I have a Potterton boiler and although the timer is off, the boiler still comes on. I had the same problem years ago, and after much investigational work, the Engineer put the fault down to an electrical one (I think). There is a humming noise if that helps. My question is, what is the problem and when the Engineer fixed it (a few ago) the problem has returned. The boiler is nearly 20 years old.
It could be the zone valve sticking open and still calling for heat, but this would mean it's wired direct, not through the timer. Almost certainly an electrical fault as the timer should control all the electrics including the zone valve, pump and boiler. Possibly a timer fault. I have a Potterton Flamingo boiler from about 1979 and it still has the original thermocouple. Try not to be persuaded by salesment to replace it! My Brother used to sell combis to people who had perfectly working conventional boilers - NOT TO ME THOUGH!
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