See if you can turn on a cold tap very slowly to replicate the fill valve, I dont fix boilers but does anyone know if a worn divertor will trigger the boiler if a cold is turned on a certain way?
Just thinking out loud really, also maybe crossflow? You've somehow tee'd the cold cistern feed into the hot aswell?
Just thinking out loud really, also maybe crossflow? You've somehow tee'd the cold cistern feed into the hot aswell?