same thing happened to me several years ago. After using lots of hot water, i would get a puddle underneath the relief valve. Plumber came out and replaced the relief valve. I questioned him, and told him that the water pressure gets pretty high right after showering, then the relief valve pops. I thought all along it was the expansion tank. Anyway, replacing the relief valve didn't fix the problem. called a different plumber who laughed at the first plumbers diagnosis and replaced the expansion tank. not a single problem since.