We built an EPA 5 star house about 5 years ago (builder got $20,000, I got all the bills including the cost to certify). Thought long and hard about going with a horizontal non-contact geothermal system. But the upcharge was $10,000 (which I could have put into more insulation/etc. and then have less need for the system - plug the hole, then find a more efficient way to fill it). But my real gripe was that for the system to have the lowest life cycle cost the length of underground loop would be minimized. So, as Len said, the system only would work down to 20 degrees F. After that an electric grid would kick in and you'd be heating with the least efficient method available as conditions get their worst. That urked me to no end.
My experience with geothermal heating/cooling is very different. I have a ground source heat pump with a closed loop of about 500 feet of tubing six feet down. Mine is a three-stage system. In stage one, the compressor runs at 60% capacity; in stage two it's at 100% capacity. Stage three is where the 10KW electric element kicks in. Since it was installed about 7 years ago we've had several instances of temperatures at about -5 degrees F, and once it went down to -10. (This is near Pittsburgh.) Never has it gotten to stage 3, and it went to stage two no more than a dozen times, and each time only for brief periods (Usually cycling between stages one and two).
My unit is a brand called Water Furnace made by Northern Leader (which is a Canadian company, I think).
I chose to go with the geothermal system at a time when my gas furnace and whole house air conditioner had to be replaced. The geo system cost only about $5000 more than a new gas furnace and traditional AC system would have. My heating/cooling annual bill is now about $1000 less than it was, so I passed the break-even cost point a couple of years ago, and now I'm reaping the reward. Of course there's also the factor that it's "green."
By the way, I looked at three companies before the one I chose to go with. One was rejected because under close questioning he showed considerable ignorance about how the system works. The other was rejected because his price was almost double the one I went with. You have to do the research.
Using the system is very convenient. I set maximum and minimum temperatures, and the system automatically either does nothing or produces heating or cooling as necessary.
I would urge anyone needing to replace a traditional HVAC to look very hard at a ground-based geothermal heat pump system.
Oh yes: I forgot to mention that when the system is heating or cooling there is some excess waste heat that it has to throw out. This goes to help the heating of water in the water heater.
