Right. I have a Potterton Suprima 60...Lucky me. On fire up its seems to run fine but it just keeps on running untill the water in the system boils and comes out the over flow. Obviously loads of banging as well as the air in the system expands. I have replaced the thermister and it worked for about a week. But now when ever it is switched on, either hot water or heating it just runs away.
Another PCB?........or something else???
Another PCB?........or something else???