I moved this thread from ICF section (like to share my ICF idea), I think the continuation of it fits better here. What we are thinking of building is an ICF rambler about 2500 SQF. dmaceld built this home. (I built my house in Parma, ID where the low temp went to -10°F these past two winters, and high has been over 100°F. I considered radiant floor, then radiant ceiling for heat and cool, and finally went with a Daikin air to air heat pump. The air handler is in the crawl space with short ducts in 3 directions. The crawl space is sealed with a concrete over insulation "rat" slab. The floor is Advantech subfloor on I-joists with hardwood and tile with no insulation. The roof is foam insulated underneath the decking and I use the attic for the return plenum. The end result is a quasi-radiant heated floor, warm or cool air circulating through the house constantly, no cool or hot drafts, and very comfortable uniform temperature throughout the house all year. I have no water system to contend with, and the Daikin system is quite reliable.) I kinda warming up to this idea. being that it's a heat pump system lower temps, shouldn't have any problems with combustibles. It would be located on a big sand pile with 17" of rain snow a year so not really a water problem. Have to do the standard radon mitigation. The only other thing that might be a health concern is the pluming. But when you think about it, if a quality instillation was done that would be a non issue. Also would add a fresh air exchange system. Does any one see a issue with this system?

I installed a french drain on both the inside and outside of the footing with a vent going up to the roof. This, along with the concrete rat slab, I expect to capture any radon that may be present and passively vent it to the outside. The only other hitch is the building codes prohibit drain plumbing cleanouts and fuel piping in the crawl space because it is considered part of the living space. Also, the interior surface of the foam had to be covered with fire resistant covering. I used 1/2" drywall. I'm all electric so not having gas piping was a non-issue. I considered, and had in my plans, a gas fireplace in the living room but the pipe for that would entered through the outside wall. I was able to arrange the drain piping so that I could extend each of the lines that needed a cleanout through the wall to outside. Slight complication to the design but not an onerous restriction.

We to are going to be all electric. I kinda think it's funny, you can't have clean out in a condition crawl space but I be leave you can ave one in a full basement. Might be wrong on that one. Dmaceld what HRV system did you use?

Why Daikin? Why ducted? Most Daikin ducted units don't even have a rated output below +14F or so, even though some of their wall-unit types can operate as low as -4F. A Fujitsu xxRLFCD mini-ducted unit would at least have a rated output at -5F. With a wall-coil type unit there are many models with a rated output at -10F, but there are no Daikins on that list (mostly a lot o' Mitsubishis & Fujitsus.) But that's not to say it'll work.

To get heat up through the ~R2 of floor the crawlspace air temp has to be running substantially higher than the conditioned space temp, and the colder is outside the higher the heat load, and the higher the crawlspace temp has to be to meet the load. This would cut substantially into both the capacity and efficiency of the heat pump. If it has to be 90F in the crawlspace to heat the place when it's -10F outside you are kinda screwed- it just isn't going to happen with any mini-split.

What is your design heat load, and at what outdoor temp?

How many BTU per square foot of floor does that work out to?

If the heat load at -10F is 25,000 BTU/hr (it might be that low or it might not, I haven't looked at the house design, but YOU need to know that number to settle on any heating solution), and you have 2500' of radiant floor, that's 10 BTU/ft^2 of floor area. To deliver that much heat the floor has to be about 5F warmer than the room temp. If your indoor design temp is 70F, the top of the floor needs to be 75F.

To push 10 BTU/ft^2 through ~R2 of floor materials the warm side needs to be 5F hotter than the warm side, which means the crawlspace has to be 80F. For the crawlspace to be 80F, the output air on the air handler has to be 125F or higher (you'd have to look up the cfm and other specs on the air handler unit). The capacity is going to be quite a bit lower than when delivering heat into a 68F room, as in the HSPF testing submittals, etc, and the efficiency will be too.

Given your temperature extremes, if it's not on this list, it doesn't have much of a chance of working:

https://www.efficiencyvermont.com/docs/for_partners/contractors/evt-cchp-qpl-bymanufacturer.pdf?v=6

Even if it IS on the list, you'll need to dig out the extended capacity charts on them and interpolate what it's capacity might be at your higher indoor temp.

If you come up with a combination that looks like it might work, be sure that there is a wall-thermostat that works with whatever that unit is, or it will be very difficult to keep room temps from under & overshooting your comfort range.

OK- looking at your other posts I see I was confused. You were using dmaceld's description of HIS house, not a house that dmaceld is building for YOU.

The 99% outside temperature  bin for Spokane is +7F, which won't be nearly as bad as -10F.

With suspended slab poured on steel deck and used tile rather than hardwood the R-value of the floor would be less than R1, and the delta-T required to get that heat out of the crawlspace and into the house above won't be as severe.  This is much easier to deal with.

It's still important during the design phase to keep track the design heat load relative to the available amount of radiant floor.  At 10BTU/ft^2 or less it'll probably work, but it's a lot easier  & better if you can bring that down closer to 5 BTU/ft^2.  If you're drifting up toward 15 BTU/ft^2 it's going to be very difficult to make this approach work.

Thanks Dana1, Sorry if there is some confusion I originally started this thread in the ICF section but I felt this fits better here. I be leave the way Dmaceld has his house designed is he has the crawl space heated but through floor registers condition air is pushed up into the living area and the bonus is since the floor is not insulated they give out a slight radiant effect. He has stated that in cold weather his crawl space is about 80F and the living space is +70F. His 2000 sf house was built around 2009 so he does have some history with it and seams to be quite happy with it.

I like to build a ICF home but looking for the best heating options with reasonable ROI. Wife has a circulation problems so the radiant floor would help her feet. I do like the idea of condition attic and crawl spaces for the health and efficiency reasons. I would like the floor to be it a full radiant system but Dmaceld's has a interesting system here.

I originally looked at hydronic radiant in a concrete slab/metal deck system with a geo water to water heat pump. The other option would be slab on grade set up again geo water to water. Lastly a suspended slab with build deck (ESP foam) but then why if your going to condition the crawl space? I'm not afraid of spending the money but looking for a descent ROI. BTW build deck and metal decking works out to about the same price shipping is the variable being foam is more expensive to ship and I can get the metal decking in Spokane.

I also wish air to water heat pump was more readily available in the US.

If you build the suspended slab over an insulated conditioned crawl space WITHOUT insulation at the slab you can turn it into an effective radiant floor with a mini-split, ala dmaceld's approach (without the duct registers). Done that way it will work BETTER than his, since the heat transfer through the floor would be more than twice as good, and you wouldn't need to keep the crawlspace at 80F to maintain 70F on the first floor, 75F would do it. That has real consequences for both efficiency and capacity.

You'd still need to insulate the floor & walls of the crawl space, but in a modest heat load house it should work just fine.

Like anything else, it'll have a better chance of success if you actually design it. Until you know the heat load and the total square footage of available radiant floor you really don't know anything- certainly not enough to design a heating solution with. If you're just hacking at it, punt: Run radiant tubing in the floor as if it were the total solution, and if the crawlspace mini-split doesn't keep up, hook up an electric boiler as the "Hail Mary" backup second stage or something. But it's cheaper and easier to just do all the math up front and KNOW that it's going to work (with some margin).

Dana1 right now I'm just thinking out loud. If this is a viable solution i would go and find a pro to help with the math. Question, Is there a reason why you couldn't or shouldn't install floor registers first floor?

If you want air conditioning you definitely want the registers. (You'll also have to figure out the condensate disposal issue.)

If you don't care about AC you can probably use a wall-blob type mini-split down there. You'd also need to provide some minimum amount of air exchange with conditioned space air to guarantee that the crawlspace doesn't stagnate with too much moisture or other gases.