Why expansion tank needed ?

[raypunzel]

Upon buying our home a few years ago, the hot water tank started leaking through the overflow tube. We were told that the tank needed replacing. The home warranty covered the cost of the tank. My question is why an expansion tank was needed along with the replacement water heater when there wasn't one plumbed in before?
BTW, it is a gas heater.
Ray

[havasu]

I'm not a bonafide plumber, but it sounds like your temperature / pressure relief valve is faulty. I've had a few go bad and are easily fixed. Just make sure you shut down the system, drain the water, replace the valve, and fill it back up.

[Nailer341]

Some of the codes changed in 2007. Some cities require the expansion tanks whether written in the state code or not.
Why do you not want the tank?

Hereis a very clear explanation of the expansion tank.