The rates you actually need
Bathrooms and shower rooms in England and Wales are expected to achieve intermittent extraction of at least 15 litres per second, which is roughly 54 cubic metres per hour.
That is the minimum, not the target. In a room with a power shower, a poorly insulated external wall or no opening window, we would specify comfortably above it.
Continuous-running fans work to a different, lower trickle rate with a boost, and are the better answer in ensuites and internal rooms that see heavy use.
A fan serving a room with no opening window needs a run-on timer so it keeps working after the light goes off.
Fan types compared
The right choice depends far more on duct length and room use than on the fan's headline noise figure.
| Type | Supply cost | Fitted cost | Noise | Best for |
|---|---|---|---|---|
| Basic axial | £20-£50 | £150-£250 | Moderate | Short duct straight through an external wall |
| Quiet axial with timer | £50-£120 | £200-£320 | Low | Most family bathrooms |
| Centrifugal / mixed flow | £90-£200 | £280-£450 | Low to moderate | Long or ceiling duct runs |
| Inline loft-mounted | £120-£280 | £350-£600 | Very low in room | Ensuites, internal rooms, quiet-critical |
| Humidity-sensing continuous | £140-£350 | £400-£700 | Very low | Persistent condensation problems |
Ducting is where fans fail
Flexible concertina ducting robs an enormous amount of airflow. Rigid duct, or at worst smooth-bore semi-rigid, is what we install as standard.
Every bend costs performance. Two right angles in a short run can halve what a basic axial fan actually shifts.
Ducting must terminate outside the building. Venting into a loft void is a damp claim waiting to happen, and we see it more often than you would think.
Duct through a cold loft needs insulating, otherwise warm moist air condenses inside the pipe and runs back down to the fan.
The external grille matters. A restrictive grille with fine mesh can undo the whole installation, so we use a low-resistance cowl.
Fitting, wiring and certification
Bathroom fan wiring is notifiable electrical work in a special location, so it needs doing correctly and certifying. Our own electrician handles this, so nothing waits on a subcontractor.
Fans over a shower or bath must carry the correct IP rating for the zone they sit in, and be supplied through the appropriate protective device.
A run-on timer needs a permanent live as well as a switched live, which is why retrofitting one to old wiring sometimes means a new cable rather than a swap.
Replacing like-for-like in an existing position is usually a half-day. Adding a new fan with fresh ducting and wiring is typically a full day.
Signs your current fan is not coping
The mirror stays fogged for more than ten minutes after a shower.
Black spotting appears in the corners of the ceiling or along silicone joints.
Paint peels above the shower, or the ceiling feels cool and damp to the touch.
A sheet of paper held to the grille does not stay put when the fan runs, which is a crude but effective airflow test.
Frequently asked questions
What size extractor fan do I need for a bathroom?
At least 15 litres per second of intermittent extraction, which is around 54 cubic metres per hour. Rooms with heavy shower use, long ducting or no window should be specified above that.
Can an extractor fan vent into the loft?
No. It has to terminate outside the building. Venting into a loft dumps warm moist air into the roof structure and causes condensation and staining.
Are inline fans worth the extra cost?
In ensuites and internal rooms, usually yes. The motor sits in the loft rather than in the ceiling above your head, so the room is far quieter for the same airflow.
Does a new fan need certifying?
The electrical work in a bathroom is notifiable, so yes. Our own electrician does the installation and you get the paperwork as part of the job.
Will a better fan stop my mould?
It usually helps a great deal, but mould is a moisture balance problem. If the room is also cold and under-heated, ventilation alone may not solve it.