Kitchen Extraction Ducting: 150mm vs 125mm Rigid vs Flexible, Run Length Limits, Approved Document F 30 l/s Requirement

Summary

Kitchen extraction ducting is one of the most commonly incorrectly installed elements in UK kitchens. The failures fall into predictable patterns: flexible duct run the full length from hood to external wall, creating excessive resistance; 100mm duct used where 125mm or 150mm is required; duct run through a void that cannot be cleaned or inspected; and termination vents too close to adjacent openings, recirculating cooking smells back into the building.

Approved Document F (2021 edition) strengthened the ventilation requirements for kitchens, setting a mandatory 30 l/s (litres per second) extract rate for ducted cooker hoods. This rate assumes a properly sized and routed duct. If the duct has too many bends or the wrong diameter, the hood motor cannot achieve 30 l/s even on maximum setting — which means the installation does not comply regardless of the hood's rated capacity.

Getting extraction right matters for the occupant's health (cooking fumes are a significant indoor air pollutant), for the building fabric (condensation and grease buildup in poorly ventilated kitchens causes mould), and for contractor liability. A Building Regulations-compliant extraction system that is properly commissioned is increasingly expected by building control officers, especially in new build and larger refurbishments.

Key Facts

• Approved Document F (2021): minimum kitchen extract rate 30 l/s (108 m³/h) for ducted hood
• Recirculation rate (carbon filter hood): minimum 13 l/s — not a substitute for ducted extract in new dwellings
• Preferred duct diameter: 150mm circular rigid (maximum airflow, minimum resistance)
• Minimum duct diameter: 125mm circular for short residential runs; 100mm only for recirculation modes
• Equivalent duct length: total resistance expressed as length of straight pipe; use to size fan
• 90° bend equivalent length: approximately 1.0–1.2m for 150mm rigid; 1.5m for flexible duct
• Maximum flexible duct: 1.5m total length, and only at the hood connection (transitional section)
• Rigid duct material: galvanised steel circular duct (most common) or rigid aluminium; PVC only where permitted by manufacturer
• Grease filters: must be cleaned every 4–6 weeks to maintain airflow; blocked filter can halve effective extract rate
• Termination distance: minimum 300mm from any window, door, or ventilation opening; minimum 1,500mm from gas boiler flue terminal
• Roof termination: use purpose-made mushroom or louvre terminal; minimum 500mm above nearest roof covering
• Backdraft shutter: all external terminals must include a spring-loaded or gravity backdraft damper
• Fire damper: required in any duct passing through a fire-separating floor or wall under Approved Document B

Quick Reference Table

Detailed Guidance

Duct Sizing and Resistance Calculation

The resistance of the duct system determines whether the hood fan can deliver the required extract rate. Fan manufacturers publish fan curves — the relationship between airflow (l/s or m³/h) and system resistance (Pa). The designer must ensure the system resistance at the required airflow (30 l/s) is less than the fan's available pressure at that flow rate.

Equivalent duct length method (simplified for domestic installations):

Assign equivalent lengths:

• Each metre of straight 150mm circular rigid duct = 1m
• Each metre of 150mm flexible duct = 1.5× multiplier (due to corrugated interior resistance)
• Each 90° elbow (rigid) = 1.0m
• Each 45° elbow (rigid) = 0.5m
• Wall terminal with backdraft damper = 1.5m
• Grease filter (clean) = approximately 1.0m

Example calculation:

• 2.5m straight rigid duct = 2.5m
• Two 90° bends = 2.0m
• Wall terminal = 1.5m
• Total equivalent length: 6.0m

For a 150mm duct system, a total equivalent length of 6m is at the limit of what most standard cooker hood fans can overcome to achieve 30 l/s. Where runs are longer or more convoluted, specify a higher-performance fan or increase duct to 200mm.

Rigid vs Flexible Ducting

Rigid circular duct (galvanised steel or aluminium):

• Lower internal resistance — smooth bore versus corrugated
• Preferred for all sections except the final connection to the hood
• Available in standard 150mm diameter in 1m lengths with slip joint or push-fit connections
• Sections must be sealed with aluminium foil tape (not standard cloth duct tape) at all joints — the seal must resist grease and temperature cycles
• Rectangular flat channel duct (e.g., 110×54mm or 120×65mm) is often used where ceiling void depth is limited — equivalent flow capacity is lower; check conversion tables

Flexible aluminium foil duct:

• Significantly higher resistance per metre — use only for final 0.5–1.5m connection at hood
• Must be pulled fully extended, not left kinked or bunched — a kinked flexible duct can reduce airflow by 50%
• All joints must be taped with aluminium foil tape — not cable ties, not standard tape
• Replace every 5 years — corrugated foil degrades and the internal surface accumulates grease

Rectangular flat duct:

• Used in kitchens where ceiling voids are shallow (e.g., flat-roof extensions with 100mm void)
• Equivalent diameter conversion: 110×54mm ≈ 100mm circular; 120×65mm ≈ 110mm circular
• Higher resistance than circular of equivalent cross-section — factor this into equivalent length calculation
• Joints must be taped; some systems use rubber gasket push-fit connections

External Termination

The duct must terminate externally with a purpose-made terminal fitting. Key requirements:

Wall terminal:

• Plastic or aluminium louvre/grille with backdraft shutter (spring-loaded flap)
• Core drill through external wall — typically 160mm core for 150mm duct; allow 5mm clearance
• Seal the annular gap between duct and core with fire-rated expanding foam (BS EN 1366-3 rated)
• Position: avoid prevailing wind direction if possible (wind loading on shutter increases back pressure)
• Distance from adjacent openings: 300mm minimum from any opening window, door, or trickle vent

Flat roof/soffit termination:

• Use mushroom-type terminal (gravity flap or spring damper)
• Raise above roof covering by at least 500mm to prevent blockage by leaves and debris
• Ensure duct run has no dips where condensate could pond — slope at minimum 3° toward hood (condensate falls back to cooker area), or toward external with a drain point

Distance from gas boiler flue:

• Minimum 1,500mm from any gas boiler flue terminal (to prevent combustion products being drawn into kitchen)
• The HHIC code of practice and Gas Safety (Installation and Use) Regulations 1998 both reference this requirement

Passing Through Fire Compartment Walls or Floors

Where the kitchen extraction duct passes through a fire compartment wall (e.g., into a garage, or through a separating floor between flats), a fire damper or intumescent sleeve is required per Approved Document B (Fire Safety):

• Fire damper: mechanical damper that closes when fusible link melts at 72°C (standard) or 165°C (kitchen); closes duct in fire conditions
• Intumescent sleeve: liner around the duct opening in the wall; expands under heat to seal the duct bore
• The fire damper must be accessible for testing and replacement — provide an access panel in the duct at the damper position

Omitting fire protection in through-floor or through-wall duct installations is a Building Regulations compliance failure. Building control will check this on inspections for new build and renovation projects.

Frequently Asked Questions

My cooker hood is noisy even on low speed — is the duct wrong?

Noise in cooker hoods is typically caused by: (1) too small a duct creating turbulence — 100mm duct on a high-capacity hood is almost always the cause; (2) flexible duct that is kinked or too long — increases turbulence; (3) a blocked grease filter — restricted airflow causes the motor to work harder and noisier; (4) a backdraft damper flap that is stiff or partially open — rattles in the airflow. Check duct diameter first, then the grease filter (clean or replace), then the damper flap.

Can I duct through an internal wall into a chimney flue?

No. You must never use an existing chimney as a kitchen extraction route unless:

• It is a dedicated, unlined flue serving no other appliance
• The flue is brick or clay-lined and the duct is sealed at the terminal
• HETAS or an equivalent specialist has assessed the flue

In most cases, ducting to the outside wall directly is simpler, shorter, and compliant. Using a flue serving or that previously served a gas boiler is prohibited under Gas Safety Regulations.

Does a recirculating hood comply with Approved Document F?

Not for new dwellings. ADH Part F (2021) requires 30 l/s ducted extraction for new kitchens. A recirculating (carbon filter) hood that does not duct to outside does not comply for new build. In existing dwellings where ducting to outside is genuinely impractical, a recirculating hood at 13 l/s minimum plus adequate background ventilation (trickle vents in windows) may be accepted by building control — discuss with your local authority BCO.

Regulations & Standards

• Building Regulations Approved Document F (2021) — ventilation requirements; Table 1.2 sets 30 l/s kitchen extract rate

• BS EN 13141 — performance testing of ventilation for buildings; fan performance measurement

• Approved Document B (Fire Safety) — fire dampers in duct penetrations through fire compartments

• Gas Safety (Installation and Use) Regulations 1998 — flue terminal separation distances

• CIBSE Guide B2 — ventilation and ductwork design guidance

• Approved Document F (2021) — MHCLG official guidance

• BESA Ductwork Specification DW/144 — Building Engineering Services Association ductwork standards

• BEAMA Kitchen Ventilation Guide — industry guidance on kitchen extract

• HETAS Technical Guidance — chimney flue use for extraction

• kitchen extract — cooker hood selection and installation

• kitchen electrics — electrical supply for cooker hood

• kitchen units installation — wall cabinet installation above hob

• part f ventilation — Approved Document F overview