paulplumb
It sounds like you from what you have described I would hazard to guess that the coil inside the hot water cylinder has failed. This would necessitate a new cylinder. Is the central heating tank higher than the cold water storage tank? Isolate the feed to the cold water storage tank. If the water level in the cold water storage tank is still rising see if the central heating feed and expansion tank is filling. If so isolate the supply to the feed and expansion tank. If the water level in the cold water storage tank then stops rising it is a pretty safe bet that the coil has gone.
