A little admission: When I wrote last week [0] about forced-air heating, I was feeling a little morally superior, a little bit scoffy [0], like I knew more about home-heating and -cooling than might actually be true. So it was with great wonder (and a small dose of humility) that I got into this bit of nerdery [0] with Lime reader DeCapeJack [0]about geothermal systems.
Geothermal systems, more technically known as geoexchange [1] systems or geothermal heat pumps [2], take advantage of the simple fact that, below ground, the earth – by which I mean the soil and rocks and whatnot we walk upon – doesn't really change temperature. Once you dig below the frost line, below where the soil usually freezes during winter, the earth keeps its internal thermostat set around 55 degrees. When it's 5 below outside, the earth is a balmy 55 a few meters below your frozen nose. When it’s a sweaty 105 in the shade up top, down deeper it's still that balmy 55 degrees.
Heating and cooling systems operate on a basic physical principle called heat transfer [3]: Heat seeks out cold and tries to warm it up. Kind of like when our dogs [4] crawl under the bedcovers at night. In winter, geoexchange systems allow the earth's relative warmth to seek out (and warm up) the cold inside your house. In summer, they allow the heat inside your home to seek out the relative cool of the earth. 
[illustration courtesy McQuay International [5]]
So how does it work? Geothermal heat pumps require some sort of access to the ground, either through deep vertical wells or a more shallow field of horizontal pipes buried below the frost line. A shallow field requires a fair amount of land space, while vertical wells need deep holes – like 60 feet deep and about four inches wide [5]. A fluid (usually water mixed with glycol) is pumped through the pipes. In winter, that fluid draws heat from the earth and uses it to bring warmth to your home's heating system – whether that's forced air or a hydronic system like radiant heat. Because the earth's temperature is already close to the temperature you want to make your house or office park or pottery studio, you need much less energy to make it so.
While the operating costs are of a geoexchange system are very low, the upfront installation costs can be very high – unless you've got access to a very large drill [6], of course. That's not to say it can't be done. DeCapeJack obviously got his holes dug. And then there's this guy [7], who drilled a 1,110-foot-deep hole beneath his Tribeca townhouse. At least nobody's ever gonna call him a cheapskate.
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Last week I told you I was going to make some pretty little drawings of our house for you using Google SketchUp [8]. Those may take a while. When Internet nerds say something is easy, it’s very different from when I say something is easy. When I say it, I mean that you could do it even while half-asleep; they mean you could do it even if you only got a B-plus in Fluid Dynamics 301 [9]. So I think I’m going to stick with sketches made on good ol’ pen and paper for now. Look for some basic outlines of the house soon.