Why does -10f mean you MUST have a gas furnace? Most modern cold temp heat pumps still have COP’s 2.5 to 3 at that ranges. Just look at the energy star database and then find a spec sheet for one of the cold wether ones.

Secondly, look into the actual cost of the unit, the largest air source ones like you mentioned are often in the 7 to 10k range, so take that, subtract the 2k fed subsidy, then the 3 to 4k of the one way heat pump (ac) it replaced. Admittedly, if your getting multiple weeks of -10 the benefits of such a high efficiency AC probably don’t have much effect.

Although, if your building new in zone 5, you probably should be looking at ground source rather than air source. It’s is actually a lot more expensive to lay the pipe and install, 20 to 30k, but that is a lot easier to do that before landscaping. Obviously the benefit is a lot higher efficiency at cold temperatures, given that the outside air temperature doesn’t have any significant impact on the output efficiency or capacity, as well as much lower energy useage.

Solar is also something to poke at if you have yard space or are planning a decent roof, especially if you’re not afraid of technical documents and at forum posts to do it yourself as diy often haves the cost. Pannel and inverter prices keep coming down, to the cost will probably be better by the time you actually build. I have supper cheap electricity (10c) and still a sub ten year net profit.

The extra 30 to 60k (depending on diy or cash grab company) is steep, but in the context of building your own house, the extra 50 to 100k for both geothermal and solar mean basically negligible utility payments for the rest of your life, outside of water if you don’t need a well, as compared to still having to pay in thirty to forty years, but now with that many years of price increases and inflation. Especially if we end up getting a carbon tax in fifteen to twenty years and utilities do what they did in Australia and massive jack up prices in protest, not that they saw anything but massive windfalls or lowered prices after it was repealed of course.