The instinct is often to buy a more powerful fan, but if the duct run is long, bent and terminating in the wrong place, a stronger fan just fights the same restrictions harder — it rarely solves the problem on its own.
A proper fix looks at the whole system: the fan, the duct, where the duct actually ends, and whether the room can draw in replacement air at all.
What has to be opened up
Accessing the duct run usually means going into the loft space or behind a section of ceiling or wall lining, depending on the route, to see its actual length, number of bends and where it currently terminates.
The correct sequence of works
Each part of the system affects the others, so it's worth addressing them together rather than one at a time in isolation.
- Shorten and straighten the duct run as far as practically possible, replacing multiple loose bends with the fewest, gentlest bends the route allows
- Terminate the duct properly outside through a wall or roof vent fitted with a working backdraught flap, never left ending inside a roof void
- Size the fan correctly for the room, factoring in the actual duct length and bends rather than the room size alone
- Provide a make-up air path — typically a gap under the door or a dedicated vent — so the fan can draw in replacement air as it extracts
Why simply upgrading the fan doesn't work
A more powerful fan pushing against a long, bent duct run still loses much of its rated performance to resistance in the ductwork, and if there's no make-up air, a stronger fan can end up drawing air back down the very duct it's trying to push it out of. The fan is rarely the actual limiting factor.
Interfaces with other trades
Where the duct route passes through a roof void, coordinating with whoever manages the loft insulation matters, since insulation packed around the duct run can trap moisture if the duct isn't properly sealed. Electrical connection for a replacement or upgraded fan needs a qualified electrician, particularly given the fan sits in a bathroom's regulated zones.
How you know the repair has worked
Test by running a normal shower and timing how long the mirror stays fogged afterwards — it should clear noticeably faster than before. Over the following weeks, check the corners that previously grew mould; if the moisture is genuinely being extracted, mould shouldn't reappear at the same rate.
Common questions
Do I need to replace the fan as well as the ducting?+
Not always — if the fan is correctly sized once the duct run is shortened and straightened, it may perform well without replacement. It's worth reassessing the fan sizing against the improved duct run before assuming a new fan is needed.
How disruptive is correcting the duct route?+
It depends on the route — loft access is usually straightforward, but a run through finished ceilings or walls may need some opening up and making good, which is worth pricing before work starts.
What counts as a proper external termination?+
A vent fitted through an external wall or roof, with a backdraught flap that opens to let air out and closes to stop cold air and insects coming back in — not simply an open duct end left inside a loft space.
Get a written quote to fix your bathroom extract
We'll correct the duct run, terminate it properly, size the fan correctly and quote the whole job in writing.