We are building a passive solar log home with solar panels to supplement electricity and radiant floor heat, both in concrete(lower level) and Warmboard. Is there any reason we could not use two tankless water heaters, one for the household hot water and one for the closed system radiant heat source? There are five of us.
Our current home in the same area has had several waterheater elements due to hard water,corrected by both a water softener and an ionic system.Our other choice is a LP gas system with two tank water heaters.I really want to be "greener" and tank heaters seem like such a waste.I know the tankless system is popular in Europe.The one I am looking at is the ESI-2000 made by Envirotech.
I appreciate your pro and con response.

Take a look at Baxi boilers. Wall hung,tankless and DHW plate HX built-in. Also modulating burner. Can be "twinned" for more capacity, if needed.

Sold to trained, certified installers only, but nationwide.

(No I do not work for them)

I originally had grand plans of using a Bossch/Aquastar 125 BLLP (the recirc heating unit, not the on demand hot water unit) to act as a boiler for the radiant heat system. I'm very glad that NRTRob convinced me to go with a Baxi instead. There's an awful lot packed into a small package with the Baxi. It simplified the design of the radiant heating and DHW systems. It's not as efficient as the condensing type boilers but it's a pretty good bang for the buck.