Tuzz, I have an open loop geothermal system made by Waterfurnace. I installed the 3.5 ton system myself for about $3000, 10 years ago. A closed loop system would have cost an additional $3000 to purchase the re-circ pump, piping and labor to bury the piping material.
If you have your own well, the incremental costs of installing an open loop system is small and installation is easy.
My well pump is 2 HP with an 80 gallon water tank, which is slightly bigger than most residential needs. The unit uses 5 gpm, when its running. It runs about 6 hours per day in the summer. It is almost enough water to keep the two acre pond full in the summer, (pond level is about 4" low due to evaporation losses).
I did not consider recirculating the pond water because this would require installing an additional re-circ pump and filtration. It may be possible to re-circulate pond water, if you are not on a well, but there will likely be problems with biological growth coating the heat exchanger. In this case its probably better to go with a closed loop and just put piping coils in the bottom of the pond.
