Kitchen Extraction and Ventilation: Part F Rates and Ducting

Quick Answer: Under Approved Document F (Ventilation), a kitchen needs intermittent extract of 30 l/s when the fan/cooker hood is sited adjacent to the hob, or 60 l/s if located elsewhere in the kitchen (alternatively a continuous mechanical extract rate of around 13 l/s). Extraction can be a cooker hood, a wall/window fan or a continuous MEV system, and it works alongside background ventilation (trickle vents, typically ~4,000 mm² equivalent area in a kitchen) and purge ventilation. Ducting should be the full diameter of the fan/hood outlet — typically 150 mm round (or 220 × 90 mm flat) for cooker hoods and 100/125 mm for wall fans — kept as short and straight as possible, with rigid duct preferred, and always ducted to outside, never into a roof void or wall cavity.

Summary

Kitchen extraction removes the steam, grease, cooking odours and combustion moisture that would otherwise condense on cold surfaces, feed mould and degrade the kitchen. Cooking is the single biggest moisture source in most homes, so the kitchen extract is the most important wet-room ventilation in the dwelling. Approved Document F (Ventilation) of the Building Regulations sets the minimum extract rates, and getting both the rate and the ducting right is the difference between a kitchen that stays fresh and dry and one that grows mould on the ceiling within a year.

This matters to kitchen fitters, electricians, builders and anyone installing a cooker hood or extractor fan. The two failure modes are under-extracting (fan too small, or recirculating instead of ducting outside) and badly ducted (long flexible runs, undersized duct, sharp bends, or — the cardinal sin — venting into a loft or cavity instead of to outside). A recirculating "ductless" charcoal hood does not meet Part F because it filters and recycles air without removing moisture; it must be ducted to outside to count.

The common misconceptions are: that any cooker hood satisfies the regs (only a correctly rated, externally ducted one does — recirculating hoods don't); that flexible duct is fine for the whole run (rigid is far better for airflow and grease cleaning); and that a bigger fan fixes a bad duct (it doesn't — a powerful fan on a strangled duct still under-performs). Size the fan to the Part F rate plus duct losses, duct full-bore to outside with minimal bends, and pair it with background and purge ventilation for a compliant, effective kitchen.

Key Facts

• Governing document — Approved Document F (Ventilation), 2021 edition (England, in force from 15 June 2022). Sets kitchen extract rates.
• Intermittent extract — adjacent to hob — 30 l/s (e.g. a cooker hood directly over the hob).
• Intermittent extract — elsewhere in kitchen — 60 l/s (e.g. a wall or window fan not over the hob).
• Continuous extract (kitchen) — minimum continuous mechanical extract rate around 13 l/s.
• Background ventilation — kitchens also need background ventilation, commonly cited as ~4,000 mm² equivalent area (trickle vents).
• Purge ventilation — an openable window (typically 1/20th of floor area for windows opening ≥30°) for rapid air change.
• Duct to outside only — extract air must discharge to outside air, never into a loft, roof space, wall cavity or other enclosed area (causes condensation and rot).
• Cooker hood duct size — typically 150 mm round (or equivalent 220 × 90 mm rectangular/flat) for cooker hoods; matching the hood's outlet diameter — don't reduce it.
• Wall/window fan duct size — typically 100 mm (4") or 125 mm (5") for higher-rate kitchen fans.
• Rigid duct preferred — rigid spiral or solid duct gives lower resistance, better airflow and easier grease cleaning than flexible; minimise flexible runs.
• Keep runs short and straight — every bend and metre of duct reduces effective airflow; size the fan/hood for the actual duct length and number of bends.
• External termination — fit a wall/roof grille or cowl with a backdraught flap/shutter to stop draughts, weather and birds; don't fit insect mesh so fine it strangles airflow.
• Recirculating hoods don't comply — charcoal-filter recirculating hoods filter odour but don't remove moisture or vent outside, so they do not satisfy Part F extract; an additional compliant extract is needed.
• Electrical — fans/hoods wired by a competent person; cooker hood over a hob must respect zone clearances and be on a suitable circuit; isolation provided. (Part P applies to fixed electrical work.)
• Combustion safety — powerful extraction in a room with an open-flue (atmospheric) gas/solid-fuel appliance can cause spillage of combustion gases; check appliance compatibility and provide make-up air (links to gas safety / Part J).
• Building Regs links — Part F (ventilation), Part L (energy — duct/fan efficiency, airtightness), Part J (combustion appliances/flues), Part P (electrical), Part B (fire — ducts crossing compartments).

Quick Reference Table

Detailed Guidance

Part F extract rates — getting the number right

Approved Document F sets the minimum kitchen extract rate by where the extractor sits. If the extractor is directly over the hob (a cooker hood positioned to capture rising steam and grease), the required intermittent rate is 30 l/s — the position does some of the work, so a lower rate is allowed. If the extractor is elsewhere in the kitchen (a wall or window fan not over the hob), it must remove 60 l/s, because it's pulling moisture across the room. Alternatively, a continuous mechanical extract system can run the kitchen at a lower continuous rate (around 13 l/s) all the time rather than on demand. Choose a fan/hood whose rated airflow at the actual installed duct resistance (not its free-air figure) meets the requirement — manufacturers quote both, and the in-duct figure is the one that matters.

Ducting — size, route and material

A powerful hood on a poor duct under-performs, so the duct is as important as the fan. Match the duct to the outlet diameter — for cooker hoods that's typically 150 mm round (or the equivalent 220 × 90 mm flat/rectangular profile), and for wall/window fans 100 mm or 125 mm. Never reduce the duct below the outlet size; a 150 mm hood throttled into 100 mm duct loses most of its performance. Keep the run as short and straight as possible — every bend (especially tight 90°s) and every metre of length adds resistance and cuts airflow. Rigid duct (spiral or solid) is strongly preferred: it has far lower resistance than flexible duct, doesn't sag or trap grease, and is easier to clean. If flexible duct is unavoidable, keep it short, fully extended and well supported. Insulate ducts running through cold spaces (lofts) to prevent condensation forming inside the duct.

Discharge to outside — never into a roof or cavity

Extract air carries moisture and grease, so it must discharge to outside air. The most damaging mistake is venting a cooker hood or fan into a loft, roof void, wall cavity or sub-floor — the moisture condenses on cold timber and roofing, causing mould, rot and ruined insulation. Terminate at an external wall grille or roof cowl fitted with a backdraught flap/shutter to stop cold draughts, driving rain and birds when the fan is off. Don't fit insect mesh so fine that it chokes the airflow. Where the duct passes through a fire compartment (e.g. a flat), fire-rated ducting or dampers may be required under Part B.

Recirculating hoods and why they don't satisfy Part F

A recirculating (ductless) cooker hood pulls air through grease and charcoal filters and blows it straight back into the kitchen. It removes some odour but does not remove moisture and does not vent to outside, so it does not meet the Part F extract requirement. If a kitchen island or layout makes external ducting genuinely impractical and a recirculating hood is fitted, you still need a separate compliant extract (a ducted wall fan at 60 l/s, or a continuous MEV terminal) to satisfy the regs. Tell the customer this up front — many believe a charcoal hood "ticks the ventilation box" when it doesn't.

Background and purge ventilation alongside extract

Extract is only one part of the kitchen's ventilation. Background ventilation (typically a trickle vent of around 4,000 mm² equivalent area in the window head) provides continuous low-level fresh air to replace extracted air and control moisture between cooking sessions. Purge ventilation — an openable window of roughly 1/20th of the floor area — gives rapid air change to clear cooking smells and summer heat. The three work together: extract removes moisture at source, background ventilation provides make-up air and ongoing freshening, and purge clears the room quickly. Skimp on background ventilation and the extract fan struggles to pull air through a sealed room.

Combustion safety and make-up air

A high-rate extractor in a kitchen that also contains an open-flue (atmospheric) gas or solid-fuel appliance can depressurise the room and pull combustion gases (including carbon monoxide) back down the flue — spillage. This is a genuine safety issue, not just a performance one. Where such appliances are present, check the extractor and appliance are compatible, provide adequate make-up air, and follow gas safety / Part J requirements (a spillage test may be needed). Room-sealed (balanced-flue) appliances are not affected the same way. Never fit a powerful extractor near an open-flue appliance without considering make-up air.

Frequently Asked Questions

What extract rate does a kitchen actually need?

Under Approved Document F, 30 l/s if the extractor is directly over the hob (like a cooker hood), or 60 l/s if it's elsewhere in the kitchen. Alternatively, a continuous mechanical extract system can run at around 13 l/s all the time. Size the fan or hood by its airflow at the real installed duct resistance, not the free-air figure on the box, because ducting always reduces actual performance.

Does a recirculating cooker hood meet Building Regulations?

No. Recirculating (charcoal-filter, ductless) hoods filter odour and blow the air back into the room — they don't remove moisture or vent outside, so they don't satisfy the Part F extract requirement. If you fit one (e.g. on an island where ducting is impractical), you still need a separate compliant extract such as a ducted wall fan at 60 l/s or a continuous MEV terminal. Always make this clear to the customer.

What size ducting should I use for a cooker hood?

Match the duct to the hood's outlet — typically 150 mm round, or the equivalent 220 × 90 mm flat profile. Never reduce below the outlet size, keep the run short and straight with as few bends as possible, and use rigid duct in preference to flexible for better airflow and easier grease cleaning. A 150 mm hood squeezed into 100 mm duct loses most of its rated performance.

Can I vent a kitchen extractor into the loft?

No — never. Extract air is full of moisture and grease; venting it into a loft, roof void, cavity or sub-floor causes condensation, mould, rot and ruined insulation. The duct must discharge to outside air through a wall grille or roof cowl with a backdraught shutter. This is one of the most common and most damaging installation faults.

Do I still need trickle vents and an opening window if I have a cooker hood?

Yes. The cooker hood provides extract ventilation, but Part F also requires background ventilation (trickle vents, ~4,000 mm² equivalent area in the kitchen) and purge ventilation (an openable window, roughly 1/20th of floor area). Background ventilation provides the make-up air the extractor needs to work and controls moisture between cooking sessions; without it a sealed kitchen starves the fan and traps moisture.

Regulations & Standards

• Approved Document F (Ventilation), 2021 edition — Volume 1 (dwellings); sets kitchen extract rates (30/60 l/s), continuous rates, background and purge ventilation. In force in England from 15 June 2022.

• Approved Document L (Conservation of fuel and power), 2021 — fan/duct efficiency and airtightness interactions.

• Approved Document J (Combustion appliances and fuel storage) — air supply to combustion appliances; extractor/open-flue spillage risk.

• Approved Document P (Electrical safety — dwellings) — fixed electrical installation of fans/hoods by a competent person.

• Approved Document B (Fire safety) — fire performance of ducts crossing compartments; dampers where required.

• BS 5440-2 — Installation of flueless/flued combustion appliances: air supply (relevant to extractor/appliance interaction).

• BS EN 13141-series — Ventilation for buildings: performance testing of ventilation components (fans, hoods, terminals).

• Domestic Ventilation Compliance Guide — supporting guidance for Part F (installation and commissioning of domestic ventilation).

• GOV.UK — Approved Document F: Ventilation — primary regulation and kitchen extract rates

• GOV.UK — Domestic Ventilation Compliance Guide — installation/commissioning support for Part F

• BEAMA — domestic ventilation guidance — UK ventilation industry body guidance

• GOV.UK — Approved Document J: Combustion appliances — extractor/open-flue spillage safety

• Cooker hood and extractor fan manufacturer technical data — airflow at duct resistance, duct sizing

• kitchen extraction ducting — duct routing, sizing and termination detail

• kitchen extract — cooker hood and kitchen fan selection

• part f ventilation — full Part F requirements across all rooms

• trickle vents and part f — background ventilation and equivalent area

• bathroom ventilation — wet-room extract rates and ducting