Thanks for the reply Dana, and the info.
I should provide more info. Radiant appeals to me because I can get it done in a slab for cheaper than a crawl space with a finished floor on it plus a separate heating system like a conventional forced air system. I know thats not the case for everyone, but it is for me. The ductless split is intrigueing but we have pets and allergy sensitive kids, so I was trying to take out the air movement from the equation. (dander and dust). Also, an aquantanace who had a split system installed says the moving air always feels a bit cold. Could be just her I guess. Also, I wanted to use the tankless for houshold water as well, so I thought thats where a buffer tank would come into play. i.e. prevent hand washing from forcing the tankless to kick on every time, not to mention the cold water sandwiches. As I say, I am not a plumber, but I'm trying to educate myself so I can make the best choice.
Yes, an air tight envelope is part of the plan, likely a low density foam/ cellulose hybrid for starters, with major caulking, so I feel pretty good about starting with as reduced a demand as possible.
We do not have the option of ntural gas, so propane or electricity are my options. I worry about the volatility of propane prices more than electricity prices in the NW, Propane is about $2.40/ gal and electricity is about $.10 /kwh which is about a wash before comparing efficiencies, but I also don't know if efficiency cclaims can be compared between propane units and electrical, and if they can be believed, especially heat pumps. I guess I've started to question all the assumptions I'd made while planning the house. I suppose thats good because I'm not locked into one idea/ solution/. But I need to come to a decision here pretty soon, and got to this site after my plumber laughed at my idea of doing infloor heat with electricity. He suggested propane and swears by Navien tankless. Thats the system he used with his in-floor system in the same neighborhood. What I've read here so far makes me lean more towards a Noritz or Rinnei, but obviously I'd have to committ to propane. The one thing I know for sure is that I don't know enough, so thanks again for the info and if you see a good heating solution somewhere in all this let me know.
With $2.40 propane as your fossil fuel and 10 cent electricity your heating cost will be at least 2.5x that of doing it with heat pumps.
At $2.40 per 91000 BTUs and a 90% average burner efficiency that works out to about 10 cents/kwh, delivered- heat, but with a heat pump you'd most likely get a COP of 2.5 in winter, better than that in the shoulder seasons.
With a continously variable interior unit the air movement issue is very slight (some use 2-speed AC motors, but the better Mitsubishis all use variable DC) and you can bump up the temps 2-3F for higher comfort and still be ahead. Placement of the interior units where they won't be blowing directly on you (at any speed) is also an important comfort factor.
Ecotope (a consulting company in Seattle) has an ongoing study of primarily retrofit heat pumps all over the PNW commissioned by the BPA, some of which is available on online if you want to seek it out. You'll get a COP > 2.5 in Bend if you size it right, and at least 2 even if you don't.
There's no particular cost-advantage to going with a propane tankless with a separate tank for HW compared to a condensing boiler + indirect. With a radiant slab and an even modestly high-R house your heating water temps on design day will never exceed 100F, and you'd need 120F+ for the tank.
Design the house for the minimum heat load, THEN decide what mechanicals make the most sense. But there are 2-ton mini-splits heating high-R homes in much cooler climates than Bend, that cost less up front than a propane tankless + tank + radiant floor.
With a Daikin Altherma air-to-hydronic heat pump and a low-temp slab you'd probably average a COP of ~3 in Bend, maybe even a bit more, but it'll be at least 2x the upfront cost of a mini-split.