For it to work well air should be drawn across the room. When air is sucked out of the bathroom, air will come in from somewhere else to replace it. Ideally the source of the replacement air (usually the door) will be diagonally opposite so that the air travels across the entire room collecting steam and erm... smells as it does so.
In reality it's not always possible to do this when the fan has to be sited on an external wall. So a ceiling fan can be a better solution.
To work out the extraction rate, calculate the size of the room in cubic metres. Depending upon whom you ask, for a bathroom generally the recommended air exchange rate seems to be somewhere between 6 and 10 changes per hour. So if your bathroom is say 25m3 a fan with an extraction rate of 150 to 250m3/hour would be required. If you do use a ceiling fan the length of duct should be as short as possible, because the air resistance it creates will lower the performance of the fan.
Also to be considered. Is the fan suitable for the location it is installed in? If in Zone 1 (less than 2.25 metres from the floor when installed above a bath or shower) it needs to be suitably rated for such a location (IXP4 as a minimum).
A 'run on' timer will help clear the room after use. We have ours on a PIR set to run on for another 20 minutes after we leave the room.
Heat is also important. Mould is caused by lack of heat, in that if the walls & ceiling are cold, any moist air that comes into contact with them will condense even with a fan running. Also, warmer air carries more moisture than cool air, and so the air being expelled by the fan will contain more moisture if it's warmer. Not ideal when we're all trying to conserve heat though, but that's the physics of it. If the problem is with the ceiling is there sufficient insulation above it?
