Geothermal well

The other day I heard heavy machinery in an old, residential neighborhood, so I stopped to see what was happening. A well was being drilled, which seemed very strange. Why would someone inside the city limits be drilling a well?
The owner of the property explained that it was not a water well, but a heating well, a well that would use the ground for heat exchange. Heat is extracted from the ground and moved into the house during the winter, and during the summer heat is extracted from the house and moved into the ground. (The principle is the same one used in a refrigerator or an air conditioner.) Because this house had a limited back yard, the owner needed to drill vertically to get enough surface area for the heat pump to be effective. The big well driller was needed because there is limestone a few feet (7, 11?) below the surface, and a smaller drill that had attempted to drill the holes had not been effective with the limestone. In this case, six holes will be drilled, and the heat exchange pipe will go down and up each of those six holes.

I recall that a few year ago a new home in the area of Bunkum Road and Airport Drive had a heat-pump system installed. Because it had a large back yard, there was no need to drill deep into the earth. Instead the back yard was dug out several feet deep, several hundred feet of piping was laid out, and then the soil was replaced.

Using a heat pump for heating and cooling has expensive set-up costs, but the operating costs are low. I wonder how common it is in Rensselaer.