[gorush]

"Heat pumps don’t work when the outside temp is below 40 degrees."

You are incorrect. We live in central Wisconsin and the heat pump was our only source of heat until adding a small fireplace for atmosphere recently. The geothermal system starts to struggle when the temp hits -20F.

[driftdiver]

Geothermal and generic heatpumps are different animals. geothermal gets its temperature differential from below ground. Standard heatpumps use the outside air.