Updated
Bathroom Fan Not Clearing The Steam
The mirror stays fogged, the walls run with water and mould keeps coming back in the corners.
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What's happening?
A bathroom fan has one job: to remove the moisture a shower puts into the air before it condenses on the coldest surface in the room. When it cannot, everything else in the bathroom suffers — sealant goes black, grout stains, paint peels off the ceiling and the room smells damp. The fan being noisy is no proof it is working: the commonest fault is a fan that spins happily while moving almost no air, because the duct is crushed, disconnected in the loft, blocked, or far longer and more twisted than the fan was rated for. Building Regulations set minimum extract rates for a bathroom, and a fan that cannot achieve its rate through the ductwork it actually has is not doing the job whatever the box claimed.
What usually causes it?
- Flexible ducting crushed, kinked or sagging in the loft, choking the airflow
- The duct disconnected from the fan or from the outside terminal, dumping moist air into the loft
- An external grille or cowl blocked with debris, paint, or a backdraught shutter stuck closed
- A duct run far longer or with more bends than the fan is rated for, so the actual extract rate is a fraction of the claimed one
- The fan undersized for the room, or an intermittent fan where continuous extract was needed
- No gap under the bathroom door, so no replacement air can get in and the fan has nothing to pull
- The overrun timer set too short, or the fan wired to the light alone, so it stops long before the room has cleared
Is it dangerous?
Not directly. The consequence is damp: persistent condensation grows mould, and damp and mould in a home is treated as a health risk, particularly for children, older people and anyone with a respiratory condition. There is a hidden building risk too, where a duct discharges into a loft rather than outside: that puts every shower's worth of moisture into the roof space, wetting timbers and insulation. Altering the fan's wiring is restricted electrical work in a room with a bath or shower, and it is covered by Part P rather than by whoever is fixing the duct.
Can I fix it myself?
Testing it is easy. Hold a sheet of toilet paper against the grille with the fan running — a working fan holds it there, a failing one drops it. Take the cover off and clean the impeller and the grille, since dust alone can halve the flow. Look at the outside terminal and clear anything blocking it. Check there is a gap under the door for replacement air. What you should not do is rewire the fan, extend ducting into a loft void, or replace a fan yourself in a bathroom zone.
When should you call a bathroom fitter?
Call a bathroom fitter if the paper test fails after cleaning, if the duct is crushed or you cannot trace where it goes, if it discharges into the loft, or if the room still fogs with a fan that appears to run. Correcting duct runs, resizing the fan for the room and the run, and getting a terminal through the wall or roof is fitting work. Where the fan needs new wiring or a new supply, that part falls under Part P and is done by someone competent to certify it.
What will the bathroom fitter actually do?
- Measure the actual extract rate at the grille rather than trusting the rating on the fan
- Trace the duct end to end and find the crush, disconnection or blockage
- Replace long flexible runs with rigid or semi-rigid duct, insulated in a cold loft, on the shortest sensible route
- Terminate properly through a wall or roof with a clear cowl, never into a loft or a void
- Fit or resize a fan appropriate to the room and the duct run, with the overrun set long enough to clear it
- Confirm replacement air can enter the room and re-test the rate before finishing
The paper test
- Close the window and switch the fan on.
- Hold a single sheet of toilet paper flat against the grille and let go.
- Held firmly: the fan is moving air, so look at run-on time and room size instead.
- Falls immediately: the flow is poor — clean the impeller and grille and repeat.
- Still falls: the duct is crushed, blocked, disconnected or far too long.
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