heating comes on when we switch water on?

thank you ian for replying to my question.

jeffery seyed
March 2010

This could be caused by a faulty mid-position valve, this valve may have a motorised control.
It controls the flow of water in some heating systems into the heat exchanger in the storage tank (for hot water) and the radiator system.
It can allow the flow into one or the other or both at the same time.

If you can determine whether this is failing due to a faulty motor it is possible that you can replace this part alone with no need to change the valve, and some versions without draining the system down even.
good luck

gruff
March 2010

may be faulty timer, more likley faulty motorised valve

Ian
March 2010