Choosing and siting a bathroom extractor fan
An extractor fan is the cheapest thing in a bathroom and the one that decides whether the expensive things survive. A room that clears its moisture in twenty minutes stays sound for decades. A room that holds it grows mould on grout, lifts paint off plaster and eventually rots the timber behind the tiles.
Work out the rating you need
Fans are rated in litres per second or cubic metres per hour, and the two are easy to confuse. For a domestic bathroom, 15 litres per second is the usual minimum, which is roughly 54 cubic metres an hour. A shower room wants more, nearer 20 to 25 litres per second, because a shower puts far more water into the air than a bath does.
Treat the number on the box as a best case measured with no ductwork attached. Real installed performance is lower, sometimes by half, which is why the next section matters more than the rating.
The ducting decides everything
A good fan on bad ducting moves less air than a modest fan on a short straight run. Each bend costs airflow, flexible ducting costs more than rigid, and a long run costs most of all.
Keep the route as short and as straight as you can. Use rigid duct where the space allows it. If flexible duct is unavoidable, pull it taut rather than leaving it concertinaed, because every ripple is resistance. Check that the duct actually reaches outside: fans vented into a loft void simply move the moisture problem into the roof, where it does more damage and is not noticed for years.
Where to put it
Site the fan as far from the main source of fresh air as practical, so the air crosses the room rather than taking a short cut. In a bathroom with a door and a window, that usually means the fan on the far side from the door.
Put it close to the shower or bath, where the steam starts, not in a far corner. Mounted in the ceiling directly above a shower, a fan catches the moisture before it spreads to cold surfaces.
The room needs a way for replacement air to get in. A fan cannot push air out of a sealed box. A gap of around ten millimetres under the door is enough and is the reason bathroom doors are often trimmed short.
Run it for longer than the shower
Most of the moisture leaves the room after the water stops, as warm wet surfaces dry. A fan switched off with the light has done perhaps half its job. A timer overrun of fifteen to twenty minutes makes more difference than upgrading the fan itself.
Humidity-sensing fans do this automatically and run only when needed, which suits a bathroom used at unpredictable times.
When to get an electrician
Bathroom wiring is zoned by proximity to water, and the rules on what may be installed where are not optional. Replacing a like-for-like fan on an existing circuit is often within reach of a competent person, but a new circuit, a new ceiling position or anything inside the shower zone is work for a qualified electrician.