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It does sound like the PCB is at fault. Does the boiler fire all the time correctly for the hot water?
You have a condensing boiler but a standard system. I take it you have a cylinder?
It may not be a boiler fault - it could be a system fault.
I am thinking the motorised valve.
When you turn the heating on the valve should open fully and fire the boiler.
When on hot water only then the valve will be closed.
But, if it always works with the room stat turned right up, do you not think it is a faulty room stat triggering too high?
No problem - let me know what he finds