there are many things that this could be so it's difficult to say. try and run the hot water when the heating is cold, feel the heating flow pipe directly beneath the boiler and make sure it stays cold. none of the hot water should be passing down this pipe with the tap on. also check the gas rate on dhw demand, it should be 24kw net (hence isar 24HE) it could also be a blocked calorifier due to sludge. hope this helps!
andy gas man
March 2009
howtomendit.com for free repair help, information and advice.
