Malcolm Gladwell’s father presents The Case for Geothermal:
Geothermal heating and cooling is based on one simple fact: that 6 feet down in the ground the temperature is the same — between 50?F and 60?F — the whole year round. This means that it is relatively cool in the summer, and relatively warm in the winter. Geothermal heating is thus quite different from solar heating: solar heating works worst when you most need it — in the cold, cloudy, snowy conditions of winter; the source for geothermal heating and cooling is not affected by the weather.For geothermal cooling, all one needs to do is to circulate water in a pipe through the ground to cool it, and use this cool water to cool the air pumped through the house in the heating ducts.
For heating, there is an extra wrinkle. Most of us prefer the temperature in the house in the winter to be nearer 70?F then 60?F, so we need to raise the temperature of the relatively warm air a little.