I am in the middle of a new tankless install, and want to be sure of one thing... is an expansion tank needed? I never had one on my tank based system, and never had issues.
I understand the basis behind requiring it with a storage based system, where the heating of the water creates pressure within the closed system. But there is no storage within a tankless system.
If you never had one on your old system than i would think one is not needed for the tankless.Generally,exp.tanks are needed when a back flow preventer or PRV with internal check are installed on the water line to the house.