Call that guy Neil Chayett(SP) on his “looking at the law” radio show, it was his house.
What I was told was the water is warm enough to prevent your pipes from freezing in the winter, and cool enough to ac the house in summer. All free, after the 20k hole is paid off.
You don't need to drill any mile down to do that in most of the USA.
The temperature of the ground six or eight feet down in most areas is the same as the average air temperature over the course of the year.
So if you dig down the appropriate distance and lay a bunch of pipe, the water that comes out the end of the system will be at (about) average air temperature for the year.
Here's a link to a map for US annual air temperatures.
http://www.maps.com/ref_map.aspx?cid=680,747,1304&pid=11583
(I don't know how to post pics.)
Designing such a system is not for the faint-hearted. Dramatically affected by soil type, water table level and a bunch of other factors. But the basic principle is valid.
If you look at the map you can see ground temperature won't provide effective AC in the South, and it sure won't heat the house in the North.