TonyV hit on many of the key points. Initial cost is relatively high, about 3 to 4 times that of an air-to-air heat pump system.
The type of loop, as Tony mentioned, is a big factor. The open loop schemes do suffer high failure rates, not only due to corrosion, but also silting and clogging due to waternborne contaminants.
Closed loop is the better way to go. Horizontal trenching is one method. This requires a trench, 4' deep or more, with 1 to 4 pipes in the trench. You will need about 400-500' of pipe per ton of heat pump capacity. With multiple pipes in the trench, this runs more on the longer side. I just did a 3.5 ton system with a 475' of pipe with 4 runs in the trench.
Slinky systems use the same pipe, with 3-5' diameter coils spread out flat over a large area. While they are easier to install, they require a large area to be dug up For example, a 3.5 ton slinky would require an excavation 3' wide, 6' deep and 475' long. In both horizontal and slinky installations, fine fill (screenings, typically 40 to 60 tons) are required around the pipe to better couple the pipe to the ground due to air pockets.
Vertical systems use a bore hole, similar to a well, with a "U" pipe loop inserted. The hole is then filled with a cement grout to hold the pipe and thermally couple the pipe to the ground. The same 3.5 ton would require 4 holes, 6" to 8" diameter, each 150' deep. Vertical has the advantage of requiring the smallest land area, but is the most expensive to install.
Last is a closed loop pond system. This uses a large and deep enough pond for the source. Pipe is laid on the bottom of the pond and runs closed loop, so the pond water is never introduced to the system. This can be the most economical to install - if you have a pond.
The pipe that is used is plastic polyethelene pipe. The thermal conductivity of the plastic is not a big issue since the thermal conductivity of the ground is much lower, and the limiting factor. Even if you used copper tubing, there would be little improvement in heat transfer. A "geothermal" pipe is sold for the application. It is basically the same as plastic well piping, but it is delivered cleaned and sealed, and is rated for 160 PSI. The loop pressure rarely exceeds 40 PSI or so, but the higher pressure rating is used to protect the pipe from physical damage. All underground joints are a thermal fusion process that when done properly are leak free. Mechanical and metal components for the joints are not recommended underground, as metal can corrode and mechanical joints can fail.
The fluid is water with an antifreeze mix. The antifreeze is required since the water exiting the unit can be below 32 degrees during high heating damands in the winter. If allowed by code, methanol is used as an antifreeze, at 20% or so concentration. It has very good properties but is toxic if it leaks. Proplyene glycol is the other antifreeze used. The main disadvantage is is becomes more viscous at cold temperatures, and the heat carrying capacity is lower. These usually require bigger diameter piping and perhaps a larger flow circulation pump, both of which add to install costs. I would recommend using it only if local codes require.
The great benefits of geo units are reduced running costs, up to 50% lower, no defrosting required, long life and reliability due to the location of the equipment in the basement and out of the weather. Also most units can generate about 40% of your domestic hot water by recouping waste heat from the system. This adds to overall efficiency of the system.
I am a big advocate of geo systems, so I would say go ahead, but in TODAY's energy environment you should expect a 10 year payback. That could improve in the coming years. Also there is a "hugh" $ 300 tax credit in the new energy bill for geo systems. Also check at:
http://www.geoexchange.org to see if your state has any programs for installing a system.
paul
