boiler overheating

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.

The other contenders are sludge, or a slow pump. If these mean that the system can't get the energy out of the boiler fast enough, it's possible that the boiler stat isn't quick enough to react to the overheating.

The boiler stat could be contributing if it has "drifted" with age.

I would check the B Stat, then look at the pump for blockage or slow speed. If all OK then it's system sludge.

cheers