Bathroom Extractor Fans: Part F Regulations, Fan Types, and Installation Guide

Bathroom Extractor Fans: Part F Regulations, Fan Types, and Installation

Bathroom and kitchen ventilation is a legal requirement under Building Regulations Part F (Ventilation), not an optional upgrade. Inadequate ventilation leads to condensation, mould, and structural damp — all of which cause ongoing damage and affect the health of occupants. Yet a significant proportion of UK bathrooms are under-ventilated: fans that are too small, incorrectly positioned, ducted into the loft rather than outside, or wired without the overrun timer the regulations require.

This guide covers what Part F requires, the types of extractor fan available, how to size them, wiring options, and the installation details that installers frequently get wrong.

Part F Requirements for Bathrooms and WCs

Approved Document F (2022 edition, in force from June 2022 for new builds and major refurbishments) sets minimum ventilation rates for wet rooms. For bathrooms, shower rooms, and WCs, intermittent extract ventilation must provide:

• Bathroom (with bath or shower): Minimum 15 litres per second (l/s) extract rate
• WC (toilet only, no bath/shower): Minimum 6 l/s extract rate
• Utility room: Minimum 30 l/s (or 8 l/s background ventilation)

These rates apply at the fan's rated airflow, which is measured in l/s or m³/h (divide m³/h by 3.6 to convert to l/s). A fan rated at 100 m³/h delivers approximately 27.8 l/s — well above the 15 l/s minimum for a bathroom.

Additionally, the fan must run for a minimum overrun period after the trigger (light switch or humidity sensor activation) has been removed. Part F does not mandate a specific overrun duration, but CIBSE guidance and standard industry practice is a minimum of 15 minutes. Most wiring regulations-compliant fan installations achieve this with an overrun timer built into the fan or wired into the switched live.

Fan Types

Axial (Propeller) Fans

The most common domestic bathroom fan. The motor drives a propeller impeller directly — air is moved axially (along the shaft axis). Axial fans are compact, inexpensive, and adequate for short, straight duct runs. Their limitation is static pressure: at high duct resistance (long ducts, multiple bends, restrictions), airflow drops significantly. Maximum recommended duct run for a standard axial fan is approximately 3–4 m with one or two 90° bends.

Leading brands: Xpelair, Vent-Axia, Manrose, Greenwood Airvac. Entry-level rated at 85–100 m³/h. High-performance models (e.g. Vent-Axia Svara, Xpelair GX9) reach 115–150 m³/h and handle slightly longer runs.

Centrifugal (Inline) Fans

Centrifugal fans use a scroll housing and radial impeller to develop higher static pressure than axial fans, maintaining airflow over longer or more complex duct runs. An inline centrifugal fan is installed within the duct run (in the loft or ceiling void) rather than at the wall or ceiling aperture. They are the correct choice for runs longer than 4 m, multiple bends, or where the duct terminates at a remote location.

The fan body can be positioned anywhere in the duct run. The grille at the bathroom ceiling is purely decorative — the actual fan unit is hidden in the ceiling void or loft. Inline fans are also quieter at the grille, since the motor is remote from the occupied space.

Low-Profile (Slimline) Fans

For installations where there is minimal ceiling void (e.g. concrete slab or close-joisted floor above), low-profile axial fans are available with a depth of 90–120 mm (vs 200+ mm for standard units). They sacrifice some static pressure capability but maintain adequate airflow for short runs.

Humidity-Controlled Fans

A humidity-sensing fan activates automatically when relative humidity rises (typically above 65–70% RH) and switches off when humidity returns to the set threshold. This eliminates the need to wire the fan to the light switch and provides more responsive ventilation — the fan runs whenever the room is damp, regardless of whether the light is on. Humidity-controlled fans are increasingly specified in new builds and refurbishments where a clean ceiling installation (no pull cord, no switch connection) is preferred.

PIR and Timer-Only Fans

Some fans incorporate a passive infrared motion sensor, activating on occupancy and running for an adjustable overrun after the space is vacated. Others have a manual run-on timer only (no humidity or PIR sensing). Both types are available as recessed ceiling units with a flush grille for a clean finish.

Sizing: Why 100 m³/h Is Not Always Enough

Most domestic axial fans are rated at 85–100 m³/h in free air — meaning zero duct resistance. In a real installation with duct length, bends, and a terminal grille, the actual delivered airflow will be lower. The fan's performance curve (pressure vs flow) determines how much the airflow drops at a given system resistance.

As a practical rule:

• Duct run up to 2 m, one bend: 85–100 m³/h axial fan adequate
• Duct run 2–4 m, two or three bends: 100–150 m³/h axial fan, or upgrade to inline centrifugal
• Duct run over 4 m or four or more bends: Inline centrifugal fan minimum; calculate system resistance and select fan accordingly

For a 150 m³/h fan delivering air at 15 l/s minimum, the room air change rate in a typical 4 m² bathroom with a 2.4 m ceiling (9.6 m³) is approximately 150/3.6 = 41.7 m³/h ÷ 9.6 m³ = 4.3 air changes per hour — well above the Part F requirement.

Wiring Options

Switched Live (Standard)

The fan is connected to the bathroom light switch via a switched live. When the light is on, the fan runs; when the light is switched off, an internal timer delays fan shutdown for the overrun period. This is the most common domestic wiring arrangement and the simplest to install.

In bathrooms (Zone 2 or outside zones), the fan must be either IP44-rated (for ceiling installations outside the defined zones) or the appropriate IP rating for its zone position. Fans mounted in the ceiling of a bathroom are typically outside Zone 1 and 2, so IP44 is the minimum. Fans must not have a local on/off switch accessible from within the bath or shower zone.

Permanent Live with Separate Switched Live for Overrun

Some fans require two feeds: a permanent live to maintain the overrun timer's power, and a switched live from the light switch that triggers the fan. This allows the fan to complete its overrun period even after the permanent live is interrupted (i.e., after the light switch is turned off at the consumer unit). Not all fans use this; check the wiring diagram in the manufacturer's installation guide.

Humidity Control (No Switch Connection)

Humidity-sensing fans are connected to a permanent live and neutral only — no switched live required. The fan's internal humidity sensor controls operation. This is the simplest wiring arrangement for ceiling-mounted fans in rooms where a wiring connection to the light switch is difficult.

Separate Fused Spur

The fan circuit can be taken from a 3 A fused spur rather than from the lighting circuit. This is common where the bathroom lighting circuit cannot easily be accessed, or where a standalone fan circuit is preferred for circuit isolation purposes.

Ducting: The Most Common Installation Error

The duct from a bathroom fan must terminate outside the building — not into the loft, not into the ceiling void, not into a soffit. Terminating into the loft is the most widespread installation error and one of the most damaging: the warm, moist air condenses against the cold roof structure, soaking insulation, causing timber rot, and potentially growing mould on the underside of the roof deck.

Correct termination options:

• Through the external wall: A core drill through the external wall with a wall vent terminal (gravity-flap type to prevent backdraught). The duct must be as short and straight as possible.
• Through the roof: A proprietary roof tile vent or lead slate terminal for pitched roofs. For flat roofs, a mushroom vent. The duct runs through the ceiling void and exits through the roof covering.
• Via the soffit: Acceptable if the soffit vent is directly connected to the duct (not just open to the eaves cavity). The duct must terminate at a through-soffit grille, not simply discharge into the eaves cavity where it will saturate insulation.

Use rigid circular PVC or aluminium duct in preference to flexible corrugated duct where possible. Flexible duct is convenient for making connections, but it has significantly higher resistance per metre (due to corrugations) and tends to sag and trap condensate if not correctly supported. If flexible duct is used, keep runs short (under 1 m), ensure it is fully extended (not compressed), and pitch it slightly toward the external terminal to drain condensate.

Insulate duct runs that pass through unheated spaces (lofts, cold roof voids). Uninsulated duct running through a cold loft will condense moisture inside the duct, which drips back into the fan and ceiling.

IP Ratings for Bathroom Zones

BS 7671 and IEC 60364-7-701 divide the bathroom into zones:

• Zone 0: Inside the bath or shower tray. IP67 minimum. No fans.
• Zone 1: Above the bath/shower to 2.25 m height. IP44 minimum. Fans not normally sited here.
• Zone 2: 0.6 m horizontally from Zone 1, up to 2.25 m height. IP44 minimum.
• Outside zones: Remainder of the bathroom. IP44 recommended as best practice (mandatory if there is a risk of water jets).

All ceiling-mounted bathroom extractor fans should be rated at least IP44. Most reputable fans sold for bathroom use meet this standard. Verify the IP rating before specifying — some cheap imports do not meet IP44 despite being marketed for bathrooms.

Installation Steps

1. Isolate the lighting circuit at the consumer unit and verify dead with a voltage tester
2. Mark and cut the ceiling aperture to the fan manufacturer's template — typically 100 mm, 125 mm, or 150 mm diameter
3. Route ducting from the ceiling aperture to the external termination point; cut wall or roof penetration
4. Fit wall vent terminal or roof vent tile at the external termination
5. Connect duct to fan body; secure with duct tape or a clamp at every joint
6. Wire fan per the wiring diagram (switched live from light switch, permanent live and neutral from lighting circuit or fused spur)
7. Fit fan body into ceiling aperture; most fans clip or screw to a mounting plate
8. Restore power and test: confirm fan runs with light, confirm overrun timer operates after light off, confirm airflow at the external terminal
9. Set overrun timer and humidity threshold (where adjustable) per manufacturer's instructions

Common Faults and Remedies

Summary

A correctly installed bathroom extractor fan is straightforward — the right fan size for the duct run, terminated outside the building (not the loft), wired with an overrun timer, and rated IP44 or above. The most expensive fan installed with a duct that terminates in the loft void will cause damp and structural damage within a few years. Get the duct termination right first, then specify a fan with adequate static pressure for the actual run length, and the installation will perform reliably for the life of the property.

At APM Plumbing & Electrical we stock bathroom extractor fans and ventilation ducting for UK trade professionals.

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APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.