Hi i wonder if anyone can shed any light on this problem (so i know what i'm talking about before i get an engineer to sort it.)..When i run the hot tap with the central heating turned off...the radiators start to warm up, and the hot water is tepid....the boiler fires up as normal..The diaphragm in the casing at the front left hand side has been changed ( made no difference)......the heating works just fine ..Bill