Bathroom Extractor Fans — Humidity Sensors, Timer Functions, Wiring, and Part F Compliance for UK Electricians

Bathroom mechanical ventilation is one of the most common jobs an electrician gets called back for — either because the original fan was undersized, wired incorrectly, or the client didn't understand why the humidity sensor doesn't work the same as a timer. This guide covers everything from Part F minimum flow rates and IP zone wiring rules through to fan selection, duct sizing, and the wiring diagrams used in practice.

Part F Building Regulations — Ventilation Requirements for Bathrooms

Approved Document F (ADF) 2021 sets the minimum performance for mechanical extract ventilation (MEV) in dwellings. For a bathroom containing a bath or shower, the minimum continuous background extract rate is 4 litres per second (l/s), with a boost rate of at least 15 l/s during use. These figures apply to the fan's rated extract capacity — not inlet free area or duct diameter.

For a utility room or en-suite with no bath or shower, 6 l/s boost is sufficient. WC-only rooms require 6 l/s boost. In practice, most 100mm (4-inch) fans are rated at 85–100 m³/h (~24–28 l/s) which comfortably exceeds the 15 l/s minimum for a standard bathroom. Where duct runs are long or the fan is remote (inline), always check the fan's pressure-flow characteristic curve against calculated system resistance.

ADF 2021 changes: The 2021 revision introduced system performance testing requirements for new-build dwellings (approved inspector or building control witnessing) and tightened specific fan power (SFP) limits for continuous ventilation systems. For intermittent fans in existing dwellings (including like-for-like replacements), ADF2010 performance thresholds remain typical in practice — but always confirm with building control on notifiable projects.

Part P — Notifiable Work

Replacing an existing bathroom fan on the same circuit, in the same position, is generally not notifiable under Part P of the Building Regulations (Schedule 4, Class A, para 1(a) — minor works). Adding a new fan on a new dedicated circuit, or extending an existing circuit to add a fan in a new bathroom, is notifiable and requires either self-certification by a registered competent person (NICEIC, ELECSA, NAPIT etc.) or building control notification and inspection.

Always leave the client an Electrical Installation Certificate (EIC) or Minor Electrical Installation Works Certificate (MEIWC) as appropriate, and update the installation schedule accordingly.

Fan Types

Axial Fans

The standard wall or ceiling-mounted axial fan is the most widely installed type. The motor and impeller are in-line with the airflow, making the unit compact and straightforward to wire. Rated capacities of 85–100 m³/h at zero static pressure are typical for 100mm units. Axial fans perform well over short, straight duct runs (under 3 metres) but their pressure performance drops sharply as duct resistance increases. For runs over 3m or with multiple bends, check the fan curve — a fan rated 100 m³/h at zero resistance may only achieve 40 m³/h at 30 Pa system resistance.

Centrifugal (Radial) Fans

Centrifugal fans use a radial impeller and have a much flatter pressure-flow curve. They are the correct choice for longer duct runs, inline installations, and systems with significant resistance (multiple bends, fine grille, wall louvre). Typical installations: through-floor ceiling voids to a roof tile vent or soffit, or long horizontal runs through a wall to an external louvre. Units like the Manrose CF100 series or Airflow iCon eco range offer EC motor options with very low SFP suitable for ADF2021 compliance.

Inline (Remote) Fans

An inline fan sits in the duct run — typically in the ceiling void — rather than at the extract point. This removes the motor noise from the bathroom entirely and allows one fan to serve multiple extract points via a manifold. Common in new-build and high-end refurbishments. Wiring is identical (switched live or permanent live + neutral + earth), but the fan may be some distance from the bathroom light switch. Always verify the control signal voltage matches the fan's trigger input specification — some inline fans require a volt-free contact rather than a 230V switched live.

Humidity Sensors, Timers, and PIR Controls

Humidity-Sensing Fans

A humidity-sensing fan contains an internal RH (relative humidity) sensor. The fan boosts when RH rises above a factory-set or user-adjustable threshold (typically 70–80% RH) and continues until RH drops back to the set point. The key point clients often miss: the fan does not switch on when the light comes on. It switches on when moisture is detected. If the shower is quick or the bathroom is well-ventilated, the fan may never boost at all. If the sensor is set too high, it may not respond until condensation is already forming.

Wiring: Humidity fans require a permanent live and neutral (not switched live). The fan must remain powered continuously to sense RH, even when the light is off. This is the most common wiring error on humidity fans — connecting to a switched live means the fan only detects RH when the light is on, completely defeating the purpose.

Always label the wiring clearly and note this on the MEIWC. If the existing switch circuit only has a switched live at the fan position, you will need to run a spur from a permanent live source — typically the ceiling rose or a junction box upstream of the switch.

Timer (Overrun) Fans

Timer fans switch on when the light switches on (switched live trigger) and continue to run for an adjustable overrun period (typically 2–25 minutes) after the light is switched off. Overrun is adjustable via a trim pot on the PCB, usually accessed through the grille. This is the most straightforward wiring method and suitable for most domestic bathrooms.

Wiring: Timer fans require a switched live (from the light switch) plus neutral and earth. The switched live is the trigger — when it goes live (light on), the fan runs; when the light switches off, the overrun timer starts. Some models offer combined switched live + permanent live connections, allowing both the boost (switched) and continuous (permanent) modes.

PIR Fans

PIR-triggered fans detect occupancy rather than light or humidity. These are less common in standard bathrooms but are used in commercial/communal bathrooms, disabled WCs (where the user may not operate the light switch), and refurbishments where no switched live is available at the fan position. The PIR triggers the fan when it detects movement; the fan continues to run for an overrun period after the last motion detected.

Combined Models

Many current fan models — particularly the Airflow iCon and Manrose MF100 series — offer field-selectable modes: humidity, timer, or combined humidity+timer. Combined mode runs on timer trigger from the switched live but also boosts independently if RH rises above threshold while the bathroom is unoccupied. This is the recommended choice for new installations as it satisfies ADF requirements for continuous background ventilation (if fan is set to continuous low + boost) while remaining responsive to occupancy.

IP Zones and Wiring Rules in Bathrooms

BS 7671:2018+A2:2022 Section 701 defines four zones in a room containing a bath or shower:

Fan placement: Most ceiling-mounted extractor fans are installed directly above or adjacent to the shower — which places them in Zone 1 or Zone 2. Confirm the fan's IP rating against the zone. A fan rated IP44 is suitable for Zone 2. For Zone 1 placement (directly above a shower tray within 0.6m of the shower head), the fan must be rated at minimum IPX4 and listed as suitable for Zone 1 — check the manufacturer's installation instructions. Most domestic axial fans are IP44 rated and zone 1 listed when installed in the ceiling void with only the grille exposed.

RCD protection: All circuits in Zones 0, 1, and 2 must be RCD protected (30 mA, Type A minimum per Reg 701.411.3.3). In practice this means the fan circuit should be on an RCBO or on an RCD-protected way in the consumer unit. If the fan is on a spur from the lighting circuit and that circuit is RCD-protected, the requirement is met. Never add a fan on a non-RCD circuit in a bathroom.

Selecting the Right Fan

Flow Rate vs Duct Run

The 15 l/s (54 m³/h) minimum is achievable by almost any 100mm fan on a short duct run. The practical limit is system resistance. Use this rule of thumb for estimating duct resistance:

• Each metre of smooth rigid duct (100mm): approximately 1–2 Pa
• Each 90° elbow: approximately 5–8 Pa equivalent length
• Wall louvre (100mm): approximately 10–20 Pa
• In-line silencer: approximately 10–15 Pa

Add up the total and check the fan's pressure-flow curve at that resistance. If the fan only delivers 8 l/s at your calculated system resistance, it will not meet Part F — upgrade to a centrifugal unit or shorten the duct run.

Noise (Sone Rating)

Fan noise is a common complaint. Standard axial fans produce 28–38 dB(A) at 3 metres — audible but acceptable. For en-suites or bedrooms next to the bathroom, specify a fan rated under 25 dB(A) or use a remote inline fan to move the noise source away from the occupied zone. EC motor fans (eg Airflow iCon eco) are significantly quieter than AC shaded-pole models and draw 2–4W vs 10–18W, making them preferable for continuous ventilation duty.

Duct Size

100mm (4-inch) duct is standard for domestic bathrooms. Use rigid PVC or galvanised steel ducting where possible — flexible duct (corrugated aluminium or PVC hose) adds significant resistance and should only be used for short final connections (under 1m). Never use flexible duct for the main run. Ensure all duct joints are sealed with foil tape; un-taped joints allow moist air to condense in the ceiling void.

Terminate externally with a wall vent with a backdraught shutter. Internal grilles with no shutter allow cold draughts and animal ingress. Where the duct terminates through a roof tile, use a purpose-made roof vent tile — flat-roof extractions to a weathered lead slate are also acceptable.

Wiring the Fan

Switched Live (Timer Fans)

The most common wiring method in existing domestic installations uses the bathroom light switch to control the fan:

Consumer unit (lighting circuit)
  → Junction box or ceiling rose
    → Line (L) → Switch (light switch)
      → Switched Live (sw L) → Fan (trigger input) and Light fitting
    → Neutral (N) → Fan neutral and Light fitting neutral
    → Earth (E) → Fan earth

In practice, the switched live and neutral are run to the fan using 1.0mm² or 1.5mm² two-core-and-earth (2C+E) from a convenient junction point — ceiling rose, rose on the light fitting, or a new junction box. The 1.5mm² cable is preferred where any doubt exists about existing circuit loading.

If the light fitting is a loop-in ceiling rose, the three-plate loop-in wiring method provides easy access to both permanent live (incoming) and switched live (outgoing to fitting) at the rose. Run a spur from the rose to a junction box above the fan, then drop tails to the fan terminals.

Permanent Live (Humidity Fans)

Consumer unit (lighting circuit)
  → Junction box
    → Permanent Live (L) → Fan permanent live terminal
    → Neutral (N) → Fan neutral terminal
    → Earth (E) → Fan earth terminal
(Note: Light switch feeds only the light fitting, not the fan)

Where the existing junction box or rose only provides switched live at the fan location, run new cable back to an upstream permanent live point. Always use a junction box (not a direct splice) and ensure the junction box is accessible.

Switched Live + Permanent Live (Combined Mode)

Some fans (eg Manrose MF100HT combined model) accept both a switched live and a permanent live. The permanent live enables continuous low-speed background ventilation and humidity monitoring; the switched live triggers the boost speed. Wire both connections accordingly. This is the most capable configuration and is the recommended approach for ADF2021 compliance in new builds.

Cable Sizing and Protection

Most bathroom fans draw under 30W (0.13A at 230V). A 1.0mm² or 1.5mm² 2C+E cable is adequate for the fan spur in almost all cases. The limiting factor is usually the circuit protection at the consumer unit — the fan will share the 6A or 10A lighting circuit breaker. Verify that adding the fan does not overload the circuit. If the fan is on a dedicated spur from a fused connection unit (FCU), the FCU should be fused at 3A.

The FCU must be outside the bathroom zones — typically mounted in the airing cupboard, hallway ceiling void, or at the consumer unit — or at a suitable location outside Zone 2 if surface-mounted. A 3A or 5A FCU with a neon indicator provides a convenient local isolator and visual confirmation that the fan is live.

Common Faults and Diagnosis

Fan Runs Continuously

On a humidity fan: check the RH setpoint trim pot — may be set too low (triggered by ambient humidity in hot weather or adjacent kitchen). Clean the sensor grille. Check if the room has adequate make-up air (trickle vent in window or under-door gap) — a sealed room will prevent the RH from dropping even when the fan is running flat out. On a timer fan: check the overrun pot isn't set to maximum (25 min overrun on a 15-minute shower cycle appears continuous).

Fan Doesn't Run

Check fuse (if FCU present). Check RCD hasn't tripped. Verify switched live is present at fan trigger terminal with a meter — not just continuity to the light switch. On a humidity fan, confirm the permanent live is present. Motor may have seized (replace the fan unit — bearing replacement is rarely cost-effective on domestic fans).

Condensation Still Building Despite Fan Running

Fan is running but flow rate is inadequate. Check duct for kinks (flexible duct often kinked in the ceiling void), blocked wall louvre, or backdraught shutter stuck closed. Measure actual flow with an anemometer or smoke pencil — compare to rated capacity. If duct run is long, upgrade to centrifugal fan. Check trickle ventilation — if the bathroom is sealed (no trickle vent, draught-sealed door), the fan will create negative pressure and struggle to extract.

Excessive Noise

Check that the fan is mounted on a rigid backing board and that the grille is seated flat — vibration transmitted to plasterboard causes resonance. Check the duct is not kinked or restricted, causing back-pressure and motor laboring. If the fan is axial and the duct run is long, the motor may be working at an unfavorable operating point — replace with a centrifugal fan rated for the system resistance.

Product Cards

Card 1 — Bathroom Extractor Fan (Humidity + Timer)

A 100mm combined humidity/timer extractor fan is the right specification for most domestic bathroom retrofits and new installs. IP44 rated for Zone 2 installation. Look for fans with adjustable RH threshold, adjustable timer overrun, and energy-efficient EC motor. Common options include the Manrose MF100HT, Airflow iCon15 eco, and Vent-Axia Solo Plus HT. Typically draws 8–14W, rated 95–100 m³/h free air delivery.

Card 2 — 1.5mm² Twin and Earth Cable (per metre)

Standard wiring cable for fan spur runs within the ceiling void. 1.5mm² 2C+E is preferred over 1.0mm² for fan circuits — the additional conductor capacity gives headroom if the fan is ever replaced with a higher-wattage unit. Grey sheathed (6242Y). Always sleeve the bare earth conductor in green/yellow sleeving at terminations.

Card 3 — 3A Fused Connection Unit (FCU)

A 3A FCU provides a local isolator for the fan and protects the cable spur. Install outside the bathroom zones (hallway ceiling, airing cupboard, or surface-mounted outside Zone 2). A FCU with a neon indicator allows quick confirmation that the circuit is live. Specify a 3A fused version — 13A FCUs are not appropriate for a 30W fan load.

Card 4 — 100mm Rigid Duct and Wall Vent Kit

Rigid PVC or aluminium duct (100mm diameter) with a backdraught-shuttered wall vent is the correct installation standard. Avoid flexible corrugated duct for runs over 1 metre. A complete kit (duct length + external louvre with shutter) simplifies installation. External louvres must be fixed securely to the external wall and sealed around the duct penetration with appropriate external sealant.

Cross-Links

• Electric Showers — Wiring, Installation, and kW Selection (#245) — bathroom electrical circuits and zone wiring
• Earth Bonding and Supplementary Bonding (#217) — Zone 1/2 bonding requirements in bathrooms
• PIR Sensors and Occupancy Detection (#205) — PIR-triggered ventilation
• Consumer Units — MCBs, RCBOs, and Split-Load Boards (#173) — RCD-protected circuits for bathroom fans
• LED Downlights and Fire-Rated Downlights (#219) — coordinated ceiling void work in bathrooms

Summary

Bathroom extractor fan installation is straightforward but the details matter: confirm the minimum 15 l/s boost flow rate against actual duct system resistance, select humidity or timer mode based on the available wiring and client requirement, ensure the permanent live is supplied to humidity fans (not switched live), check the IP zone and use an IPX4-rated fan for Zone 1 or Zone 2 positions, and provide RCD protection on the circuit. Leave the client a certificate and adjust the fan sensitivity settings before you leave — a properly commissioned bathroom fan will run correctly without callbacks.

Shop Extractor Fans at APM Electricals

Browse our full range: Extractor Fans | Bathroom Accessories

APM Electricals
24 Western Avenue, Acton, London W3 7TZ
Phone: 020 8702 8080
Web: www.apmi.uk
Same-day collection available for West London trades.