i have a gloworm space saver boiler. The room stat is set to 15 degrees and the tank stat set at 55 degrees. If i increase the room stat to anything higher than 17 degrees then after the heating has been on a while, the hot water tank starts boiling and the hot water vents into the expansion tank in the attic and out of the overflow onto the garage roof. stat on hot water tank is new and everything else seems ok ie room stat..is boiler stat at fault or any other suggestions please.