In St. George, Utah, they don’t use furnaces to heat their homes, they use heat pumps. They work very well, even when the temps drop down into “zero” territory.
Question! What kind of heat pumps are being used, air to air, or ground to air? Also do you have any data/idea on relative temperatures? We have an air to air heat pump for a home at 3600 ft. in northern CA and it holds house ,1500 sq. ft, temp at 69-70F down to about 32-35 degrees.
I’m in Florida. I heat my house with a heat pump, as do 90% of Floridians. However, the efficiency of a heat pump is quite poor once the outside temps dip below about 25F.
I do like using the outside coils of the AC to heat water. But I think you would get a surplus of hot water.