Hello gentleman, and if your not a gentleman I would still appreciate your weighing in on this... it doesn't make a bit of sense to me. I just installed a third water heater in our house after a couple water heater failures in fairly rapid succession. The plumber that installed our first new water heater when we bought the house, did not incorporate an expansion tank into the installation. I decided I shoud to install the newest water Htr exactly like recommended in the installation instructions, and per code requirements with a new 5 Gal expansion tank. Or house has copper water distribution lines. We've lived in our house 5 Yrs, and proir to the installation of this new expansion tank we had never heard any pipe hammer. I installed the expansion tank in the 1" main line within 5 Ft of the new water Htr. Suddenly when turning on the tub hot water proportioning valve in the closest bathroom, there's a new pipe hammer. Any ideas???