Extractor fans are a Building Regulations requirement in any new or renovated bathroom, shower room, or kitchen without adequate natural ventilation. Get the type or sizing wrong and you get condensation, mould, and a potential compliance issue. This guide covers the key fan types, the minimum extract rates set by Approved Document F, and what installation typically costs.
Why Ventilation Matters: Approved Document F
Part F of the Building Regulations (Approved Document F, 2021 edition, England) sets minimum ventilation rates for all habitable and wet rooms. For intermittent extract fans — the standard domestic solution — the minimums are:
| Room | Minimum extract rate |
|---|---|
| Bathroom (with or without WC) | 15 litres/second (54 m³/h) |
| Shower room | 15 litres/second |
| WC (separate) | 6 litres/second (22 m³/h) |
| Kitchen | 30 litres/second (108 m³/h) adjacent to hob; 13 l/s elsewhere |
| Utility room | 8 litres/second |
These rates apply to new builds, material changes of use, and extensions. For like-for-like replacements in an existing property you are not strictly obligated to upgrade, but it is good practice to meet or exceed these figures.
[!tip] Checking a fan’s airflow Fan performance is quoted at zero static pressure (free air). Once the fan is connected to ducting, real-world flow drops. Always add 15–25% to your target airflow when choosing a model to account for duct resistance — especially if the run is longer than 1.5 m or has bends.
Fan Types
Axial (Wall or Ceiling, Short Duct Run)
The most common domestic type. A motor with axial blades pushes air straight through a short spigot and out through an external grille. Best suited to external walls where the duct run is 1.5 m or less. Cheap and easy to install; performance drops noticeably with longer ducts or multiple bends.
Typical cost: £15–£50 for the unit; £80–£130 to install (assuming wiring already in place).
Centrifugal (Inline, Long Duct Runs)
An impeller spins air outward, generating higher static pressure. Better able to overcome the resistance of longer or more complex duct runs (up to 5–6 m with appropriate diameter). Usually installed in a ceiling void or roof space, with a decorative grille at the room end and an external termination at the other.
Typical cost: £40–£150 for the unit; £150–£300 to install (includes accessing ceiling void and running duct).
Humidity-Sensing Fans
An integrated humidity sensor triggers the fan automatically when moisture rises (typically set to activate above 70–80% RH) and runs it until the room drops to the baseline. This eliminates the need to rely on occupants switching the fan on and ensures continuous control during long showers. Ideal for family bathrooms.
Typical cost: £40–£100 for the unit.
Timer-Run-On Fans
Switch-activated but fitted with an overrun timer so the fan continues for a set period (typically 5–20 minutes) after the light is switched off. The fan wiring is linked to the lighting circuit. A lower-tech solution than humidity sensing but effective for predictable-use bathrooms.
Typical cost: £20–£60 for the unit.
Heat-Recovery Ventilation (MVHR / dMEV)
At the premium end, MVHR (Mechanical Ventilation with Heat Recovery) whole-house units recover up to 90% of the heat from extracted air. dMEV (decentralised mechanical extract ventilation) provides a continuous low-level extract from wet rooms. Both are much more expensive than a single extractor and are most cost-effective in new builds or major refurbishments with high levels of airtightness.
MVHR system cost (installed): £3,000–£6,000+ depending on house size.
Choosing the Right Fan: Sizing Calculation
To size a fan for a standard bathroom:
- Calculate room volume: width × length × height (in metres) = m³.
- Multiply by the recommended air changes per hour. Part F uses a fixed extract rate, but the traditional rule of thumb is 8–12 air changes per hour for a bathroom.
- The higher of the two figures governs.
Example: Bathroom 2.5 m × 2.0 m × 2.4 m = 12 m³. At 10 air changes/hour = 120 m³/h = 33 l/s. Part F minimum is 15 l/s. The air-changes calculation gives a more demanding figure here — use that, choosing a fan rated at least 33 l/s in free air (aim for 40+ l/s to allow for duct losses).
Ducting: Getting It Right
Poor ducting undermines even a well-chosen fan.
- Duct diameter: 100 mm is standard for bathrooms and WCs; 125 mm for kitchens and higher-flow applications.
- Duct length: keep runs as short and straight as possible. Each 90° bend is equivalent to roughly 1.5 m of straight duct for resistance purposes.
- Duct type: use rigid PVC or aluminium rigid duct where possible. Flexible corrugated duct has significantly higher resistance and should be limited to final connections.
- External termination: fit a proper external wall or soffit vent with a back-draught shutter, not just a louvred brick vent. The shutter prevents cold air and insects entering when the fan is off.
- Never duct into a roof void — this creates condensation and mould in the void and contravenes the regulations.
Bathroom Wiring Zones
Bathrooms are divided into IP-rated zones under BS 7671. Extractor fans must meet the IP (Ingress Protection) rating for the zone where they are installed:
| Zone | Location | Minimum IP rating |
|---|---|---|
| Zone 0 | Inside bath or shower | IP X7 |
| Zone 1 | Directly above bath/shower to 2.25 m | IP X4 (IPX5 if jets used) |
| Zone 2 | 0.6 m outside zones 0/1 horizontally | IP X4 |
| Outside zones | Rest of bathroom | IP X0 minimum |
Most bathroom ceiling fans are installed outside Zone 1 (i.e., not directly above the bath) and must carry at least IP44. Installation in Zone 1 requires IP44 minimum; most manufacturers recommend IP45 or better for overhead shower positions.
Installation Costs (2026, England)
| Scenario | Typical cost |
|---|---|
| Replace existing fan (same location, wiring in place) | £80–£130 |
| New fan installation with new switched wiring | £150–£250 |
| New fan with humidity sensor | £180–£280 |
| Inline centrifugal fan (ceiling void, with duct run) | £250–£400 |
| Full bathroom ventilation to Part F standard | £200–£350 |
Installation in bathrooms is notifiable work under Part P. Use a registered electrician, or notify your local authority before proceeding.
Common Mistakes
- Ducting into a loft — always terminate externally.
- Fan too small — cheap fans often deliver only 50–80 m³/h in free air, which falls to 30–40 m³/h once ducted. Check the spec sheet.
- No back-draught shutter — in winter, cold air reverses through an unshuttered duct and the room feels draughty.
- Fan on a separate switch — occupants forget to turn it on. Link to the lighting circuit with an overrun timer, or use humidity sensing.
- Flexible duct kinked in the ceiling void — a kinked flex duct can reduce airflow by 50% or more.
