geothermal exchange heat pump. Geothermal is one of the most environmentally friendly ways to heat and cool a house since it only uses electricity to run a water pump and a blower, but getting there is really messy.
We are installing two WaterFurnace ES030S1ANB units, coupled with Trane 4TEE3F31B1000A variable speed air handlers. We didn't choose these particular components, we chose the most reputable company in our area and this is what they recommended.
In our area (close to the water, sandy soil, with our average daily temperature) generally speaking you need one 250 foot deep well per ton of heating/cooling, and two tons of heating/cooling per 1000 square feet of living space. Our house is 2800 square feet so we ended up with six wells. These are being connected in a zoned system, with 3 wells servicing each of the WaterFurnace units. This is a closed loop system, meaning that there is no outflow of water. You can think of it like your refrigerator or the cooling system in your car. Instead of using air to cool things off this uses the ground.
Our lot is only 67 feet wide and in total the wells cover a space of about 10 x 20. The heating/cooling field will extend up to 20 feet out from that. The loops are joined together about 3 feet underground, then connected underground to the heat pump. When it's all done we will have two relatively small sealed boxes outside that house the water pumps and heat exchangers. This replaces the usual air conditioning compressor eyesore. Inside we will have two air handlers in the attic, suspended from the rafters so they don't vibrate the ceiling and cause noise. We will have no gas or electric powered supplemental system, all our heating and cooling will be done using only the electricity needed to power the water pumps and air handlers.
As for cost, generally speaking it's about 20% more than a conventional system, and you can expect a reduction in your utilities of 20% - 50%, depending on your usage. Our situation is a little different. In our case we're also having new ducting and new inside air intakes. That adds about $8,000 to the cost, and needed to be done regardless of the system we chose. Altogether we spent about $26,000 on the WaterFurnace units, the air handlers, the ducting and all the other interior work. The wells were another $11,000.
By way of comparison the company that installed and serviced our conventional heat pump quoted a little over $20,000 just to replace the conventional furnaces and compressors. They didn't see any problem with the ducting or air intakes. The main trunkline of the ducting was original to the house (1962ish) and in pretty bad shape. The air intakes weren't even connected to the air handlers so they were sucking in air from the attic instead of from the house.
So at the end of the day we could have gone with a conventional system for a little over $20K, added the $8K for ducting and the other work, and been done with it. Our total for the geothermal unit including all this extra work will be closer to $40K, and we expect to see about a $120/mo drop in our utilities.
It's a long ROI (27 years), but the upside is that there is one. With a conventional system the savings would have been so slight we never would have fully recouped the investment. The new system requires extremely minimal maintenance, it's done right by reputable people I trust, and it will theoretically last forever. The guy we bought the system from said his company has been installing these systems in our area for over 30 years and the only part he has ever replaced is a handful of water pumps. And I like to think we're earning some karma points by helping reduce our environmental impact and consume less fossil fuels. :-)
Side note: one additional point for us was that this only needs electricity to run a pump and a blower. At some point down the line I am considering adding a solar panel to handle that, so in the event of a hurricane we will still have heat and air even if the power is out for an extended period. Even without the solar panel, we were told we can run the entire system off a car battery for a day or so.
Updated 7/7/07 - Added details and fixed some parts that I got clarification on after talking to the guy doing the install.