Questions on Radiant Heat With Water Heaters To the Editor: Good article on using water heaters for radiant heating ("Using Water Heaters for Radiant Heat," 11/98). It answered several questions I've been asking myself for awhile. I still have one question: What's the function of the expansion tank? Certainly there will be changes in the volume of the system as hot water warms up various parts of it, but the same applies to any plumbing system. I've always assumed the difference was made up by flow back into or out of the supply mains. Bob Fankhauser Portland, Ore. Bill Clinton responds: Traditionally, plumbers have indeed relied on volume increases flowing back into the mains and have not installed expansion tanks on water