Bathroom Extractor Fans — 100mm vs 150mm, Timer, Humidistat, and Part F Building Regulations for UK Electricians

Every bathroom and shower room in a UK dwelling must have adequate ventilation under Part F of the Building Regulations. For electricians, fitting a correctly specified extractor fan is one of the most common domestic jobs — and selecting the right control type, size, and IP rating is essential to pass Building Control inspection and avoid callbacks.

This guide covers how to size an extractor fan for a bathroom or utility room, when to use a timer versus humidistat control, IP zone requirements, and how to wire a fan on a dedicated circuit or switched live.

Part F Building Regulations — What's Required

Approved Document F (Ventilation) sets minimum extract rates for habitable spaces. For bathrooms and shower rooms in new-build and renovation work, the requirements are:

• Bathrooms (with bath or shower): minimum 15 litres/second (l/s) intermittent extraction, or 8 l/s continuous
• Utility rooms and sanitary accommodation: minimum 6 l/s continuous or 30 l/s intermittent
• Kitchens: 30 l/s adjacent to the hob, or 60 l/s for recirculation cooker hoods
• Overrun requirement: fans must continue to run for a minimum 15 minutes after the triggering event (light switch off or humidity drop)

In practice, a 100mm (4 inch) fan rated at approximately 85–100 m³/hour meets Part F for most domestic bathrooms. A 150mm (6 inch) fan is required for larger bathrooms, utility rooms, or where duct runs are long and resistance is high.

100mm vs 150mm — Which Fan Do You Need?

The most common domestic bathroom fan is 100mm (4 inch). These are compact, quiet on modern models, and straightforward to retrofit into a spigot hole cut with a 100mm hole saw. Most 100mm fans deliver 85–110 m³/hour of airflow, comfortably exceeding the Part F 15 l/s (54 m³/hour) minimum.

A 150mm (6 inch) fan is typically specified where:

• The bathroom is larger than 8 m² (ensuite bathrooms, main family bathrooms in larger homes)
• The duct run exceeds 3 metres, or includes more than two 90° bends (each bend reduces effective airflow by approximately 15%)
• The room is a utility room with washing machine and tumble dryer — 30 l/s is required
• Commercial or landlord HMO properties where Building Control requires higher extract rates

When in doubt, specify a 150mm fan for utility rooms and calculate duct resistance for long runs. Under-specifying results in condensation, mould, and a failed inspection.

Control Types: Standard, Timer, Humidistat, and Pullcord

The control type is the most important selection decision for a bathroom fan. Part F requires a minimum 15-minute overrun, which means the fan must keep running after the light turns off. The three main options are:

Standard (Switched Live)

The simplest installation — the fan runs only while the light switch is on. This does not meet Part F overrun requirements as a standalone installation, but is acceptable where a separate overrun timer is incorporated into the switch circuit, or where the fan is used in a WC without a shower. Standard fans are also used in commercial extract systems where continuous running is controlled separately.

Timer (Overrun)

A timer fan contains an internal overrun relay that keeps the fan running for an adjustable period (typically 5–30 minutes) after the switched-live drops. This is the most common installation type for domestic bathrooms and fully satisfies Part F. Timer fans require a permanent live (brown), switched live (typically grey or brown-sleeved), and neutral (blue). Some models accept a single switched live input and derive the overrun from an internal capacitor — check the wiring diagram before installation.

Humidistat

A humidistat fan activates automatically when relative humidity rises above a set threshold (typically 70–80% RH, adjustable via internal pot). It runs until humidity returns to baseline, without needing a switched-live input at all — the fan can be wired to permanent live and neutral only. This is the preferred option for:

• Bathrooms where the light switch is outside the bathroom (cord-pull switch zone)
• En-suite bathrooms where occupants may shower with the light off
• Landlord installations where overrun compliance needs to be trigger-independent

Combination timer + humidistat models cover both scenarios — the fan runs either on humidity trigger or overrun timer, whichever is applicable.

Pullcord

A pullcord fan incorporates a mechanical on/off pull switch directly on the fan body. This is required in bathroom Zone 1 (directly above the bath or shower) where a standard rocker switch is not permitted. Pullcord models are also used in older properties without a convenient switched-live feed to the fan position. They do not inherently provide overrun unless the unit includes a timer mechanism.

IP Ratings and Bathroom Zones

BS 7671 and IET Guidance Note 7 divide bathrooms into zones based on proximity to water:

• Zone 0: Inside the bath or shower tray — IP67 minimum. No fans installed here.
• Zone 1: Above the bath or shower to 2.25 m height — IP44 minimum. Wall-mounted fans above shower heads fall in this zone.
• Zone 2: 0.6 m horizontally from the bath rim or shower enclosure edge — IP44 minimum.
• Outside zones: No specific IP requirement from wiring regulations, but a general purpose IP20 fan is acceptable only if located well clear of the shower area.

In practice, all reputable bathroom fans are rated IP44 or above — this is the de facto minimum for any bathroom installation. Some ceiling-mounted fans above shower enclosures carry IPX5 (protected against water jets) for added margin.

12V fans (powered via a transformer/controller unit) are acceptable in Zone 1 installations where a SELV (Separated Extra Low Voltage) supply is used. The transformer must be located outside all zones.

Wiring a Bathroom Extractor Fan

Extractor fan installation in a bathroom is notifiable work under Part P of the Building Regulations when it involves new wiring or connection to the electrical installation. The work must be carried out by a competent person registered with a scheme (NICEIC, NAPIT, ELECSA) or notified to Building Control.

Dedicated Circuit

The preferred method for a new installation is a dedicated circuit from the consumer unit via a 3A fused spur or FCU (fused connection unit). A 1.0mm² or 1.5mm² twin-and-earth cable is run from the FCU to the fan. The FCU provides isolation for maintenance and limits the current draw if the fan motor develops a fault. The FCU switch also acts as the permanent isolation point required by BS 7671 Regulation 462.1.

Switched Live from Lighting Circuit

A common retrofit method — a 1.0mm² T&E or three-core cable is run from the lighting switch back-box or ceiling rose to the fan. The permanent live and neutral come from the lighting circuit; the switched live comes from the light switch. This saves running a new circuit but requires the fan to share the lighting circuit protective device. Ensure the MCB rating and cable CSA are compatible.

Three-Core Cable for Timer Fans

A timer fan with separate permanent and switched live terminals requires three-core and earth cable (brown/black/grey plus earth). The black and grey cores should be sleeved with brown to identify them as line conductors per BS 7671 Table 51. Always check the fan's wiring diagram — connections vary by manufacturer and model.

Product Selection Guide

Installation Tips

Duct Routing

Keep duct runs as short and straight as possible. Each 90° bend reduces effective airflow by approximately 15%; a 1.5 m duct equivalent is added for each bend. Insulate duct runs through cold roof spaces with 25 mm duct insulation to prevent condensation forming inside the duct and dripping back into the fan. Always terminate the duct at an external wall vent or roof cowl — never into a loft space or wall cavity.

Isolation

Install a fused connection unit (FCU) with a 3A fuse on the supply to the fan. The FCU provides local isolation for maintenance and protects the fan motor from overcurrent. Locate the FCU outside the bathroom zones (Zone 0, 1, 2) — typically on the landing side of the wall, in the airing cupboard, or within the consumer unit circuit itself.

Testing After Installation

After wiring, test the fan operation in each mode: timer overrun (switch off the light and confirm the fan runs for the set period), and humidistat trigger (breathe on the sensor and confirm activation). Document the installation on an Electrical Installation Certificate or Minor Works Certificate as required by BS 7671 and Part P.

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