Amie, I finally looked up Baxi Luna. I was thinking just of heating water, not heating the house. When I did my brief searching, I was thinking of something like this:

Hubbell Ceiling Hung Horizontal Water Heater

Of course I'm interested in tankless, but I don't think I'd find it satisfactory in the winter. Having a tank like the one I linked to would at least let us put it in a space that otherwise wouldn't really be utilized.