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Bathroom Extractor Fans — 4 Inch, Timer and Humidistat Guide for UK Electricians
Bathroom extractor fans are a mandatory installation under Part F of the Building Regulations for rooms without openable windows, and a strongly recommended upgrade in rooms that do have windows but suffer from persistent condensation. For UK electricians and contractors, choosing the right fan means understanding control options — standard, timer, humidistat, and motion sensor — as well as compliance requirements and brand compatibility with your existing preferred suppliers.
APM Electricals stocks 4 inch (100mm) and 6 inch (150mm) extractor fans from Manrose, DETA, Airflow, CED, Knightsbridge and Airmaster, covering every specification from a basic pull-cord replacement to a LED backlit humidistat unit. This guide covers the key decision points.
Part F Compliance — What You Need to Know
Building Regulations Part F (Ventilation) requires mechanical extract ventilation in bathrooms and shower rooms. The minimum extract rate for a bathroom is 15 litres per second (54 m³/h). For utility rooms and WCs, rates vary — check the full Part F document or the manufacturer's data sheet for confirmation that the fan meets the required flow rate for the specific room type.
For new builds and notifiable work, the fan installation may need to be air pressure tested. Most 4 inch (100mm) fans from the brands in this guide are rated at 85–100 m³/h free air, well above the Part F minimum. Where a humidistat model is used, the fan typically continues running until humidity drops to a set threshold, offering better performance than a timer in high-usage bathrooms.
Control Types — Standard, Timer, Humidistat and Motion Sensor
Standard (On/Off)
Wired to the light circuit or a separate switch. The fan runs while the switch is on and stops when it is switched off. Suitable for replacement of existing standard fans and for loft conversions or extensions where budget is the primary constraint. No overrun — the fan cuts immediately when the switch is off.
Timer
The fan continues to run for a set period (typically 2–25 minutes, adjustable) after the switch is turned off. This is the most common upgrade on a standard bathroom fan and the default choice for most new installations. Ensures the fan removes residual moisture after showering or bathing rather than stopping the moment the occupant leaves.
Humidistat
The fan monitors relative humidity and activates automatically when it exceeds a set threshold (typically 60–80% RH, adjustable). Runs until humidity returns to the lower set point. Can be combined with a timer. This is the preferred option for en-suites and bathrooms where the fan needs to operate independently of occupancy — for example, when a shower has been left running or in a holiday let where guests may forget to use the switch.
Motion Sensor
Activates when movement is detected in the room and includes an adjustable overrun timer. Useful in commercial applications, cloakrooms, and accessible bathrooms where a wall switch may not be practical. Typically higher cost than timer-only models.
4 Inch vs 6 Inch — Which Size for the Job?
The vast majority of domestic bathroom installations use a 4 inch (100mm) fan. This is the standard aperture in UK bathrooms, compatible with 100mm flexible ducting and standard 100mm wall/ceiling cores. A 4 inch fan delivers 85–100 m³/h, more than sufficient for any domestic bathroom.
6 inch (150mm) fans are used in larger bathrooms, combined kitchen-diners, utility rooms, and commercial WC facilities where a higher extract rate is required. They use 150mm ducting and a larger core. APM stocks the DETA 6" 150mm Extractor Fan Timer (£36.92) for these applications.
Featured Products
Manrose 4" 100mm Extractor Fan Standard
Price: £15.99 inc VAT
A straightforward on/off 4 inch extractor fan from Manrose — one of the UK's most trusted ventilation brands. Wires to the lighting circuit or a dedicated switch. Rated at 85 m³/h, it exceeds the Part F minimum for bathroom extract. White fascia, 100mm spigot. The go-to choice for simple like-for-like replacements and budget-conscious installs where a timer is not required.
- 4" (100mm) aperture — standard UK fitting
- On/off control — wires to light or switch
- 85 m³/h extract rate — Part F compliant
- White fascia panel
- Suitable for bathroom, en-suite, WC
DETA 4" Extractor Fan Timer
Price: £22.99 inc VAT
The DETA 4 inch timer fan is the default upgrade over a standard on/off model. The adjustable overrun timer (2–25 minutes) ensures the fan continues extracting moisture after the switch is turned off, significantly reducing condensation and the risk of mould in bathrooms and shower rooms. DETA is a reliable mid-market brand stocked widely by UK electrical wholesalers. Compatible with standard 100mm ducting and grilles.
- 4" (100mm) aperture
- Adjustable timer overrun (2–25 min)
- Wires to lighting circuit or dedicated switch
- White fascia
- Suitable for all domestic bathroom applications
DETA 4" Extractor Fan Timer & Humidistat
Price: £36.99 inc VAT
DETA's combined timer and humidistat model is the recommended choice for en-suites, holiday lets, and any bathroom where the fan needs to operate reliably regardless of whether occupants remember to use the switch. The humidistat activates the fan automatically when humidity rises above the set threshold (adjustable) and runs until conditions return to normal. The timer overrun adds a further period of extract after the switch is turned off. Dual control provides maximum moisture management.
- 4" (100mm) aperture
- Humidistat with adjustable trigger threshold
- Timer overrun adjustable
- Can be wired to activate independently of the light switch
- White fascia
- Ideal for high-moisture-risk bathrooms and en-suites
Knightsbridge 4 Inch 100mm LED Backlit Extractor Fan — Black
Price: £49.99 inc VAT
A premium 4 inch extractor fan with a backlit LED fascia in a matt black finish — the specification choice for modern bathroom refurbishments where the fan needs to complement designer sanitaryware, black accessories and dark-scheme bathrooms. Knightsbridge's LED backlit model combines contemporary aesthetics with full functionality: timer, adjustable speed, and a low-profile fascia. Stocked at APM for electricians fitting out high-spec residential bathrooms.
- 4" (100mm) aperture
- LED backlit fascia — matt black finish
- Timer overrun included
- Suitable for modern/designer bathroom schemes
- Compatible with standard 100mm ducting
View product — Knightsbridge 4" LED Backlit Extractor Fan Black
Wiring Notes for Electricians
Zone Ratings (IP Rating)
In a bathroom, extractor fans are typically installed in Zone 1 (directly above the bath or shower, up to 2.25m from the floor) or Zone 2 (0.6m beyond Zone 1). IP44 is the minimum IP rating required for Zone 1 and Zone 2. Most 4 inch bathroom fans are IP44 or above — always verify the data sheet before installation in wet zones.
Timer Fans — Wiring Configuration
A timer fan requires two live feeds at the fan: a permanent live (for the timer circuit) and a switched live (from the lighting circuit or dedicated switch). If only a switched live is available, the overrun timer will not function correctly. Always check the wiring diagram supplied with the fan before installation — wiring configurations vary between manufacturers.
Humidistat Fans — Separate Circuit Recommendation
For humidistat models that need to activate independently of the light switch, wire the fan to a permanent live with the humidistat controlling the operation. This allows the fan to run when the bathroom is unoccupied if humidity remains elevated. A separate fused spur (3A) from the consumer unit is the cleanest installation method.
Ducting
All 4 inch fans in this guide use 100mm ducting. Use rigid UPVC or semi-rigid aluminium ducting where possible for best performance and minimum restriction. Flexible ducting should be kept as short and straight as possible — excessive bends and length will reduce the effective extract rate. Ensure the external termination point uses a backdraft shutter (gravity flap or motorised) to prevent cold air ingress when the fan is off.
Frequently Asked Questions
Do I need an extractor fan if the bathroom has a window?
Under Part F, a bathroom with an openable window does not require mechanical ventilation as a compliance requirement. However, in practice, windows are rarely opened in winter, and persistent condensation in bathrooms with windows is very common. An extractor fan with a humidistat is strongly recommended in any bathroom used for showering, regardless of window provision.
Can I replace an existing extractor fan myself?
Like-for-like replacement of a bathroom extractor fan (same position, same ducting, same wiring) is Part P notifiable work in England and Wales. This means it must be carried out by a registered competent person (e.g. NICEIC, NAPIT, Elecsa registered) or notified to Building Control. A qualified electrician can self-certify under Part P.
What is the difference between an inline fan and a wall fan?
An inline fan is mounted within the duct run (typically in a loft or ceiling void) rather than at the point of extract. They are quieter in the bathroom as the motor noise is remote from the extract grille. Wall-mounted and ceiling-mounted fans have the motor at the extract point. For maximum noise sensitivity, consider a Manrose or Airflow Aura model rated for quiet operation.
How long should a timer be set to run after the switch is turned off?
15–20 minutes is the standard recommendation for a bathroom used for showering. For a WC, 5–10 minutes is typically sufficient. Adjust the timer pot (accessible after removing the fan fascia on most models) to suit the occupancy pattern of the installation.
Order Extractor Fans from APM Electricals
APM Electricals supplies 4 inch and 6 inch bathroom extractor fans from Manrose, DETA, Airflow, Airmaster, CED, Knightsbridge and more. All prices include VAT. Next-day delivery available on stocked items.
View the full extractor fan range at apmi.uk.
APM Electricals
Unit 3, The Old Vinyl Factory, Blyth Road, Hayes, Middlesex, UB3 1DD
Phone: 020 8574 0077
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