My hydronic heating is not in the floor. It is in the ceiling space between the floors and has an electrical fan to blow the hot air through the vents. Most of the Eichler homes have hydronic heating in the floors.
My. Still building Eichler homes (!). I was in California when these were first being built. In an art/architecture class they were hailed as an inovation in affordible architecture. There were the first mass produced homes to have an atrium. Good to see that a good thing has persisted.
Not building them--maybe renovating but it's been many years since all the Eichlers were built. Newer homes out here now are mostly stucco-covered tract homes. People still like Eichlers, a well-maintained one can command a nice price if you're selling it, and there are lots of people who've bought ones that got a little run down over time and they're trying to restore them to their original glory but they haven't been built in many years.
So were these homes supposed to be energy efficiant etc. I googled them and see this mentioned anywhere. Or did they just use the hydronic radient heating? More info please. I like the designs I saw with the atriums etc. I'm assuming this was another Eichler trademark.
There are many older Eichler neighborhoods in the SF Bay Area. Eichlers designs were some of the first to "bring the outdoors inside". They were energy efficient for their time (and many still are) and relatively inexpensive to build as well as distinctive and attractive in design. Eichler had also designed some low income housing projects in San Fransicso that were quite attractive to look at and didn't yell "tenement". Sadly I think these have since been torn down and replaced with an uglier model.
Quite tragic that Eichler himself died in poverty.
Re: hydronic heat from a tankless water heater. I am not a plumber - but since no one else answered - my best guess is that it wouldn't be cost effective because of the volume of water needed. I'm guessing that your tankless heater would be running all the time to keep up the the flow and electricity is not an efficient fuel source to heat water. Maybe someone who knows about plumbing will answer your question.
I believe the hydronic floor systems are self contained so the system has to heat the water only one time, from then on temperature is maintained by a thermostat. Some of them are also used for cooling.