reply to Subaru
Re: Water Hammer: Possible Causes?
said by Subaru:They didn't in the past, but when city water services started installing check valves (to prevent back flow-say I pipe in a well or somehow taint my water lines), these became necessary.) Fill a cold water heater, and the water expands. In the past, it just pushed back into the city water supply. With the check valves, you PSI goes through the roof. Hence the need for an expansion tank. said by Bob:
A bit of a long-shot, but... If you have an expansion tank on your water heater, make sure it hasn't failed and filled with water.
Hmm I never knew water heaters used expansion tanks.. I know the furnace here uses it but I don't see one for the water heater.