As part of the next big projec - the bathroom - I've been doing some thinking about the shower and how to run the extractor fan. It goes without saying that we need a fan, as the window in there is tiny and there's no other way for the steam to escape. My only issue was how the fan was going to be turned on.
One way of turning on a shower extractor fan is to have it connected to the bathroom light. However, this relies on the light being turned on in the first place. Which isn't ideal. Another option is a manual over-ride pull-cord switch. Again, this doesn't gaurantee the fan is on when the shower is on. Another better option is to have a humidi-stat switch that turns the fan on where there's a certain amount of water vapour in the air. How trustworthy these are I don't know.
The fool-proof option I've gone for is to use a flow switch. This switch simply fits to the 15mm copper hot water supply running in to the bathroom. As soon as there's a demand from either bath, shower or tap, depending on where it's placed, you can have the fan kick in.
I bought mine from here and paid about 45 quid for it. Whether it's as good as it sounds I've yet to discover. I'll report back when I know.