My Ravenheat RSF100ET is some 7 years old and has had various problems but the latest one has troubled my limited skills. Recently it has started heating the radiators whilst hot water is in use, and I know that is either the 3 way valve or the DHW pressure switch. But now even with the timer off, and the main switch on DHW only, the heating is staying on all the time. Resetting the mains switch or even power cycling the supply does not stop the heating. After leaving the supply off for longer period, it seems to be okay, until it starts the heating yet again! I can't work out what would run the heating all the time.