Shower fans... again

[shewantsmybody..inabag]

I've been searching for an answer but not found it yet.

I have an extractor fan for the bathroom, it drops from 240v to 12 volt with timer. I want to wire it in to the shower wiring so it runs when the shower pull cord is on, ie forcing the fan to be used when the shower is used (and remaining on for a period after). to turn the fan off they will have to switch the shower off at the pull cord.

The reason I want it like that is because of the big window in the room, the lights are only used during darkness and trying to get them to use the light for the fan to work is impossible. The room is getting very damp.. They don't like opening the window either.
I understand that I need the pull cord switch but will also put a fused switch in the airing cupboard so it can be independently isolated.

Is it possible/legal/safe to do it like this or is there another safer/legal way to do it.

 

[bernardgreen]

Use a humidity sensor to control the fan. Covers all eventualities of steam in the bathroom.

Wire the fan via an isolator to the lighting circuit and add the humidity sensor as per instructions supplied with it.

The only thing this will not cover is use of the toilet and removal of smells.

A passive infra red movement sensor controlling the fan would provide for that.

It helps a lot if there is an air vent in the lower area of the door. An extractor sucking from a "sealed" room does not move as much air as it should do.

I have for one friend fitted a switch operated by the door being opened to ensure a period of extraction happened whatever happened in the room.