I just read the info on thermal expansion. Thanks. One thing I do not understand. If I have a backflow preventer in my new tank, that would explain why I can not keep a tpr valve in this new tank, but how does an expansion tank on the cold water side help this situation. It seems that if I have thermal expansion problem created from a backflow preventer valve in the tank, that the backflow preventer would prevent the expansion tank from absorbing the additional pressure when it occurs. It seems that expansion tank would have to be on the hot side.