How do I service an extractor fan?

Beyond cleaning: the duct, the shutter, the bearings and the run-on timer — the parts that make a fan weak rather than dirty.

Difficulty: beginner
Time: 1 hr
Read: 4 min
Safety: warning

Short answer

Cleaning the grille and impeller restores most fans. If it is still weak, the problem is almost always the ducting — a crushed flexible hose, a long run, or an external grille with a stuck backdraught shutter. Those are where servicing differs from cleaning.

Most advice on extractor fans stops at cleaning the dust off, which fixes a good proportion of them. This is about the rest: the fan that has been cleaned and still barely moves air, where the fault is downstream of the unit.

Safety

Isolate the circuit at the consumer unit before removing any fan cover, not just at the light switch — many bathroom fans are wired to run on after the light goes off and are still live. If the fan is hard-wired rather than plugged in, and you are not confident isolating it, this is an electrician's job. Never work on a fan while standing in a bath or shower tray.

Step by step

Isolate the circuit properly.At the consumer unit, not the light switch. Bathroom fans with a run-on timer stay live after the light goes off, and that surprises people at the worst moment.
Test the airflow before and after.Hold a single sheet of tissue against the grille with the fan running. It should hold there on its own. Doing this first and last tells you whether the work achieved anything.
Clean the grille and impeller.The cover usually unclips. Wash the grille in warm soapy water and dry it fully. Clean the fan blades with a brush and a barely damp cloth — dust on the blades unbalances them and cuts airflow substantially.
Check the impeller spins freely.By hand, with the power off. Stiffness, grinding or wobble means worn bearings, and on a domestic fan that means replacement rather than repair.
Inspect the ducting — this is the usual culprit.Flexible hose crushed behind the unit, sagging in a loft with water pooled in a dip, kinked around a joist, or an unnecessarily long run with several bends. Each one costs a large share of the airflow.
Straighten and shorten the duct run.Pull the flexible hose taut rather than leaving it concertinaed — a compressed hose has far more resistance than an extended one. Rigid duct is much better than flexible where the run allows.
Check the external grille and backdraught shutter.Go outside and look. Painted-over louvres, a shutter stuck with dust, cobwebs, or a bird's nest. A stuck shutter reduces a fan to almost nothing and is invisible from indoors.
Check the duct actually goes outside.A fan ducted into a loft rather than through the roof or wall is a common installation fault. It puts the whole moisture load into the loft, and it is worth fixing rather than servicing.
Insulate the duct in a cold space.An uninsulated duct in a loft fills with condensation, which runs back into the fan and drips through the ceiling. Lagging it prevents a problem people usually attribute to a roof leak.
Check and set the run-on timer.Most fans have an adjustable overrun, often a small dial inside the casing. Fifteen to twenty minutes after the light goes off is what actually clears a bathroom; the factory setting is frequently much shorter.
Check air can get into the room.A fan can only extract what it can replace. A tightly sealed door with no undercut starves it. A 10mm gap under the door makes a measurable difference.
Consider whether it is undersized.A fan rated for a small bathroom in a large one with a long duct run will never keep up. Extraction rates are quoted in litres per second or cubic metres per hour, and a properly sized replacement is inexpensive.
Retest with the tissue.If it still will not hold a sheet of tissue after all of the above, the fan is worn out or wrongly specified. Replacement is straightforward and cheap.

Tips

Do the outside grille first. It takes two minutes and is the most commonly missed cause of a fan that has been cleaned twice and still does nothing.
Rigid duct instead of flexible is the single biggest airflow improvement available if the run can take it.
Kitchen hoods recirculating through a charcoal filter extract nothing — they filter smells. If a kitchen steams up, check whether it is ducted at all.

Common mistakes

Isolating at the light switch only — Fans with a run-on timer stay live after the light is off. Isolate at the consumer unit before removing any cover.
Leaving flexible duct concertinaed — A compressed hose has many times the resistance of an extended one. Pulling it taut can transform the airflow with no other change.

If it doesn't work

Cleaned and still barely extracting

Cause: Ducting or the external grille — Fix: Straighten and shorten the duct, and check the outside shutter opens freely.

Water dripping from the fan or the ceiling below

Cause: Condensation in an uninsulated loft duct running back — Fix: Insulate the duct along its whole length and give it a continuous fall towards the outside.

Fan noisy or grinding

Cause: Worn bearings or an unbalanced impeller — Fix: Clean the blades first. If it persists, replace the unit — domestic fan bearings are not serviceable.

Bathroom still steamy despite a working fan

Cause: Overrun too short, or no air getting into the room — Fix: Set the overrun to fifteen to twenty minutes and make sure there is a gap under the door.

Questions people ask

Why is my extractor fan weak even after cleaning?

Almost always the ducting or the external grille — a crushed flexible hose, a long run with bends, or a backdraught shutter stuck with dust or paint. Both are outside the unit and invisible from inside the room.

How do I test whether an extractor fan is working?

Hold a single sheet of tissue against the grille with the fan running. It should stay there on its own. If it falls, the fan is not moving enough air.