Bathroom Extractor Fan Wiring: Timer, Humidity Sensor, Overrun & Zone Compliance

Quick Answer: A bathroom extractor fan must be connected to a switched supply via an overrun timer (15–minute minimum overrun per Approved Document F). In Zone 1 (directly above the bath or shower), the fan motor must be IPX4 minimum. Fans in Zone 2 (within 600mm horizontally of Zone 1) must be IPX4 minimum. Never switch-plate a fan directly — use a FCU (fused connection unit) with a 3A fuse or wire to the light supply via a timer module. Part P notifiable if new circuit is installed.

Summary

Bathroom extractor fans are mandatory in bathrooms with no openable window (Approved Document F) and strongly recommended in all bathrooms to control moisture and prevent mould. Getting the wiring right requires understanding three things: the zone rules (IP rating requirements by position), the control method (overrun timer vs humidity sensor vs PIR), and whether the supply is new (notifiable) or a modification to an existing circuit.

The most common wiring mistake is connecting the fan directly to the light switch without any overrun. This means the fan stops the moment the light is switched off — when moisture is still present in the room — providing almost no effective ventilation. The minimum compliant approach per Approved Document F is an overrun timer that keeps the fan running for a set period after the light is switched off.

Key Facts

• Approved Document F requirement — Intermittent extract: 15 l/s (litres per second) for bath/shower rooms. Continuous extract: 8 l/s. Minimum overrun function required
• Zone 0 — Inside bath/shower enclosure. No standard fans permitted (SELV 12V specialist products only)
• Zone 1 — Above the bath to 2.25m or above the shower enclosure to 2.25m. IPX4 minimum. Most recessed ceiling fans are suitable
• Zone 2 — Area extending 600mm horizontally from Zone 1 boundary, and from 0–2.25m height. IPX4 minimum
• Outside zones — Standard IP-rated products. Minimum IP20 for light fittings, no specific IP for fans but good practice is IPX4 in bathrooms generally
• Control methods — Timer (runs for preset period after switch-off), humidity sensor (triggers on humidity rise and stops when RH falls), PIR-linked, occupancy-linked, or speed-controlled continuous
• Overrun timer setting — Minimum 15 minutes recommended by Approved Document F. Many fans have adjustable timers (5–30 minutes)
• Supply options — Spur from lighting circuit (most common, 5A fused connection unit), new radial circuit, or unswitched spur for always-live humidity sensor fans
• Isolator — For service access, a local isolator (FCU or pull-cord isolator) is desirable. A standard switch is not an adequate isolator
• Ducting — Not electrical but critical: minimum 100mm diameter flexible or rigid duct. Maximum run approximately 3m flexible (more with rigid). Terminate outside with external grille (not into loft space)

Quick Reference Table

Detailed Guidance

Understanding the Supply Options

From the lighting circuit (most common): The fan is connected to the lighting circuit. This requires access to the lighting circuit cable — typically at the ceiling rose, junction box, or in the loft. The fan needs:

• Switched live (live only when light is on) — for the trigger signal
• Permanent live — for the overrun timer to function when the light is off
• Neutral
• Earth

In older loop-in wiring, three-plate ceiling roses, or junction boxes, identify which conductor is switched live (goes only to light fitting) and which is permanent live (from supply). A multimeter confirms this.

If the lighting circuit only has the older two-wire (live + neutral) at the switch position, a separate permanent live must be run or a different control method used.

New unswitched spur (for humidity sensors): A humidity-sensor fan does not need a switched live — it operates entirely from the permanent live supply, activating when bathroom humidity rises. Supply is a simple two-core and earth unswitched spur from a convenient point. Fuse at 3A if from a ring final (via FCU) or use 6A on a dedicated circuit.

Fused connection unit (FCU): A 3A FCU (switched or unswitched depending on control method) provides the fan supply. FCU protects the fan from the higher current of the lighting or ring final circuit. For a basic timer fan: switched FCU (switched live activates the fan, permanent live feeds the timer).

Wiring a Timer Fan

A timer fan (e.g., Manrose, Greenwood, Xpelair standard range) has terminals for:

• L (permanent live)
• SL (switched live — from the light switch)
• N (neutral)
• E (earth)

Connect:

• Permanent live from supply to L
• Switched live (live only when light switch is on) to SL
• Neutral from supply to N
• Earth to E

The timer module inside the fan detects when the switch live disappears (light switched off) and runs the fan for the preset overrun period.

Adjusting the overrun timer: Most fans have a small potentiometer (screwdriver-adjustable dial) inside the fan housing, preset to 2–5 minutes from the factory. Set to minimum 15 minutes to comply with Approved Document F. Do not set longer than 30 minutes — this frustrates occupants.

Wiring a Humidity Sensor Fan

A humidity fan (e.g., Manrose CHRONO range, Xpelair WX range) monitors relative humidity (RH). When RH rises above the trigger threshold (typically 75–85% RH, adjustable), the fan activates. When RH drops back below the lower threshold, the fan stops.

Humidity fans require permanent live and neutral only — no switched live needed.

Terminal connections:

• L (permanent live from supply)
• N (neutral)
• E (earth)

Set trigger humidity and stop humidity using adjustable dials inside the fan. Many humidity fans also include an overrun timer and a boost function when a light switch is connected.

Zone Compliance

Mount fans in Zone 1 or Zone 2 only if they meet IPX4. Most reputable brands (Manrose, Xpelair, Greenwood, Airflow) state the IP rating on the product. IPX4 means splash-proof (water from any direction). Most ceiling-mounted bathroom fans are IPX4 as standard.

Zone 0 (inside the shower enclosure) must not be used for standard fan motors. A SELV (12V) extractor or a remotely located motor with a Zone 0-rated air inlet is required for in-enclosure extraction.

Ducting

Electrical installation without correct ducting is ineffective:

• 100mm minimum diameter duct. Never use 75mm — it restricts airflow and voids any compliance with Approved Document F
• Terminate outside the building with a wall grille or through-tile vent. Never terminate into the loft space — this causes condensation and damp in the loft
• Rigid duct is more efficient than flexible. Use rigid UPVC or galvanised steel for long runs
• Insulate duct in cold areas (loft) to prevent condensation in the duct causing dripping
• Shortest, most direct route possible. Each 1m of flexible duct and each 90° bend reduces airflow

Part P Notification

• New circuit from consumer unit to power the fan: Part P notifiable
• New spur from existing ring final outside the bathroom: Not notifiable (minor addition to existing circuit)
• Replacement of existing fan (same mounting position, same wiring): Not notifiable
• New installation of fan in a special location (bathroom) where a new circuit is not required: Technically notifiable as it's in a special location (bathroom), but in practice this is rarely enforced for like-for-like additions. Use a registered electrician to self-certify to be safe

Frequently Asked Questions

Can the fan be wired to a pull-cord switch inside the bathroom?

Yes. A bathroom-rated pull-cord switch (Zone 2 or outside zone depending on position) can be used as the control for a fan. Wire the fan to the pull-cord switch using a switched live and permanent live arrangement as above. In cloakrooms with no natural light, pull-cord fans are common.

My humidity fan runs constantly — how do I stop it?

The humidity sensor threshold is set too low, or the bathroom is genuinely very humid (poor ventilation in adjacent areas). Check the trigger RH setting inside the fan (typically an adjustable potentiometer). Set the trigger RH higher (e.g., from 75% to 85%). If the fan still runs continuously, the bathroom may have a structural moisture problem — investigate.

Can I put two bathrooms on one fan?

No — each bathroom requires its own dedicated fan and duct run. Sharing one fan with dual duct runs creates back-draught problems: when one duct is at higher pressure (wind-loaded external grille), the backdraft blows into the other bathroom. Use twin-duct fans with non-return valves only in consultation with the fan manufacturer's specification.

Do I need a local isolator for the fan?

There is no absolute regulatory requirement for a local isolator, but it is good practice (and will be required for future maintenance access without switching off the whole lighting circuit). A switched FCU or a pull-cord isolator meets this requirement. In practice, most installation electricians include a nearby FCU or isolator as standard.

Regulations & Standards

• Approved Document F:2021 — Ventilation requirements: 15 l/s intermittent for bathrooms, overrun requirement

• BS 7671:2018+A2:2022 — IET Wiring Regulations: Zone classification (Section 701), IP rating requirements, RCD protection in bathrooms

• BS EN 60529 — IP rating classifications (IPX0 to IPX8)

• Approved Document P — Part P notifiable work classification for bathroom electrical work

• Manrose Fan Installation Guides — Wiring diagrams for timer and humidity fans

• Approved Document F 2021 — Full ventilation requirements for dwellings

• Xpelair Technical Support — Fan specification and zone compliance data

• bathroom zones — Full BS 7671 zone classification for bathrooms

• bathroom ventilation — Ventilation requirements and duct sizing

• kitchen extract — Kitchen extraction requirements for comparison

• part p notifications — What is and isn't notifiable under Part P