Just a shot in the dark,I know it's combi but do you have a hot water storage cylinder on the system,sometimes this is done if there is more than one bathroom in the house.
If so the coil may have gone and a new cylinder is required.
Black water suggests the central heating water is getting in to the domestic water.Be very careful this could call health problems if there is inhibitor in c.h.water.
Glen
October 2009
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