Im studying plumbing at the moment and from what your saying it sounds like the 3 way valve needs replacing.
The boiler heats water and from there it goes to your airing cupboard, if hot water is required for central heating then the water goes to your rads, if hot water is priority it goes into a coil inside your hot water cylinder that heats up the water inside it.
So i would assume it has something to do with this 3 way valve, could be blocked or needs replacing.
vic gouldthorpe
March 2009
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