Right, I agree with you. The math I did was if you wanted to completely eliminate the old, and use computers to heat the house with a radiant floor heating / hot water combo system. That would be pretty sweet. But, very expensive and unnecessary for most folks.

What you propose is a very good intro level solution. Try making a small-ish hot water system that is axillary to the main system. If you coupled that system with an on-demand tankless system to pick up the slack - say when someone actually takes a shower...I'll bet that would be the only time it would ever turn on. Dishes, laundry, etc - when not done at the same time, would all be covered by your aux system.
Cool stuff to think about...