Also, people often don't realize that the temperatures that matter thermodynamically are degrees Kelvin, not Celsius. 30 below 0 is 243K; 20 above (room temper

Also, people often don't realize that the temperatures that matter thermodynamically are degrees Kelvin, not Celsius. 30 below 0 is 243K; 20 above (room temperature) is 293K. The coefficient of performance for a Carnot heat pump at those temperatures is 293 / (293 - 243) ~= 5.86; that means that one joule of work will transfer 5.86 joules of heat. This isn't great, but it's less of a difference than people would expect given our subjective experience of how cold -30C is. The coefficient of performance for a heat pump trying to heat 0C ambient temperatures up to room temperature (or alternatively, an air conditioner cooling 40C temperatures to room temperature) is 293 / (293 / 273) = 14.65, so your heat pump in Calgary will be maybe 2.5x less efficient than one in NYC. Drill below the frost line, like you suggest, and you'll get similar efficiencies regardless of where you are.