thermal expansion tank for water heater?

do i need to add a thermal expansion tank to my new hot water heater? old one was 50 gall electric and did not have one. new tank is 80 gall electric?
thanks
Scott

I've seen very few old houses that actually had them. It is recommended for any closed system. Since most modern water supplies have a check valves to prevent water from flowing back into the public water system it is highly recommended.

When water heats it expands. Unless there is a check valve in the intake supply line, this expansion is absorbed by the water main. Some city meters have a check valve built in, and a pressure regulator valve also is a check valve. It either of these are present, then you have what is termed a closed system. Without an expansion tank in a closed system, the expanded water creates a huge pressure rise in the water heater and results in the TP valve tripping. The size of the tank would not make any difference, so it sounds like you do not have a closed system and do not need an expansion tank.