If you have public water and not a well, a expansion tank is necessary and code required because essentially you have a closed system because of back flow check valves in the water supply.The tank is designed to absorb the excess pressure of the hot water because of thermal expansion. If not installed it stresses the system with too much pressure, causing premature failure of the HWH. If you have a well it already has a bladder tank so therefore one is not usually required.
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