The paper test
Switch the fan on and hold a single sheet of toilet paper against the grille. A healthy fan holds it there without help. If it falls, or only clings at one edge, the fan is barely moving air whatever noise it is making.
If the paper holds firmly but the room still steams up, the fan is working and the problem is elsewhere: run time, replacement air, or the room being far bigger than the fan was specified for. That distinction saves people buying a new fan that changes nothing.
Ducting is the usual culprit
Most disappointing fans are strangled by their ducting. Flexible ducting that sags into loops holds condensation, and a water filled dip blocks airflow almost completely. Long runs, several tight bends, or ducting crushed under loft insulation all cut performance dramatically.
Outside, check the external grille. Wasp nests, painted over louvres, backdraught shutters seized shut with dust, and grilles fitted behind render are all things we find regularly. If you cannot feel air leaving the outside grille while the fan runs, the fan is pushing against a closed door.
The fix is usually rigid or semi rigid ducting on the shortest sensible route, supported so it falls slightly towards the outside, and a clear external terminal.
Replacement air, the part everyone forgets
A fan cannot pull air out unless the same volume can get in. Modern draught proofed bathrooms with a well sealed door and no undercut can be almost airtight. The fan then works hard and achieves very little.
The standard remedy is a ten millimetre gap under the door, measured above the finished floor covering. It sounds trivial. It often doubles the effective extraction rate. Test it by propping the door open a couple of inches with the fan running and seeing whether the mirror clears noticeably faster.
Run time and fan size
Steam does not clear during the shower, it clears afterwards. A fan wired to the light with no overrun stops the moment you leave, at exactly the point where the room is at its wettest. A timer overrun of fifteen to twenty minutes makes more difference than almost any upgrade.
Size matters too. Building regulations expect at least fifteen litres per second from a bathroom fan, and small budget axial fans in a large family bathroom with a power shower simply cannot keep up. A centrifugal or mixed flow fan copes far better with long duct runs. If your fan is an axial unit pushing through five metres of flexible duct, no amount of cleaning will make it adequate.
When to bring someone in
Wiring, replacing or relocating a bathroom fan is notifiable electrical work in a bathroom zone and should be done by someone competent. It is also the sensible moment to upgrade the ducting rather than reconnecting the old flexible run.
We deal with ventilation as part of bathroom installations and as a standalone improvement where mould keeps returning. If your sealant blackens within a year of being replaced, ventilation is where the money should go, and we will say so before quoting for anything else.