Hot Tap Water...Faster

I haven't studied up on the on demands but I did broaden my knowledge on our boiler replacement a few years back. 3500 square feet and it heats the house heating only a couple gallons at a time. I got a Weil McLain energy efficient boiler. In essence it is a "on demand" design. Saved me $100 a month average on my heating bill over the old 6' long cast iron boiler we had.