At our continuing education class on Thursday of last week, To be Green or Not to be Green, a student inquired about historic tankless water heaters v. their modern efficient & green cousins that are now promoted by the energy efficiency movement. While I was sure about the effect of the difference, that the modern electric tankless models are quite more efficient, I was not sure about the specifics. So I researched until I found this article, which I think is quite informative. Click here to learn more.
Basically, the historic version included boiler-integrated tankless coils, which uses a home's space heating system to heat water as explained here whereas the modern cousin is an on-demand tankless model that only provides water when needed as explained here.