My brand new hotwater heater keeps releasing 8oz of water from the pressure valve. We replaced it but the problem perssists. The warranty company is saying that we need to install a thermal expansion tank for the overflow water. Is this normal? What happens when the expansion tank fills up?

One of the three things are happening:

1. Pressure valve in bad
2. You have too much pressure in your plumbing system
3. Thermal Expansion

1. We always replace pressure relieve valve with every new heater installation. Did you replace yours ?
2. Pressure in your house should be set at around 55 psi. Check your pressure. Adjust as necessary
3. If Thermal expansion occurs, install Thermal Expansion tank

Back to you. Milo

Bump

Thermal expansion tank does not fill up with water. Thermal expansion tank is a tank of air. When water is heated it expands, the pressure in the tank goes up and water flows into the expansion tank. When you open a faucet pressure goes down and water flows out of the expansion tank. Generally you only need an expansion tank if you have some device on the cold water line, like a pressure regulating valve, that prevents water from moving back up the cold water line.
So, yes this is normal and the expansion tank does not fill up. It is maintenance free. Easily installed by any plumber or anyone who can solder pipe, assuming copper pipe.

Thank you very much for the information.Very Helpful! Also Thank you for fixing the admin problem I was experiencing.