I've just moved into a new flat (its about 10 years old) and have a problem with the hot water system. There was evidence of a leak from the expansion tank in the loft and signs of damp but no obvious cause without taking the tank out and checking. However, last night got woken up by the smoke alarm close the to tank. The hot water cylinder had been timed to come on at night, had obviously heated up and had filled the expansion tank with boiling hot water, the steam setting off the alarm. I turned off the cylinder and timer to stop it heating and wasting the water and its cold now, and doesn't seem to have made any noticeable extra leak marks but probably could've if i'd left it on.

So basically, whats the most likely cause for the expansion tank to be filling up with hot water? Its an electric immersion cylinder, the expansion tank has been relocated to a higher position than initially installed when built, and it just supplies hot water not central heating.

I've had a check on google but there're quite a few conflicting ideas.

Any ideas??