The fan comes on, you can hear it working, but the mirror stays fogged for ages after a shower and the mould in the corners keeps coming back no matter how often it's cleaned off.
This is a very common and telling pattern: a fan that runs but doesn't actually clear the room is usually a fan fighting against a duct run that won't let it move enough air, rather than a fan that's simply broken.
Separating what you notice from what's wrong
The fogged mirror and returning mould are both signs that moist air isn't leaving the room fast enough, not that the fan has failed outright. A fan can be spinning perfectly well and still moving very little actual air if the ductwork it's pushing against is restrictive.
Causes, in order of likelihood
The duct run is usually the first place to look, because it's the part most commonly compromised during fitting and most often ignored afterwards.
- A long run of flexible ducting with several bends, each one adding significant resistance the fan has to overcome
- A fan that was undersized for the room, or for the length and number of bends in its duct run
- No make-up air path — nothing under the door or elsewhere for replacement air to enter as the fan extracts
- Ducting that terminates inside a roof void instead of properly outside, simply dumping the moist air into the loft
How to check which cause applies
Trace where the duct actually goes, if you can get access from the loft or an external wall. Establish whether it terminates outside through a proper vent, or simply stops in the roof space.
Count the bends in the run. Flexible ducting with multiple tight bends is far more restrictive than a short, straight, rigid run, even at the same length.
Check for a gap under the bathroom door. If the door fits tightly to the floor with no gap and there's no other vent into the room, the fan may be struggling to draw in replacement air at all.
Compare the fan's rated extract rate to the room's size, factoring in the duct length — a fan sized correctly for a straight three-metre run performs very differently bolted onto a long, bent one.
What a survey of the extract system should record
An assessment worth paying for should record the duct's full route, its length, the number and tightness of any bends, and exactly where it terminates — inside a roof void or genuinely outside through a vent with a working backdraught flap.
It should also note the fan's rated extract performance against the room's volume and the effective duct length, since a fan that looks adequately rated on paper can still under-perform once real-world duct resistance is accounted for.
Seasonal behaviour worth noting
Steam clearance problems are often most obvious in winter, when the bathroom is colder and condensation forms more readily on cold surfaces, even though the underlying duct and fan issue is present all year round.
A fan that seems to cope adequately in summer but leaves the room fogged in winter hasn't actually improved — it's simply working against a smaller temperature difference for part of the year, while the ductwork restriction stays exactly the same.
Common questions
Could the fan itself just be worn out?+
It's possible, and worth checking as one factor, but a fan running audibly and still failing to clear the room points more strongly to the duct run or make-up air than to the fan motor itself.
Is ducting into the loft dangerous?+
It's a common and serious problem — moist air dumped into a cold roof void condenses on the timbers and insulation, which can lead to mould and, over time, rot in the roof structure itself. It should be corrected rather than left.
Does the bathroom door gap really matter that much?+
Yes — without a way for replacement air to enter the room, the fan is working against negative pressure and can extract far less air than its rating suggests, even if everything else is correctly installed.
Book a free inspection of your bathroom extract system
We'll trace the duct run, check where it terminates and assess the fan sizing, and set out what's actually stopping it working.