How to Choose a Bathroom Extractor Fan: Timer, Humidistat and IP Ratings Explained

Extractor fans are on almost every domestic job — bathroom, en-suite, shower room, WC. But there's more to specifying them than grabbing the cheapest 4" unit on the shelf. Timer settings, humidistat control, voltage, IP ratings and run-on times all affect whether the fan actually does its job. This guide covers what to look for and when to use each type.

Why Ventilation Matters in Bathrooms

Approved Document F of the Building Regulations sets minimum ventilation rates for bathrooms and WCs. For intermittent extract ventilation in a bathroom, you need a minimum of 15 litres/second (54 m³/hour). A WC requires 6 litres/second. Most standard 100mm (4") fans comfortably exceed these figures — the typical 100mm unit delivers around 85–100 m³/hour.

But delivering the right air volume is only part of the spec. If the fan doesn't run long enough after use, or doesn't respond to humidity, moisture stays in the room. That's how you get mould, condensation, and damp that leads to callback complaints six months later.

Types of Bathroom Extractor Fan

Basic On/Off (Switched with Light)

The simplest option — fan runs when the light is on, stops when it's off. Meets building regs in theory but in practice most people flick the light off before the moisture has cleared. Fine for WCs on short visits, not ideal for showers or bathrooms with high humidity.

Timer Fan

The standard residential spec. The fan runs while the light is on and for a set period (typically 2–20 minutes, adjustable) after the light is switched off. That run-on period is what clears residual steam after a shower. Most timer fans have a small potentiometer on the back to adjust the overrun time — set it to match the room size and typical shower duration.

Timer fans are the minimum sensible specification for a bathroom with a shower. They're straightforward to install (standard 2-wire lighting circuit, no need for a separate switched live in most cases), and the overrun relay is built in.

Humidistat Fan

A humidistat fan monitors relative humidity in the room and switches on automatically when it rises above a set threshold (typically 65–80% RH, adjustable). It runs until humidity drops back to the set point.

This is the right choice for bathrooms used frequently or where the occupant might not switch the light on at all — overnight toilet trips, for example. It's also the better spec for shower rooms where condensation is the primary concern.

Humidistat fans can be wired in several ways — to the permanent live (so they're always available to respond to humidity), or to the switched live (so they only activate when the light is on). Permanent live is more effective for overnight use.

Timer + Humidistat

Combines both functions — the fan overruns on timer after the light goes off AND activates on humidistat independently. Best for high-use family bathrooms. Slightly more complex to wire but gives the most complete moisture control.

IP Ratings: What You Need Where

Bathrooms are divided into zones under BS 7671:

• Zone 0 — inside the bath or shower tray. No fans here.
• Zone 1 — above the bath/shower to 2.25m. IP45 minimum.
• Zone 2 — 600mm either side of the bath/shower edge. IP44 minimum.
• Outside zones — standard IP20 acceptable, but IP44 is good practice throughout the bathroom.

Most standard bathroom extractors are rated IP44 or IP45 — check the datasheet before fitting. Fitting an IP20 unit inside a zone 2 is a non-compliant installation.

100mm vs 150mm: Which Size?

100mm (4") is the standard for domestic bathrooms and WCs. 150mm (6") is used for larger rooms, kitchen extract, or where you need higher airflow — typically commercial or large open-plan bathrooms. For a standard domestic bathroom, 100mm is correct.

Check the spigot size before ordering ducting — a 100mm fan needs 100mm flexible or rigid duct to the outside. Using undersized duct creates back-pressure and reduces the effective airflow significantly.

12V vs 240V

Standard bathroom fans run on 240V mains. 12V fans exist for specific applications — typically in shower cubicles close to the water zone where extra safety is required, running from a separate safety isolating transformer. If you're specifying a 12V unit, the transformer needs to be outside the bathroom zones.

Products We Stock

• 4" Decorative Extractor Fan with Timer — White — standard timer fan for domestic bathrooms, adjustable overrun, clean white finish
• 4" Extractor Fan with Timer and Humidistat — Matt Black — combined timer/humidistat for high-use bathrooms, matt black for modern installs
• Airflow Aura-Ecoair 100mm Timer Fan — Airflow brand, energy-efficient EC motor, timer with adjustable overrun
• DETA 4" Shower Fan Kit with Timer — complete kit including fan and grille, timer controlled, suitable for shower rooms

Browse the full Extractor Fans collection for the complete range. Same-day collection from our trade counter — 24 Western Avenue, Acton W3 7TZ. Call 020 8702 8080 to check stock.

Quick Installation Notes

• Switched live needed for timer fans — most timer fans need a switched live from the lighting circuit to trigger the overrun relay. Check the wiring diagram before you start.
• Keep duct runs short and straight — every bend and metre of duct reduces airflow. Use rigid duct where possible; flexible duct should be fully extended, not bunched up in the void.
• Terminate outside — duct must discharge outside the building, not into a roof void or wall cavity. Use a gravity flap or louvred grille to prevent backdraught.
• Notifiable work — installing or replacing a fan in a bathroom is notifiable work under Part P if it involves new wiring. Either self-certify (if competent person scheme registered) or notify building control.