[ursa minor]

A little background, I'm a commercial HVAC contractor. Geothermal systems that are properly engineered and installed can be an extremely efficient method of heating and cooling. They're very well suited to colder climates because they take advantage of the relatively constant temperature of the ground water, usually around 50 degrees F. That being said, they are very complex mechanical systems that consist of a well field, pumps, water source heat pump unit (or units) and controls. For the system to work properly, all parts of the system need to be properly sized and maintained. New construction is the most common place that these systems are seen and as mentioned in several posts above, they work best when combined with a good building envelope (super insulation, premium windows, etc.).

While it would not be impossible to retrofit a geothermal system into an older home, it would typically be five to six times the cost of installing a more conventional high efficiency gas or propane conversion furnace. (See the "not in our lifetime" payback comments above.) Another alternative to consider would be a premium efficiency oil fired boiler / hot water heating system as it's sometimes it's easier to route piping and baseboard in an older home than to fit duct work.

I'll also second the comments of several other posters that finding a contractor with a proven track record of installing and maintaining geothermal systems would be extremely important. Good luck and whichever type of system you choose, be sure to get estimates from reputable contractors that will stand behind and service their work.