The pressure in your system is getting too high. Quite a likely cause is that the pressure vessel has lost some of its intial charge and there is not enough air in it to accomodate the expansion in the system when hot. If the system has not been checked for a few years this is a definite possibility. To be sure - you will need to drain the system down so there is no pressure at all showing on your gauge, attach a pump to your pressure vessel and pump that up to the recommended pressure (I think roughly 12 psi, but don't quote me). Then you can refill the system and hopefully all will be well.
By the way your system should have a pressure relief valve that opens at approx 3 bar to prevent anything blowing up but every time it opens the system loses water as well as pressure!
Hope some of that helps.
Richard, January 2009
