[jeff5may]

Ok, here's my take on your build:

Since you have much higher cooling demands than heating, a ground or air source heating/cooling system should be sized for your cooling demands. Since it rarely dips to freezing temperature outdoors, an efficient air-source unit would have no problems heating during the chilly weather. No defrosting equals no wasted energy, so a variable speed outdoor unit (or a few single-zone minisplit systems) would be the most cost-effective to install. During those sweltering summer days, a ground loop would be more effective, but at a very high upfront cost. With limestone, you can't just grab a dozer or backhoe and dig a trench; dynamite, drills, jackhammers, etc. are necessary and push up both the material and labor tremendously. A few boreholes might be cheaper than a slinky field of equal capacity.

As for heating water, your space heating method would dictate how to approach the solution. Using a phase change heating/cooling system, a desuperheater would provide hot water whenever the outdoor unit operated. This approach ties in with a dedicated heat pump water heater or solar pv panels. Many ecorenovators have published their experiments, trials, installs, and real-world, real-time results with these combined systems, many preferring to spend extra upfront on solar pv panels over thermal collection strategies. Main advantages are things not needed: regular frequent periodic maintenance, freeze protection, thermal store, extra pumps and plumbing, etc.

With natural gas being very economical in your area, it would be stupid not to use it for at least a backup energy source. Besides heating, it may make sense for backup power generation as well. I don't know how often you have power outages, but where I am they happen a few times a year. An automatic backup generator does a better job than solar pv unless you have a really large collection array. Cheap insurance against mother nature.

This says nothing about your construction and insulation, others before me have covered that subject well. Main idea is anything worth building is best done correctlythe first time, and anything worth insulating is worth superinsulating. Spend a little extra once, save forever after.