The paper test, five seconds
Switch the extractor on and hold half a sheet of kitchen paper near the grille without touching it. If the paper sticks on its own, the unit is pulling. If it falls, the motor spins but no air is moving.
It is the only check needed, and almost nobody does it because the noise gives the impression the unit is working.
Why it stops pulling
Three causes, in order of frequency. Grille and blades loaded with greasy dust, which in a bathroom mixes with moisture and sets into a hard layer. A duct obstructed or crushed somewhere along its run. And an outside outlet blocked by a nest, a leaf or a clogged bird guard.
None of the three stops the motor turning. All three stop the air leaving.
The extractor that leads nowhere
There is a fourth case, more common than it sounds in eighties and nineties properties: the extractor discharges into a suspended ceiling or a poorly ventilated shaft instead of outside. In that case it does move air, but it only moves it somewhere else.
The vapour condenses again a few metres away, and damp appears at a point with no obvious connection to the bathroom. Checking where the duct actually ends is part of the diagnosis.
What changes in winter
In summer you ventilate by opening a window. From November to March nobody opens the bathroom window after a shower, and the extractor goes from being a supplement to being the only way vapour leaves.
A hot shower with no real extraction leaves several litres of water in the air of the property. Repeated daily for four months, that is the mark in the bedroom corner that appears in February.
What gets repaired and what gets replaced
A thorough clean of the grille, the blades and the first stretch of duct restores the flow in most cases, with no parts at all. If the duct is crushed or the outlet badly oriented, that becomes a job.
And if the unit switches on with the light, check it keeps running for a few minutes after you switch off: an extractor that stops with the light has no time to do its work.
