I have been asked about a problem with a combination cylinder - the hot water is coming up and into the cold water storage tank of the cylinder. This is after fitting a mixer tap in the kitchen. When either the cold or hot water to the mixer tap is isolated the problem does not occur. Am I correct in assuming that due to the high pressure from the mains cold supply is fed back to the hot part of the cylinder and pushing this water back up into the cold tank. Ball cock in the cold tank is working correctly. Any thoughts?