when only heating is selected on my program timer the radiators all work fine and are hot. But when both heating and hot water are on the water gets hot but all the radiators go cold. When the hot water is up to temperature on the tank stat the radiators still don't get hot until I turn the heating off then the hot water off then on and off again. Then turn the heating on again and it works ok. I have a 3 position zone valve and this makes a banging noise when the water is first turned on. Could it be the valve that is faulty because I have had to replace it twice now in the past 3 years. The current one is about 13 months old. Or could it be the pump that's the problem?