Air source heat pumps don't make sense in areas where winter temps frequently fall below 32 degrees. Once you get below freezing, you will be running the auxillary heat a lot. If you decide to go the air source route, you will want a second stage gas fired auxillary heat. I live in the mid-atlantic region in the mountains and selected GT. That choice was coupled with a blown cellulose insulation package. My other choice was a propane heat/electric AC with a foam insulation package. Both options were roughly the same install cost. Both would have provided similar energy efficiency. At the time we selected, propane was 3.75 a gallon and rising (with no end in sight) so we went with GT. The delta cost for the GT system (above the cost for a HE propane heat and electric AC system) was $15,000 for the heat pumps and loop plumbing (including pumps) plus $17,000 for digging the pit. The pit excavation should not have cost as much as it did but it is a 12 ton system and the hole was gigantic (120' x 48' x 6'). We live on a mountain and hit a bunch of boulders that required special equipment to get out. Rocky soil also required a 1' base of stone dust to cover the loop piping. The total cost for the GT HVAC system including all ducting was $50,000 plus the pit excavation cost of 17K. It is a large investment and I would only recommend it if you plan to live in the house a very long time. My heating bills are great - maybe 300 dollars maximum a month during peak winter for an 8500 square foot home (5900 living and the rest a condiitoned but unfinished basement). Make sure that whatever equipment you choose can be maintained by more than 1 very qualified HVAC company who has many years of GT experience. In my area there is only 1 GT company who has over 20 years of experience but I am stuck with them for maintenance. There have been times when I wanted to fire them but cannot. Local competition is a good thing. Also - if you build a tight home make sure that you consider ventilation and dehumidification in the HVAC design. I wound up adding both a whole house dehumidifier and energy recovery vent after we moved in becuase we had basement moisture issues as well as stale air in the winter. Tight homes are great for energy efficiency but can create health issues too. It is easy to deal with as long as you plan for it. Good luck with the build.

All heat pumps are electric. However, not all heat pumps use electric backup heating strips for when the heat pump cannot keep up. In my case, I have natural gas furnace backup. As I understand it, this is considered a hybrid system. Talk to your installing company about what the difference would be between installation price of the HP only and the HP with a furnace backup. If you can swing the diference, that is the route to go. I'm in Olympia and have PSE. Who is your provider? Concerning the Trane equipment... I have a nearly three year old Trane XL14i 3.5 ton with a 4 ton indoor unit and a 90% gas furnace. I almost never use the gas furnace. Less than 90 therms total last year. In our climate, I personally would go with an 80% furnace (since your utility is covering the install of the HP only, anyway go the less expensive route.). Heating specs on the 16i aren't that good. Go with the 15i. Make them match the indoor and outdoor coil and print you an AHRI certificate proving the match and HSPF, EER and SEER figures. Some indoor/outdoor matches offer better performance than others. Concentrate on the Heating Capacity BTUH numbers at 47 degrees and 17 degrees. We are in a heating climate and those are the numbers that count around here. Make your contractor show you those numbers for both the 15i and 16i combinations. When installation happens, make sure you are getting the combination you were told. Model numbers will be stamped on the equipment. I don't think you can get a 90% furnace anymore. The more efficient, the more expensive. My point being that our electric rates are very favorable and a HP is much more efficient than gas. If you set your lockouts right, you'll have no problem. Mine are set so that the gas furnace doesn't come on until it's 25 degrees outside. I have energy bills that my neighbors drool over in this neighborhod of custom 3,000 sq. ft homes.

I guess you are out in the boonies with no nat gas. I can't help with equipment. It is a relatively low volume kind of demand for equipment, I'm sure. You may be forced into the industrial/commercial area and if you are not big enough, you may be out of luck. Assuming that you can get a suitably sized unit, if your hydronic system is designed to deliver heat with a low enough water temperature, it can probably be done efficiently. What is the water temperature currently set to? Try heatinghelp.com and terrylove.com. I've seen some good hydronic heat/boiler threads in both places. If you have to stick with an electric boiler, you might look at the installation of a large storage tank and going to day/night rates. You'd have to run some numbers to see if it will work for you.

Aside from cost, payback etc. the OP is asking an immensely complex and technical question well beyond what most can imagine. The OP would be better served choosing 1-discreet technology, such as a HP DHW tank and leaving and it at that. Looking at the link provided by pookieman that is most interesting still does not even begin to provide the proper detailed engineering specs to properly select the right tank. There are interacting/interdependent specs of heat exchanger size & materials (conductance/heat exchange), temperature differentials between Heat Exchanger and Entering Cold Water Temperature, heat input from source (boiler) and gpm flow rates (as well as flow resistance). Change any one parameter and every other then changes. Finding the ‘Sweet Spot’ is a very technical challenge and job. Gone are the days of a basic type of indirect tank being able to fit most situations. There’s a big difference between a boiler producing 180˚F water for an aluminum fin hydronic system and a boiler producing 140˚F water for cast iron radiators or even less for in-floor radiant. Then there is also Ground Source Geothermal and Solar Thermal. You can imagine what might have to go into selecting the appropriate tank for proper heat transfer, recovery rate, flow rate, efficiency, etc. in addition to ‘how much hot water you’re going to need’. It’s easy to install ‘something’; configuring the right system is quite something else. IMPO SR