Extractor Fan Not Working: Power, Timer, Humidity Sensor & Motor Fault Diagnosis

Summary

Bathroom and kitchen extractor fans have a limited service life. The motor is the weakest component — typically rated for 30,000–50,000 operating hours in theory, but in practice the cheap motors fitted in most budget fans fail within 5–10 years in high-humidity environments. When a fan stops working, the fault is almost always in one of three places: the power supply, the control module (timer or humidity sensor), or the motor itself.

Diagnosis is straightforward with a multimeter. The sequence is: power → control → motor. If power is present and the control module is functioning but the motor doesn't run, the fan unit needs to be replaced or the motor replaced (where feasible with higher-end brands).

Key Facts

• Most common failure mode — Motor failure (seized or open-circuit brushes)
• Second most common — Timer module or humidity sensor PCB failure
• Power supply — 230V AC from FCU (fused connection unit, typically 3A) or direct from lighting circuit
• Timer fan wiring — Needs permanent live (L), switched live (SL), neutral (N), and earth (E). If SL is missing, the overrun timer cannot function
• Humidity fan wiring — Needs permanent live (L), neutral (N), and earth (E) only. No switch connection required
• Typical current draw — 0.2–0.5A for most domestic fans. Fused at 3A (FCU) or protected by the lighting circuit MCB
• IPX4 requirement — Fans in Zone 1 (above bath/shower to 2.25m) must be IPX4 minimum per BS 7671
• Duct blockage — A blocked external duct grille causes the motor to work harder and overheat, shortening life. Check the external grille first
• Fan run direction — Some fans can be wired with reversed polarity — fan runs but pulls air into the room rather than extracting. Check airflow direction on installation

Quick Reference Table

SYMPTOM: Fan does not run at all
     |
     ├── Is the light/switch on? (for timer fans)
     │   If no, turn on → does fan now run?
     │   Yes → working normally
     │   No ↓
     ├── Test voltage at fan terminals with multimeter
     │   No voltage → trace supply back
     │   │   Check FCU is switched on and fuse is intact
     │   │   Check wiring centre or lighting circuit MCB
     │   230V present ↓
     ├── Is it a timer/humidity fan?
     │   Yes → Bypass the control module
     │   │   Bridge permanent live to the motor terminal directly
     │   │   Does motor run now?
     │   │   Yes → Control module (timer/humidity PCB) failed — replace module
     │   │   No ↓
     ├── Test motor winding resistance
     │   Open circuit (∞ Ω) → Motor failed — replace fan
     │   Very low resistance (< 10Ω) → Short circuit → replace fan
     │   Reasonable resistance (50–500Ω for shaded pole; 100Ω for PSC) ↓
     └── Motor stiff/seized? — turn impeller by hand
         Stiff → Motor bearing failed — replace fan
         Turns freely → Capacitor failed (PSC motor) — replace capacitor

Detailed Guidance

Confirming the Power Supply

With the power isolated, check the wiring terminals at the fan:

• L (permanent live) — Should show 230V with respect to neutral when the circuit is live, regardless of whether the light is on
• SL (switched live) — Should show 230V only when the light switch is on
• N (neutral) — 0V to earth, typically 1–2V to live
• E (earth) — 0V to neutral

If neither L nor SL shows voltage, the circuit is dead. Check:

1. The FCU (fused connection unit) if one is fitted — is the fuse intact (3A)? Is the FCU switched on?
2. The lighting circuit MCB in the consumer unit — has it tripped?
3. The junction box or ceiling rose if the fan is wired into the lighting circuit — is the connection secure?

Testing the Control Module

Timer and humidity modules are electronic PCBs that sit between the fan terminals and the motor. They can fail without any visible damage.

To bypass the control module:

1. Isolate the power
2. Open the fan cover
3. Identify the wires entering the fan body (from the wall) and the wires going to the motor
4. Temporarily connect permanent live directly to the motor live terminal
5. Restore power

If the motor runs when powered directly: the control module has failed. Replace it. Many manufacturers sell replacement timer/humidity modules as service parts — consult the fan's installation manual for part numbers.

If the motor does not run when powered directly: the motor has failed.

Testing the Motor

Shaded pole motors (most common in cheap domestic fans) — Simple AC induction motor with no capacitor. Test: apply 230V directly to motor terminals. Should run immediately. If it hums but doesn't spin, the bearing is seized — tap the case and check if it frees. If seized, replace the fan.

Permanent split capacitor (PSC) motors (better quality fans, some Manrose, Airflow, Vent-Axia high-end) — These use a run capacitor. If the capacitor fails, the motor hums but won't start. Test the capacitor with a capacitance meter. A 2µF capacitor measuring under 1µF has failed. Replacement capacitors are available cheaply (£2–£5).

Brushless DC motors (premium fans, low-energy) — Require control electronics. If the motor driver board fails, the motor won't run. These are typically not economically repairable — replace the fan unit.

Checking for Duct Blockage

A blocked duct is a common cause of fan failure (motor overheats) and also a cause of ineffective ventilation even when the fan appears to be running.

Check:

1. External grille — remove and clear any debris, spider webs, or damaged flap valve (non-return flap)
2. Flexible duct — check for kinks (common where duct passes through joists or walls)
3. Duct length — excessive flexible duct length (over 3m) significantly restricts airflow. Replace with rigid duct

Airflow test: Hold a sheet of tissue paper near the fan intake grille. If it is drawn firmly against the grille, airflow is adequate. If it flutters weakly or falls away, airflow is poor — check duct.

Replacing the Fan

Like-for-like replacement of an existing fan in the same position is not Part P notifiable. The fan connects to the existing wiring.

Steps:

1. Isolate the power at the FCU or MCB
2. Remove the existing fan — typically by unclipping the cover and unscrewing the fan body from the mounting plate
3. Disconnect the wiring, noting which terminal each wire came from (photograph before disconnecting)
4. Fit the new fan's mounting plate to the existing hole/backbox
5. Connect wiring to the new fan's terminals in the same arrangement
6. Set the timer/humidity controls as required (usually via small adjustable dials inside the fan)
7. Restore power and test

If upgrading to a different model: Check the duct connector diameter (100mm is standard, 125mm for higher flow rates). Check the wiring terminations match the new fan's requirements.

Commissioning and Testing After Replacement

After fitting the new fan:

1. Switch on the light — confirm fan starts (or wait for humidity response if humidity type)
2. Switch off the light — confirm fan overruns for the set period (adjust timer if necessary — minimum 15 minutes per Approved Document F)
3. Check external grille — confirm flap valve opens when fan runs and closes when stopped (prevents draughts when fan is off)
4. Replace covers

Frequently Asked Questions

My bathroom fan runs but makes a grinding noise. Is it repairable?

A grinding noise from an extractor fan almost always indicates a worn bearing. On cheap fans, replacement is more economical than repair. On higher-quality fans (Vent-Axia, Airflow), bearings can sometimes be sourced and replaced — but this requires dismantling the motor and is only worthwhile if the fan is relatively new or expensive.

How long should an extractor fan last?

Budget fans (Manrose SFE, Xpelair basic range, own-brand): 3–8 years in a humid bathroom environment. Mid-range fans: 8–15 years. Premium fans (Vent-Axia Silent Fan, Airflow Icon): 15+ years. The key longevity factors are motor quality and ducting — a good fan on a badly blocked duct will fail early.

Can I fit a more powerful fan to extract better?

Yes, but check the duct will handle the increased flow. A 100mm duct is rated to approximately 45 litres/second — most domestic fans run at 15–30 l/s, so the duct is rarely the constraint. However, a much larger fan on a restricted duct will just create more noise and work the motor harder without increasing airflow significantly.

Regulations & Standards

• BS 7671:2018+A2:2022 — Zone classification and IP rating requirements for bathroom fans

• Approved Document F:2021 — Minimum extract flow rates: 15 l/s for bathrooms, 30 l/s for kitchens

• Approved Document P — Part P notifiable work classification (new circuits, not like-for-like replacement)

• Manrose Technical Support — Wiring diagrams and part numbers for Manrose fans

• Vent-Axia Technical Support — Vent-Axia service and parts

• Airflow Technical Support — Airflow fan range and replacement guides

• bathroom fan wiring — Correct wiring method for bathroom extractor fans

• bathroom ventilation — Ventilation requirements under Approved Document F

• kitchen extract — Kitchen extraction requirements