my boiler is over heating even when its just the water that's on?

I busted my heating for an hour last night.. it worked ok then again later on but this time the boiler over heated an heated the water in the cold water tanks. turned off but left the water on timer to come on this morning but age boiler over heated again heating the cold water tanks again building the pressure which shouted hot water out the over flow pipes.

Karen Towler
October 2012