Extractor Fans: Types, Bathroom Regs & Sizing

Building Regulations guidance calls for extract ventilation to the outside in the kitchen, utility room, bathroom and WC of a new home.¹ When you refurbish an existing kitchen or bathroom, any existing fan must be kept or replaced, but a room that never had one does not have to gain one.¹ 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.¹

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:

The 30 l/s kitchen rate applies only to a cooker hood that extracts to the outside; any other kitchen extract fan needs 60 l/s.¹

These rates apply to new homes and material changes of use, and also when you add a wet room or replace part of the ventilation system, including an extract fan, in an existing home.¹ If a replacement fan uses the existing cabling, the work does not need to be notified to building control.¹

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 most of the heat from the extracted air: makers quote figures of 90% or more.² 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): quotes commonly run from about £3,000 to £6,000 or more, depending on house size.

Choosing the Right Fan: Sizing Calculation

To size a fan for a standard bathroom:

1. Calculate room volume: width × length × height (in metres) = m³.
2. 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.
3. 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.¹ 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 adds noticeable resistance, so keep bends to a minimum.
• 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: moist air dumped there causes condensation and mould in the void, and Approved Document F calls for extract ventilation to the outside.¹

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:

Outside zones 0, 1 and 2 there is no specific water-resistance requirement, although IP44 is usually treated as the minimum for a bathroom appliance.³ A fan inside zone 1 must be a type the manufacturer confirms is designed for that location, or be supplied at safety extra-low voltage (SELV), and it needs a higher rating where it is likely to be exposed to water jets.³

Installation Costs (2026, England)

Fitting a fan in a bathroom is notifiable under Part P only where it needs a new circuit, or where it sits inside the special location, which runs from finished floor level to 2.25 m and 0.6 m out from the edge of the bath or shower tray, or 1.2 m from the shower head where there is no tray. Where it is notifiable, use a registered competent person who self-certifies the work, or notify your local authority before starting.⁴

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 cut airflow dramatically.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.