An open-loop geothermal is based on a standard drilled well where it pumps water up from the bottom of the well and passed through a water-to-air heat exchanger

An open-loop geothermal is based on a standard drilled well where it pumps water up from the bottom of the well and passed through a water-to-air heat exchanger (heat-pump) and uses it to heat in the winter/cool in the summer. Water is returned to the top of the column ... about 10% is bled off to bring in fresh water. Wells are typically your standard artesian well drilled down to 400' to 600', the geothermal needs about 12 GPM or more to be effective. If there's not enough water flow, a closed loop system can be used where a "coil" is placed in the well and the well is filled with a bentonite grout. Those are interesting because they can use glycol to run to temps below freezing. In New England these things are very effective. The issue is the cost. The wells typically cost about $10-15K to drill, I think you get about 1-ton of cooling/heating per 100' on average. Also, there is a large (and unnecessary) premium on these ... there's just no reason why they should cost more than a regular forced air system (aside from the well drilling, but many people in rural areas in NE have artesian wells anyway). The heat-pump units are comparable in cost to a high efficiency gas boiler ($5-8K). EDIT: Add info on "bleeding" the well.