Bathroom Extractor Fan Guide: Selection, Sizing, Installation and Approved Document F

Quick Answer: Approved Document F (Volume 1: Dwellings, 2021 edition) requires mechanical extract ventilation in any room containing a bath or shower: minimum 15 L/s intermittent extract, or 8 L/s continuous extract. Installation must duct to outside air (not into roof or wall cavity), include backdraught shutter, and ideally have a 15-minute overrun timer. Bathroom fan electrical work is subject to BS 7671 Section 701 (bathroom zones) and notifiable under Approved Document P. Typical installed cost: £100-£400 for standard intermittent fan, £400-£900 for continuous fan with humidistat or heat-recovery.

Summary

Bathroom extractor fans are one of the most commonly misunderstood and badly-installed bathroom components. Underspecified fans don't clear steam; cheap fans fail within 2-3 years; ducting through roof voids without termination causes condensation in the loft; fans installed in the wrong zone breach electrical regulations. Customers often experience years of mould, peeling paint, and persistent damp because a £40 supermarket fan was fitted without ducting to outside.

This article covers the regulatory minimum (Approved Document F), the practical higher specification for good performance, fan types (intermittent, continuous, humidity-controlled, heat-recovery, in-line), correct ducting practice, the electrical compliance requirements (BS 7671 Section 701, Approved Document P), and the consequences of getting it wrong. It is the technical reference for plumbers, electricians, builders and renovators specifying or installing bathroom ventilation.

Key Facts

• Approved Document F (Volume 1: Dwellings, 2021) — Building Regulations Part F; ventilation requirements
• Intermittent extract — fan runs when needed (e.g. light switch trigger, humidistat); minimum 15 L/s
• Continuous extract — fan runs all the time at low rate; minimum 8 L/s, boost to 15+ L/s for room use
• dMEV (decentralised mechanical extract ventilation) — single continuous extract fan per wet room
• MVHR (mechanical ventilation with heat recovery) — whole-house ventilation with heat exchanger
• PIV (positive input ventilation) — whole-house alternative; pressurises with filtered outside air
• L/s — litres per second; the standard ventilation unit
• Fan power — typical bathroom fan 5-30W
• Backdraught shutter — prevents air ingress through fan when off
• Duct run length — limits fan performance; check manufacturer's performance curve
• Cooker hood — separate ventilation requirement (Approved Document F)
• BS 7671 Section 701 — bathroom zones for electrical
• Approved Document P — bathroom electrical work notifiable
• IPX4 / IPX5 — fan ingress protection ratings appropriate for splash zones
• SELV — separated extra-low voltage (12V or 24V); allowed in Zone 1 with appropriate protection
• Humidistat — automatic activation at preset humidity (typically 55-75% RH)
• 15-minute overrun — fan continues after light off; clears residual humidity
• Trickle vent / background ventilation — passive ventilation through windows; works alongside extract

Quick Reference Table

Detailed Guidance

Sizing the fan

Step 1: Determine the room category (bathroom with bath/shower).

Step 2: Apply Approved Document F minimum — 15 L/s intermittent or 8 L/s continuous.

Step 3: Consider real-world performance. Manufacturer-quoted extract rate is at zero static pressure (free air). With ducting, actual delivered air is reduced. Rule of thumb:

• Fan quoted at 15 L/s; with 4m duct + 1 bend → actual ~10-12 L/s
• Fan quoted at 25 L/s; with 4m duct + 1 bend → actual ~17-20 L/s

For a 15 L/s requirement, specify a fan rated 20-25 L/s to compensate for duct losses. Check manufacturer's performance curve.

Step 4: Match fan style to install:

• Axial fan — short duct runs (<3m); cheap; noisy under load
• Centrifugal fan — longer duct runs; better performance under back-pressure; quieter
• Inline duct fan — fan in the duct, not at the room; quieter; ideal for long runs

Choosing fan features

Modern fan options:

• Pull-cord — old-school manual switch; not recommended; non-compliant in some Zone 1 positions
• Light switch trigger — fan activates with light; standard intermittent install
• Humidistat — fan activates above set RH; useful for non-light-triggered use
• Overrun timer — fan continues after light off; 5-30 min; clears residual humidity
• Speed control — variable for noise vs performance
• PIR (passive infrared) — fan activates on movement; alternative to light switch
• Smart / Wi-Fi enabled — control via phone; some integrate with smart home

Recommended baseline: 15 L/s+ axial fan with humidistat + overrun timer + 15-minute timer + backdraught shutter, IPX4 rated. Branded options: Vent-Axia Silhouette, Greenwood, Manrose, Xpelair, Airflow Aventa.

Installation — wall vs ceiling

Wall-mounted fan — direct duct through external wall to outside via external grille. Easier install; shorter duct. Limitations: must be on external wall; visible external grille; sometimes vulnerable to wind-driven cold air.

Ceiling-mounted fan — ducted through ceiling void to exit at soffit, roof tile vent, or external wall. More flexible position; cleaner internal appearance. Longer ducts mean greater performance loss; ensure ducting is rigid where possible and termination is to external air (not into roof void).

Inline fan in ceiling void — fan mounted in the duct between ceiling grille and external termination. Quieter (fan motor in void); better for long runs. More complex install.

Ducting — the most common installation error

The fan must duct to outside air. NOT:

• Into roof void
• Into wall cavity
• Into soffit space without proper termination
• Just "out somewhere"

Why this matters: humid air vented into a roof or cavity condenses on cold surfaces, causing damp, mould, and rotting of timber. This is a leading cause of mysterious damp in bathrooms with seemingly working fans.

Correct ducting:

• Rigid plastic or aluminium duct where possible (less restrictive than flexible)
• 100mm diameter for standard bathroom fans
• 150mm diameter for high-output kitchen fans
• Flexible duct only for short final connections; minimise length (each bend ≈ 0.5m straight)
• Slope downwards from fan to outside (allows condensate to drain outside, not back into fan)
• External termination via:
  • Wall grille (cowl with backdraught flap)
  • Soffit vent
  • Roof tile vent (Manthorpe, Hambleside Danelaw)
• All joints sealed with foil tape
• Insulated duct where running through unheated voids (prevents condensation inside the duct)

Electrical installation

Bathroom fans are notifiable electrical work (Approved Document P):

1. Decide power source — typically from the lighting circuit (light switch trigger) or a dedicated spur from the consumer unit
2. Run cable in safe zones (not within 50mm of plastered surfaces unless mechanically protected)
3. Cable specification — usually 1.0 or 1.5mm² twin-and-earth
4. 30mA RCD protection required on the circuit
5. Connection via 3-pole isolator switch outside the bathroom (or fused connection unit)
6. Fan position must comply with BS 7671 Section 701 zones (see table)
7. Manufacturer wiring instructions strictly followed for switched live, neutral, permanent live (for overrun)
8. Test for insulation resistance, polarity, earth continuity
9. Notify Building Control or self-certify via competent person scheme

Continuous extract (dMEV) — modern best practice

For modern airtight homes, decentralised mechanical extract ventilation (dMEV) is becoming the preferred approach:

• Fan runs continuously at low rate (typically 4-8 L/s)
• Boosts to higher rate (15+ L/s) on light switch, humidistat or PIR
• More effective at maintaining low background humidity
• Quieter at low rate (sensors-driven boost)
• Slightly higher upfront cost; ~£20/year electric to run continuously
• Required for high-spec / Passivhaus / airtight new-build

Brands: Vent-Axia Lo-Carbon, Greenwood CV2GIP, EnviroVent Filterless.

Heat recovery — single-room and whole-house

Heat recovery ventilation pre-warms incoming fresh air using outgoing stale air, recovering up to 90% of heat in the exhaust:

• Single-room heat recovery (sHRV) — like Vent-Axia Lo-Carbon Heat Save; one unit per bathroom; ducting through external wall; supply and extract through same penetration
• Whole-house MVHR — central unit serving all wet rooms and habitable rooms; complex install; new-build standard; typical cost £3,000-£8,000

For retrofit, single-room HRV is a good upgrade for a bathroom in an airtight home or where heating cost is a concern.

Common installation errors

• Fan ducted into roof void → loft damp, mould, rotting timber
• Fan undersized for the room (12 L/s in a humid family bathroom) → persistent moisture
• Flexible duct used for the entire run with multiple sharp bends → fan delivers 30% of rated rate
• No external termination cowl → wind reverses through fan into bathroom
• Fan in Zone 1 without SELV or IPX4+ rating → electrical compliance fail
• Light switch with no permanent live for overrun → fan switches off with light, doesn't clear residual humidity
• Inadequate trickle ventilation in the room (sealed windows, no air admission point) → fan starves of replacement air; performance crashes
• No isolator switch for service → can't safely disconnect for replacement

Trickle ventilation and air admission

Extract fans need air admission — air must enter the room to replace the extracted air. Sources:

• Trickle vents in window frames (default in modern UPVC and aluminium)
• Door undercut (10-15mm gap below internal door)
• Wall vent or grille for older properties

A bathroom with sealed window (no trickle vent), tight-fitting door, and a powerful fan will create negative pressure that limits performance — sometimes pulling air back through the fan itself.

Frequently Asked Questions

Can my customer keep the existing fan if it works?

If it's compliant (15 L/s or 8 L/s continuous, ducts to outside, electrically safe), yes. If it ducts into a roof void, is undersized, or is electrically non-compliant, replace as part of any refurbishment. Even working older fans may be inefficient or noisy.

Why does my customer's bathroom still get mouldy with an extractor fan running?

Common causes:

• Fan ducts into roof void (not to outside) — humid air builds in loft, condenses, drips back
• Fan undersized for actual room size and use pattern
• Fan switched off too soon after shower (no overrun)
• No air admission (window closed, no trickle vent, sealed door) — fan can't extract
• Existing damp from other source (e.g. cold-bridge condensation)
• High humidity from drying laundry indoors

Diagnose before specifying replacement.

Pull cord or remote isolator?

Modern best practice is a remote isolator (3-pole switch) outside the bathroom, with the fan triggered automatically by light/PIR/humidistat. Pull cord inside bathroom is acceptable but typically considered older-spec.

How long does an extractor fan last?

Cheap supermarket fans: 2-4 years before bearings or shutter fail. Mid-range branded fans (Vent-Axia, Greenwood, Xpelair): 7-15 years. Premium dMEV: 15-20 years. Maintenance: annual filter clean (where applicable), check backdraught shutter, listen for bearing noise.

Can extractor fans be tested for compliance after installation?

Yes — Approved Document F requires commissioning. For new builds and major refurbishments, the air flow rate must be measured and recorded. Use an anemometer or vane flow meter at the grille. Building Control sometimes requires the commissioning sheet.

Regulations & Standards

• Approved Document F (Volume 1: Dwellings, 2021) — Ventilation requirements

• Approved Document P — Electrical safety; bathroom electrical notifiable

• BS 7671 — Wiring regulations; Section 701 bathroom zones

• BS EN 13141 — Performance testing of components for residential ventilation

• BS 5925 — Code of practice for ventilation principles and designing for natural ventilation

• BS 8313 — Code of practice for installation of accessories of buildings

• PAS 2030 / 2035 — Retrofit ventilation considerations

• The Building Regulations 2010 (as amended) — Schedule 1, Part F

• Approved Document F (Volume 1: Dwellings) — Building Regulations

• Vent-Axia technical guidance — fan selection and ducting

• Greenwood Airvac — continuous extract products

• BEAMA Building Ventilation Guide — industry guidance

• BPF / BFRC Ventilation Guidance — building physics

• bathroom ventilation — broader ventilation context

• bathroom planning guide — planning ventilation alongside layout

• damp survey what to expect — diagnosing damp where ventilation is suspect

• bathroom zones — Zone compliance for fan installation

• wet room installation guide — wet room ventilation specifics