ventilation publications — Engineering association

• Gas Safe Register — Gas hob installer guidance

• British Standards Institution — BS 5440-2 — Gas appliance ventilation

• kitchen extract — Companion article on extraction strategy

• induction hob installation — Hob types and ventilation impact

• integrated appliance installation — Hood install alongside other appliances

• kitchen electrics — Power circuits for extract fans and hoods

• bathroom ventilation — Wet room ventilation by comparison