by eggers »
Fri Jul 23, 2010 6:27 pm
suggests to me some kind of valve fault. i assume you do not have a combi boiler.
Is the system an "s-plan" system, with 2 or more zone valves? Under that circumstance i think you will find the hot water valve is not bringing the boiler on, but with heating on the heating valve does bring the boiler on, and as the hot water valve is already open, you are heating the tank aswell.
If it's a combi boiler, chances are the diverter valve is at fault, which boiler do u have?