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Originally Posted by Who
This may be semantics but I'll waste some keystrokes anyway. Technically speaking, a house that is well suited to geothermal is one that has been designed so that it can be heated with a very small heat differential. If using hydronics for the geothermal for example you are designing so that you can heat the house with lower water temps. That is exactly inline with what you need for an active solar system, and in essence, geothermal is solar heating, it's just that it is a stored form of solar energy. With the lower temperature requirements, the ability to harvest BTUs from wherever are much better because you don't have to rely so much on all of the energy spent on making harvested energy sources to have high enough temperatures to be useful.
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It's not the house that is designed that way so much as the system. Any building, regardless of size and construction, can be heated with geothermal if the coil and recovery/exchange system is large enough.
Also, although people sometimes say that geothermal is a form of solar heating, that is not totally true. Although it is certainly true that the ground is heated by the sun, the penetration isn't that deep. The fact is that once you get a few feet under the ground, the temperature is constant all year long, and the deeper you go in the earth the warmer it becomes. I believe that the typical ratio is about .5 to 1 degree for every 500 feet down you go. So it's probably more accurate to say that heat comes from above and below.
Compared to solar, geothermal is cheap -very cheap. Payback is usually much less than 10 years, depending upon where you live.