The Air Has Problems,
The Earth Has Solutions

Using the earth, instead of the air, eliminates the problems that happen to air-source heat pumps when the temperature goes below freezing. Unfortunately, below about 35° F, the air is changed in several ways that make very little heat available for air-source heat pumps to use. The combination of frost build-up, and much drier air (holding much less heat), prevents an air-source heat pump from being able to transfer enough heat to keep your home warm. And there are a lot of places that go below 35° F in the winter. Must we abandon heat pumps then?

No, there is another avenue for success! The ground 4 to 6 feet below us doesn't get that cold. It stays around 40 to 50° F all year.* Since heat pumps work extremely well at 35 to 60° F, installing the heat exchanger in the earth where the temperature maintains 40 degrees, even in the middle of winter, means geothermal (ground-source) heat pumps completely avoid the problems of air-source heat pumps, and allow us to take advantage of all the free heat in the earth!

*Equally important is that ice that coats an earth-surrounded heat exchanger doesn't slow the movement of heat into it, while ice that coats an air-surrounded heat exchanger severely limits the amount of heat that can move into it. This is because gases (the air) have much lower conduction coefficients than solids do.