How do I fix a noisy or weak extractor fan?

Diagnosing an extractor fan that rattles or barely extracts, and the cleaning and ducting checks that usually fix it.

Difficulty: intermediate
Time: 1 hr
Read: 2 min
Safety: caution

Short answer

Test the fan with a sheet of tissue paper — if it does not hold it against the grille, it is not extracting properly. Clean the blades and grille, check the ducting is not crushed or blocked, and confirm the external vent flap opens freely.

A bathroom fan that runs but does not extract is a very common cause of persistent condensation and mould. The fan itself is rarely at fault — dust on the impeller, crushed ducting and a stuck external flap account for most cases.

Safety

Isolate the fan at the consumer unit before removing the cover — a bathroom fan is usually on the lighting circuit and the switch may not fully isolate it. If the fan is wired through a timer or transformer and you are not confident, this is electrician work. Never work on a fan with wet hands or from an unstable platform.

What you'll need

Screwdriver
Cloths and a brush
Tissue paper for testing
Step ladder (optional)
Replacement ducting (optional)

Step by step

Test the extraction with a sheet of tissue paper held against the grille.With the fan running it should hold the paper. If it drops, extraction is inadequate.
Isolate the fan at the consumer unit before removing the cover.The wall switch may not isolate it if it is on a timer or shares the lighting circuit.
Remove the grille and clean it and the impeller blades.Dust and lint on the blades reduce airflow substantially and cause imbalance, which is what makes them rattle.
Check the ducting behind the fan is not crushed, kinked or disconnected.Flexible ducting compressed in a loft or squashed behind a ceiling is extremely common.
Check the ducting run is as short and straight as possible.Every bend reduces performance. Long flexible runs with several bends are often the real problem.
Go outside and check the external vent flap opens when the fan runs.Flaps stick shut with paint, dirt or a nest. A stuck flap stops extraction entirely.
Confirm the duct terminates outside, not in a loft or cavity.A fan venting into a roof space causes a far worse damp problem than it solves.
If the fan is old, noisy and weak after all that, replace it — ideally with a humidistat model.A humidistat fan runs until the room is actually dry rather than for a fixed period.

Tips

A fan with a timer or humidistat is a worthwhile upgrade, because most bathroom moisture leaves the room after the person does.
Rigid ducting outperforms flexible by a wide margin. If you can replace a long flexible run with rigid, do.
Fan performance is rated in cubic metres per hour — check the rating suits the room size rather than assuming any fan will do.

Common mistakes

Assuming a fan works because it makes noise — Noise and extraction are unrelated. The tissue test is the only real check.
Venting into a loft — It moves the moisture into the roof space and causes condensation and rot in the structure.
Long flexible ducting with multiple bends — Each bend and every metre of flexible duct cuts performance substantially.

Questions people ask

How do I test whether my extractor fan is working?

Hold a sheet of tissue paper against the grille with the fan running. If the fan cannot hold the paper in place, it is not extracting adequately whatever noise it is making.

Why is my bathroom still damp with the fan running?

Usually blocked or crushed ducting, a stuck external flap, or a fan that stops too soon. Most moisture leaves the room after you do, so a fan wired to the light switch is doing half a job.