Kitchen Extraction and Ventilation: Approved Document F Compliance Guide

Quick Answer: Approved Document F (2021) requires kitchen extraction at minimum 30 litres/second (ducted) or 60 litres/second (recirculating) when the cooker hood is running, or 13 litres/second continuous background ventilation. Recirculating hoods need activated carbon filters to remove odours and separate ventilation for moisture. Duct diameter must be minimum 125mm; maximum duct run 2m per 90° elbow before derating the motor. All notifiable electrical work requires Part P compliance.

Summary

Kitchen ventilation sounds simple — a fan blows air out — but in practice it sits at the intersection of Building Regulations (Part F), electrical safety (Part P), gas safety (near hobs), fire safety (duct penetrations), and heat loss (energy penalty of uncontrolled air leakage). Getting it wrong shows up in condensation problems, persistent cooking smells, and occasionally a failed Building Control inspection.

Approved Document F was substantially revised in 2021 to bring in tighter standards and new requirements for new-build and replacement works. The key change: separate explicit flow rates for ducted versus recirculating hoods, mandatory minimum background ventilation even when the hood is off, and guidance on mechanical ventilation with heat recovery (MVHR) in high-performance homes.

For kitchen fitters and plumbers fitting extract ducting, the most common problems are: undersized duct runs that restrict the motor and reduce flow, poorly sealed ducts that leak warm air into the ceiling void, and recirculating hoods fitted without adequate background ventilation to handle moisture.

Key Facts

• Approved Document F (2021) — ventilation regulations for dwellings; fully revised, applies to all new builds and replacement ventilation systems
• Minimum extract rate (ducted hood) — 30 l/s (litres per second) measured at the grille
• Minimum extract rate (recirculating/filter hood) — 60 l/s measured at the grille (doubled because it doesn't extract moisture, only odours)
• Background ventilation — minimum 13 l/s continuously, or an intermittent fan with 15-minute overrun
• Duct diameter — minimum 125mm round (equivalent to 150×100mm rectangular); smaller duct increases resistance and reduces flow
• Duct run deration — each 90° bend adds equivalent resistance of approximately 1.5–2m straight duct; long or tortuous runs significantly reduce flow; calculate effective duct length
• Duct material — semi-rigid aluminium, rigid PVC, or rigid metal galvanised ducting; flexible ribbed ducting reduces flow significantly and traps grease — use only for short connections
• Duct sealing — all joints must be sealed with duct sealant or aluminium tape (not standard household tape); unsealed ducts leak warm moist air into voids
• Fire considerations — duct penetrations through fire compartment walls (kitchen to garage, kitchen to loft above ceiling) require intumescent dampers
• External termination — cowl termination to outside; minimum 30mm from combustible materials; external wall grilles must prevent pest entry; height above ground minimum 1.8m (to avoid blocking by vegetation)
• New build (Part F 2021) — additional requirement for Approved Calculation Method to prove adequate ventilation; simpler prescriptive route available for standard configurations
• Air quality sensors — humidity-controlled extract fans are permitted as an alternative to timer-controlled in new buildings
• Carbon monoxide — open-plan kitchens with gas appliances need CO detection; standard kitchen ventilation does not substitute for this requirement

Quick Reference Table — Extract Rates by Hood Type

Detailed Guidance

Choosing Ducted vs Recirculating

Ducted extraction is always the better technical solution. It removes cooking moisture, odours, heat, and airborne particles. The duct run to outside is the complication — it requires a route through the external wall or ceiling and beyond.

Recirculating extraction is used where a duct run to outside is impractical: island hobs with no overhead route, period properties, or new builds where airtightness makes extraction difficult to balance. Recirculating hoods use activated carbon filters to remove odours; grease filters remove particulates. They do NOT remove moisture. In a high-airtightness home (new build), a recirculating hood must be combined with a dedicated MVHR system or a humidistat-controlled background fan in the kitchen, otherwise condensation problems will occur.

Carbon filter maintenance — activated carbon filters saturate and stop working. Replacement is typically every 3–6 months with heavy kitchen use. If the carbon filter is overdue, the hood still runs but does not remove odours. Advise clients.

Duct Sizing and Flow

The fundamental rule: duct resistance determines whether the motor achieves its rated flow rate. A 30 l/s hood motor measured on the bench may only achieve 15 l/s when installed in a 6m tortuous duct run with 3 bends.

Duct sizing:

The relationship between velocity, diameter, and flow rate: Q (l/s) = π/4 × d² × v × 1000 (where d is in metres, v in m/s)

For most domestic ducted hoods: 125mm is the minimum; 150mm is better and significantly reduces resistance; 100mm should not be used.

Duct run length and bends:

• Effective duct length = actual straight run + (number of 90° bends × 1.5m equivalent)
• Maximum recommended effective duct length for standard hood motor: 5–6m
• Each extra metre of effective duct length reduces flow approximately 3–5%; at 10m effective, flow may drop 30–40%

For long duct runs (over 6m effective), either:

• Upgrade to an inline fan (remote motor in the duct run, allowing the motor to be positioned close to the outside grille)
• Upgrade the hood to a high-power model rated for the specific duct configuration

Duct Route and Installation

Common route options:

1. Through external wall behind the cooker — shortest run, lowest resistance; requires core drilling; external face requires wall grille
2. Up through ceiling, across loft, out through roof or gable — common in terrace houses; risk of duct joints leaking in loft; seal all joints with aluminium tape
3. Through ceiling into adjacent bedroom wall above — noise issue if the duct passes close to sleeping areas; consider insulating the duct
4. Island hood up through ceiling — always a longer run; insulate duct in loft to prevent condensation inside the duct

Condensation in ducts — warm moist extract air passing through a cold duct will condense inside the duct. Symptoms: dripping from the hood on cold mornings, water pooling at the base of a wall grille. Solutions: insulate the duct wherever it runs through cold spaces; ensure the duct runs continuously to fall toward the outside (not toward the hood); fit a condensate drain at the base of any section that cannot fall outward.

Fire dampers — where the duct penetrates a fire-rated wall or floor (particularly kitchen over garage, kitchen into a party wall loft), an intumescent fire damper rated to match the fire resistance of the penetrated element (typically 30 or 60 minutes EW or EI) must be fitted. The damper must be accessible for maintenance and replacement. This is a Building Regulations Part B requirement.

Electrical Installation

Extract fans and cooker hoods are Part P notifiable work when:

• Replacing an extract fan in a kitchen (a special location)
• Installing new electrical supply for a hood in a kitchen

Work must be carried out by a competent person registered with an approved Part P competent person scheme (NAPIT, NICEIC, ELECSA), or Building Control must be notified and the work inspected. The electrical connection must be via a fused spur or double-pole switch accessible near the hood; a BS 1363 socket is not acceptable as the primary connection to a hardwired appliance.

For hoods with integral lighting (standard), the LED or lamp circuit is typically low-voltage (12V or 24V) from an integral driver — no additional Part P implications for the lighting itself.

Replacing an Existing Hood

When replacing an existing extract hood:

1. Check the existing duct size — if the replacement hood requires 125mm and the existing duct is 100mm, either replace the duct or use a 100mm-rated motor
2. Check the existing electrical connection — many older hoods used a 13A plug behind the unit; this is now considered poor practice but not prohibited; a fused spur (3A or 5A) is better
3. For like-for-like replacement of same duct size and electrical connection, Part P notification may not be required — check with the competent scheme or Building Control
4. Confirm the new hood will fit the existing cabinet opening; ceiling-mounted and wall-mounted hoods have different chassis sizes

Frequently Asked Questions

Does a cooker hood need planning permission?

Not for a standard domestic installation. However, where the external duct grille is visible on a listed building or in a conservation area, Listed Building Consent or permitted development constraints may apply. Check with the local planning authority.

Can I duct a cooker hood into the chimney breast?

Only if the flue is confirmed as disused and sealed, and the duct terminates above the sealed section. Never duct into an active flue — the extraction flow can be reversed by chimney draw and carbon monoxide from any connected appliance can enter the kitchen. Confirm the chimney is fully disused and the cap/sealed flaunching is in place before routing through it. Some chimneys also have shared flues serving multiple floors — always confirm sole use.

Why does my cooker hood not seem to pull much air?

Most commonly: the charcoal/grease filters are clogged (clean or replace), the duct run is too long or has too many bends (check effective duct length), or the external termination grille is partially blocked. Some modern airtight houses also create a negative pressure problem — the extract fan pulls more air out than can enter through background ventilation, and the differential pressure stalls the fan. Opening a window 20mm or fitting a background trickle vent resolves this.

Regulations & Standards

• Approved Document F (2021) — Ventilation: fully revised requirements for extract ventilation in kitchens, Part F Section 1 (new dwellings) and Section 2 (existing dwellings)

• Part P Building Regulations — electrical safety: cooker hood electrical connections in kitchens are in a special location requiring competent person sign-off

• Part B Building Regulations — fire safety: duct penetrations through fire-rated elements require intumescent dampers

• BS EN 61591 — household range hoods and other cooking fume extractors: performance classification and testing

• Gas Safe Register — gas appliances in the same space; if fitting a hood adjacent to a gas hob, no Gas Safe implications for the hood itself unless the duct interferes with gas appliance ventilation clearances

• Approved Document F 2021 — full regulatory text for kitchen ventilation requirements

• NHBC Technical Guidance — Ventilation — practical implementation of Part F for new builds

• BEAMA Ventilation Guide — industry body guidance on domestic mechanical ventilation

• kitchen extract quick reference — flow rate summary and duct sizing table

• kitchen extraction ducting routes and installation — duct routing options in detail

• kitchen electrical layout and Part P compliance — electrical supply requirements

• bathroom ventilation requirements — parallel Part F requirements for bathrooms