Fault Diagnosis: Extractor Fan Not Working

Summary

Bathroom and kitchen extractor fans fail more often than any other domestic ventilation product. They run wet, dusty, and unattended for years; the failure rates climb sharply after 8 years. A call-out for "the fan's stopped" is usually a 30–60 minute diagnosis followed by either a £40 cleanout or a £180–£280 replacement.

This article is a structured fault tree covering intermittent fans (timer or humidistat triggered, the most common UK domestic type), continuous-running fans (dMEV — decentralised mechanical extract ventilation), and centralised MEV systems. It covers the safety procedure for isolation, the diagnostic logic, the common physical failure points, and the regulatory environment for replacement (Part F ventilation rates, Part P electrical notification, Part L energy efficiency on replacement).

The temptation is to replace any unresponsive fan immediately ("the motor's gone"). Diagnose first — the call-out fee plus a £45 minute spent checking the switched live and the outside grille often saves the customer £250 in needless replacement and earns repeat business.

Key Facts

• Triple-pole isolator — the mandatory means of disconnection for a bathroom fan circuit; switches all three conductors (L, switched L, N). Outside the bathroom (typically in the loft or on the landing).
• Switched live — the second live wire to a fan that powers the timer. Many fans need both permanent live AND switched live; if only switched live is connected, the timer won't function.
• Timer overrun — typically 5–25 minute run-on after the light/switch is turned off; adjusted by a small potentiometer or DIP switch on the fan body.
• Humidistat — sensor that triggers the fan when relative humidity exceeds a threshold (typically 60–80% RH adjustable); some fans have humidistat + timer combined.
• Intermittent fan — runs only when triggered (switch, PIR, humidistat); meets Part F minimum extract rates only when running
• dMEV (decentralised MEV) — continuous low-rate fan running 24/7 (typically 6–13 l/s baseline, boost to 15+ l/s); single-room unit
• MEV (centralised) — single fan unit (loft or cupboard) ducted to multiple wet rooms; runs continuously
• Cooker hood (kitchen) — 30 l/s minimum with hood, 60 l/s minimum without hood (Part F)
• Bathroom extract rate — 8 l/s minimum (Part F, Table 5.1)
• Utility room / WC — 6 l/s WC, 8 l/s utility room (Part F)
• Wet room / shower room without window — 15 l/s recommended; humidistat strongly advised
• Part F (Volume 1, 2021) — Means of ventilation, domestic
• Part P — Electrical safety; new circuits in a dwelling notifiable to Building Control or via a Competent Person Scheme (NICEIC, NAPIT, ELECSA, Stroma, etc.). Like-for-like replacement on existing wiring is not notifiable.
• Part L — Energy efficiency; replacement fans should meet the SAP-default specific fan power (typically ≤0.5 W/(l/s) for intermittent, ≤0.7 W/(l/s) for continuous)
• IP rating in bathrooms — fan must meet zone requirements: Zone 1 IPX4, Zone 2 IPX4. Most modern bathroom fans are IPX5 and zone-suitable.
• Ducting — 100mm or 150mm flexible/semi-rigid; 100mm is standard for domestic. Maximum recommended duct run typically 6m for 100mm flexible, longer for rigid.
• Condensate trap — required on horizontal ducting in cold lofts to catch condensate that would otherwise drain back to the fan motor.

Quick Reference Table

Detailed Guidance

Safety first — isolation procedure

Before opening the fan body or working on the wiring:

1. Identify the means of isolation. For a bathroom fan, this is the triple-pole isolator (typically a 3-pole rotary switch on the wall outside the bathroom, in the loft, or in the airing cupboard).
2. Switch off at the isolator.
3. Verify dead with an approved voltage indicator — test L–N, L–E, N–E on both the permanent and switched live conductors.
4. Lock off if practical (small isolation tag with the customer's name).
5. Only then remove the fan cover.

If you cannot find the isolator, switch off the lighting circuit at the consumer unit (the fan is almost always wired off the lighting circuit) and re-verify. If still live, isolate the whole dwelling at the main switch.

Decision tree

Extractor fan not working — start here
│
├── Is anything happening when the trigger is on (light, switch, humidistat)?
│   │
│   ├── No sound, no movement
│   │     ├── Check isolator switched ON
│   │     │     ├── Was off → switch on, test → working? Done.
│   │     │     └── Was on → continue
│   │     ├── Isolate, open fan, voltage-test at fan terminals
│   │     │     ├── No voltage on permanent L → upstream supply fault
│   │     │     │     → trace lighting circuit / fuse / isolator
│   │     │     ├── Voltage on permanent L, none on switched L → switch / wiring fault
│   │     │     │     → check light switch, check wiring from switch to fan
│   │     │     └── Voltage present on both → fan module failed
│   │     │           → replace module OR replace fan
│   │
│   ├── Humming but not spinning
│   │     ├── Capacitor or motor winding fault
│   │     └── Fan is not field-serviceable → replace
│   │
│   ├── Spinning but no air at grille outside
│   │     ├── Check outside grille — flap stuck shut, debris, paint sealed
│   │     │     → clear / replace external grille
│   │     ├── Check ducting — disconnect at fan, blow air through
│   │     │     ├── Blocked solid (lint, mortar, bird nest) → clear
│   │     │     ├── Partial blockage → clear, inspect length
│   │     │     └── Water in duct → cold loft condensation, fit condensate trap, insulate
│   │     └── Duct OK → blades fouled with dust → clean
│   │
│   └── Spinning but weak airflow
│         ├── Blade caked with dust → strip, wash, refit
│         ├── Duct partially blocked OR over-long
│         │     → Inspect duct run, replace flexible with semi-rigid for length >3m
│         ├── Duct leaking at joints (negative pressure loss)
│         │     → Seal joints with foil tape or jubilee clips
│         └── Fan rated below required Part F rate
│               → Upgrade to higher-rated fan (8 l/s bathroom, 15 l/s utility/wet room minimum)
│
└── Won't turn OFF
    ├── Humidistat triggering on ambient humidity → adjust setpoint
    ├── Timer module stuck → power-cycle, then replace module
    └── Wiring fault (switched L permanently live) → re-trace

Wiring — permanent L, switched L, and neutral

Most modern bathroom fans need three conductors plus earth:

• Permanent live (L) — power to the fan body 24/7, runs the timer overrun
• Switched live (SL) — comes from the light switch (or the lighting circuit's switched output); tells the fan when the light is on
• Neutral (N) — return
• Earth (CPC) — protective

A common fault on older installations is that only the switched live is wired (no permanent live). The fan runs while the light is on but the timer overrun doesn't work because the timer needs permanent power. Fix: run a permanent live from the lighting circuit junction box to the fan.

For a fan with humidistat (no light-switch trigger), only permanent L and N are needed; the humidistat does the switching internally.

For a triple-pole isolator: switches all three (L, SL, N) at once. Required for any bathroom fan circuit per BS 7671 to give a complete means of isolation.

Intermittent vs dMEV vs MEV — what to specify on replacement

If the existing fan is an intermittent (runs on light switch / humidistat / timer):

• Replace like-for-like with a modern intermittent unit at Part F rate (8 l/s bathroom, 15 l/s wet room without window, 60 l/s kitchen without hood).
• Check fan SFP (specific fan power) — should be ≤0.5 W/(l/s) for Part L compliance.

If the customer wants better long-term ventilation and reduced condensation/mould, upgrade to dMEV:

• Decentralised MEV runs continuously at low rate (typically 6–13 l/s), boosts to 15+ l/s on humidity trigger.
• Eliminates the "fan only runs when you remember the light" failure mode.
• Slightly higher install cost (£180–£300 unit vs £40–£120 intermittent).
• Lower noise (continuous low-rate is quieter than intermittent high-rate).
• Better Part F compliance.

Centralised MEV (one unit serving multiple rooms via ducting in the loft) is a whole-house refurb decision, not a like-for-like fan replacement.

Cooker hood (kitchen) specifics

Kitchen extract requirements differ from bathrooms:

• With cooker hood ducted to outside: 30 l/s minimum (Part F)
• Without cooker hood (e.g. ceiling extract only): 60 l/s minimum
• Recirculating hood (carbon filter, no duct): doesn't count for Part F — a separate ventilation provision is required

If a customer reports "kitchen extractor not working" and it's a recirculating hood, the carbon filter is often the culprit (saturated, blocked). Replace filter (£10–£30) before condemning the unit.

A ducted hood with weak airflow is usually duct blockage (grease build-up in the metal ducting), an outside grille fouled with cooked-on grease, or the filter under the hood blocked solid.

Common physical faults

• Motor seized — bearings collapse, motor hums or sits silent. Not serviceable on modern fans. Replace unit (£40–£180 + labour). Lifespan: 8–15 years typical.
• Capacitor failed — fan hums but doesn't start. Capacitor is a small can wired across the motor; some fans allow capacitor replacement (rare on modern units). Usually replace whole fan.
• Backflow flap stuck — external flap on the outside grille seizes shut from paint, dirt, or corrosion. Fan runs but no air gets out. Clean or replace grille (£8–£30).
• Ducting blocked — bird nest, mortar dropout from when the duct was installed, lint accumulation. Clear from the outside grille end with a rod or vacuum.
• Condensate in duct — horizontal duct in cold loft, warm moist air condenses and pools. Drains back to fan motor, causes failure. Solution: insulate duct, add condensate trap at low point.
• Wrong-side wiring — fan installed with L on N and vice versa. Modern fans usually still run but timer doesn't function correctly. Re-check polarity.

Replacement: Part P, Part F, Part L summary

• Part P — Like-for-like fan replacement on existing wiring: not notifiable. New fan on a new circuit (e.g. running a dedicated supply for a continuous-running fan): notifiable. Sole traders not on a CPS must notify Building Control and pay the LABC fee.
• Part F — Replacement fan must meet the relevant Part F extract rate for the room.
• Part L — Replacement fan should meet SFP target; specify a Part L compliant unit.

For a domestic bathroom like-for-like replacement, the only paperwork is the customer's invoice. No notification, no certificate beyond the install commissioning.

Frequently Asked Questions

Can I replace a fan myself without Part P notification?

If you're a competent person within the dwelling (homeowner) and the replacement is like-for-like on the existing circuit, yes — Part P does not require notification for replacement of accessories on existing wiring. If you create a new circuit or run new cable to a new fan position, Part P applies and the work must be notified.

Why does my fan run on after the light is off?

This is the timer overrun, an intentional feature. Most fans run for 5–25 minutes after the switched live drops to clear remaining humidity. Adjust the overrun on a small potentiometer or DIP switch on the fan body (consult the manual).

How much air should an extractor move?

Part F minimums: bathroom 8 l/s, WC 6 l/s, utility 8 l/s, kitchen with hood 30 l/s, kitchen without hood 60 l/s. These are minimum rates and the fan should be rated equal to or higher. Boost rates on humidistat-controlled fans are typically 15–25 l/s.

Why does the fan drip water onto the bathroom ceiling?

Condensate. The fan duct runs through a cold loft, warm moist bathroom air enters the duct, cools, condenses to water on the duct walls, and drips back. Fix: insulate the duct with proprietary ducting lagging, or fit a condensate trap at the lowest point of the duct run (it catches water in a small reservoir that evaporates between uses).

Does a window in the bathroom mean I don't need a fan?

For new installations under Part F, an openable window of adequate area (1/20th of floor area is the historic rule) can sometimes substitute for mechanical ventilation, but most new builds and refurbs install a fan regardless. For a wet room or shower room without an openable window, mechanical extraction is required.

My fan is rated higher than Part F — why is the room still steamy?

Likely a duct problem. Specified airflow is at zero static pressure; with ducting added, real-world flow drops 30–60%. Long runs of flexible ducting, sharp bends, and partial blockages collapse the real flow rate. Use semi-rigid or rigid duct, minimise bends, keep runs short.

Regulations & Standards

• Building Regulations Approved Document F (Volume 1, 2021) — Means of ventilation, domestic

• Building Regulations Approved Document P — Electrical safety in dwellings

• Building Regulations Approved Document L (Volume 1) — Conservation of fuel and power, domestic

• BS 7671:2018+A2:2022 (the IET Wiring Regulations) — bathroom zones (701) and electrical installations in special locations

• BS EN 60335-2-80:2003+A2:2010 — Safety of household electrical appliances: particular requirements for fans

• BS EN 13141-4 — Performance testing of components/products for residential ventilation: fans

• Workplace (Health, Safety and Welfare) Regulations 1992 — workplace ventilation context (commercial)

• Approved Document F: Ventilation — gov.uk

• Approved Document P: Electrical safety — gov.uk

• Approved Document L: Conservation of fuel and power — gov.uk

• BS 7671 (the Wiring Regulations) — IET

• HVCA / BESA technical guidance on ventilation — Building Engineering Services Association

• bathroom fan wiring — wiring detail for bathroom extractor circuits

• bathroom zones — bathroom zone classifications for IP ratings

• kitchen extraction and ventilation — kitchen extract rate selection

• extractor fan installation pricing guide — replacement pricing context

• condensation on windows — companion ventilation fault