Working on the design of a 3100 sqft home, Zone 6, HDD 7500. Construction type is double stud r-40 walls, ICF basement foundation, r-15 under basement slab, r-60-70 in the vented attic. Combination of dual pane (shgc .57 approx. 9% glass to floor area) on the south and triple pane, u .21 on the north. Goal for tightness is <.1 @ 50 pac. The 3100 is split on the main floor and basement. The basement walks out to the north with only 3 bedroom size windows and a door. The current heat load for the total several of my programs are spitting out is about 19k BTU total. My thought was to put 1 12k btu Mitsubishi hyper heat unit on the main floor in the center and one in the basement. I plan on using stained concrete floors in the main basement space with carpet in the bedrooms. On the main floor the plan was to use bamboo floors in the main spaces, and carpet again in the bedrooms. However we are starting to shift to putting light weight concrete in the main spaces and keeping carpet in the bedrooms. Even after beefing up the floor joists and added a second layer of 3/4" subfloor, it still makes 1.5" stained concrete cheaper then the wood floor option. We have dogs, and out current wood floors are getting beat up so we are thinking concrete might be nice. I also would like to get some solar storage gains out of my windows and the slab as well. However we need a form of backup for the 2-5% of the year the temps dip below what the hyper heats can produce (which is -17) but they also drop big time in output at those temps. The thought was to put a few electric baseboard heaters in the house to pick up the slack. However I got to thinking more about making the slab on the main level a radiant slab. The area of the slab would be approximately 950 sqft. Carpeted areas would be on wood subfloor, no slab. Would it make sense to make this slab radiant slab to double as a heat source and cover the backup heat for that area (electric base boards still in bedrooms)? My thought since the heat load would be so small (approx 6000-7000 btu) a large water heater could cover this and the DHW. I would design the lines to run north/south across the house hopefully running through the sun warmed slab on the south to make it more efficient at heating the north slab. Any thoughts on this idea? Is this over kill? My thought was it would be minimal costs due to the low heat load and going the water heater I would already have route vs a boiler route.

We have dogs and concrete. We love it. Ours is also a small 1.5" slab ( we are 1200 sq feet) I would run the pex in their no matter what. There is really nothing better than warm concrete floors in the morning. We don't run the zones in our bedrooms because we like it cool at night. That does create a problem if you use the bedrooms a lot during the day or have guests that like it warmer. I have often thought that a small electric base board would solve this problem. The heating of the water could go several ways and the experts here know more about the current solutions.

"Worth it" is highly subjective- how much are you willing to pay for the additional comfort? Plumbing PEX into the floor and using an electric boiler (or fossil-fired HW heater) would work, and would leave you the option of using a different heat source at some later time (such as R744 refrigerant heat pumps that can be made to operate lower outdoor temps than you can get with R410A.)

Unless you go with a commercial unit, an electric HW tank probably isn't going to cut it. A residential type ~6kw unit is only ~20KBTU/hr total, roughly your design day load, which leaves you no overhead for DHW. You'd need at least a 9kw unit, and 10kw would be even better, and you're talking ~$2-4KUSD for the commercial hot water heater, ~$1.5K for a lesser-duty lesser warranty version (eg: http://www.pexsupply.com/AO-Smith-DEL-50D-50-Gallon-Dura-Power-DEL-Commercial-Electric-Water-Heater-Lowboy ) Then there's the isolating heat exchangers & pumps, etc, and as a system it'll be as expensive as another mini-split, maybe more.

You might consider costing out a Daikin Altherma (R410A just like the H2i mini splits, but hydronic output rather than air) and low temp slab radiant floors as the total solution rather than the pair of 1-ton mini splits + backup system. The compressor on the Altherma kicks off at outdoor temps below -4F and cranks on the resistance elements, but it's seamless operation and pre-engineered as a system. In a high-R house with slab radiant heating the output temps will be lower (and as-used COP higher) than you'd get with the mini-splits. It may come in at close to 2x the installed cost of a pair of 1-ton Mitsubishis with baseboard backup, but the comfort levels would be somewhat higher when it's really cold out.

the altherma does not kick off at -4 deg F. it can run down to -20. output is pretty low at those super low temps though.

I believe you, but it's at odds with their cheezy graphs on the FAQ page:

http://www.daikinac.com/residential/altherma-faq.asp?sec=products&page=53#q72

it's only RATED down to -4 deg F. big difference.

AHA- got it! (Thanks Rob!)

The Mistubishi H2is are typically rated at ~73% of the stated-rated output at -13F, but I'm wondering how low it has to go before it has a sub-unity COP. I suspect they'd still put out something at -20F, but at that point electric baseboards/boilers are as-efficient.

Thanks Dana
The heat load for the radiant slab would be around 6-7k BTU, not the full 19k. The total house load is basically split 10k main level, 9k lower level.
The radiant heating would only be for 75% of the main level. Rest of the level would have baseboard backups. Also it would serve as thel backup to the hyper heats, assuming heat load in theory could be less then 6-7k by a bit. Because of this lower load had me wondering if I could use the water heater. Even a dedicated waterheater to the radiant slab would not cost that much to install, and much cheaper cost then a boiler.
Any thoughts on this?

If you contrast an air-to-water heat pump with radiant floor you can't live without it! Radiant yes, heat source is highly dependent on the cost and availability of fuel with a hard look at local support for mechanical maintenance.

If you have windows in the house the floors will always be the next coldest surface and the one closest to your coldest body part-your feet.

We use high efficiency condensing combination water/space heating systems all over the US& Canada.
If you have gas or solid fuel hydronics can't be beat.