Expansion Tanks are somewhat new necessary accessories for Water Heater installations. They are usually steel and have a rubber bladder in them to hold water. Water when heated expands. That additional volume of water needs to go somewhere or it will stretch the piping walls or break the heater. often it will cause a leak on weak joints or cause a relief valve to open. That relief valve might remain open because of dirt in the lines. Before the excess would simply push back into the city water supply.
That has changed.
Cities are putting water check valves or one way valves at the meters now to prevent water contamination from the customer lines - a good thing - but the result is you must install a expansion tank.
Additionally, these expansion tanks have a effective life expectancy of about 5-7 years before the internal bladder fails make a very dangerous water logged tank weighing heavy on the supporting pipes.
Talk to me about retrofitting or replacing your tank. Ken Anderson