someone wrote:

Geothermal "heat pump system". In the middle of winter you can use 20
degree air as a heat source to warm the cooling coils or you can use 40
degree water. The water is 20 degrees warmer to start with and a much
better thermal conductor. Which do you think is more efficient?

That is a new one on me, I'd never heard of using cool water for heat. As
for which is more efficient, I wouldn't know. The efficiency of getting 75
degree warm air out out 40 degree water, after paying to pump the water to
the surface is not an intuitively obvious question to me.

--
Tony Sivori