A: Two different avenues come to mind. Heat traps are the first measure. Their purpose is to prevent hot water getting into the pipes by convection. They can either be bought in the form of short pipe nipples with a flap inside, or they can be made from long flex connectors. In any event, don't use the type that has a small "marble" inside as it can rattle. You'll find more info on heat traps elsewhere on this site. The other thought is that there might be a cross connection. This could allow hot water to leak into the cold lines. Check for it by shutting off the water supply to the heater, (only briefly for this test) and opening only a hot tap. It should run only a few seconds and then stop. If it keeps running, cold water is likely getting into the hot side and you need to find out where. In no event should you simply put a check valve on the cold inlet to the heater. Hope that helps. -- Larry (7/12/2006)