Thats ridiculous that the extra gas heats a home with a tank, exactly
the oposite will happen with an open flue to a chimney, its called
draft.

Savings, at what price a therm, certainly not what is charged now. Look
hallerb you can run your numbers any way you wish to make yourself feel
better you don`t own one, but I have proof of a 4yr payback, my utility
bills. You need to talk to owners of Ng tankless.