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I built a weekend/vacation home about three years ago and explored geotherm heat as an option. In general theory, the only difference between a traditional air to air heat pump and a geothermal (water to air) heat pump is that the traditional unit uses an electric fan to blow air through what amounts to an big radiator; and the water to air unit uses an electric pump to push an anti-freeze solution through a long tube that is buried horizontally in a trench, vertically in a well, or sunk to the bottom of a pond. Again in theory, the geothermal heat pump should be more efficient because it is transferring heat or cold to a relatively constant soil or lake temperature while the conventional heat pump is transferring the heat or cold to the outdoor air which varies in temperature much more--depending on climate. Reliability issues between a traditional heat pump and a geothermal heat pump amount to how fast an electric fan will fail as opposed to how fast an electric pump will fail or a leak will develop. Other parts of the system are virtually identical. The geothermal system should cost more because of the hundreds or thousands of feet of tubing that must be buried or sunk in a lake and the antifreeze solution that must be run through the tubing. The length of tubing required will depend on the soil type. Wet soil transfers heat better, so requires a shorter amount of tubing. The cost of tubing and antifreeze for a typical system should cost under $1500. The cost of burying this tubing in trenches, wells, or on the bottom of a lake will vary with differnces in soil conditions, labor rates, equipment rates, etc. I eventually chose to go with air to air heat pumps. I bought the equipment locally and my builder set it in place. I paid a local licensed heating contractor about $500 for the hookup. If I would have gone geothermal, I would have had to hire an installer from out of the immediate area. The estimates for geothermal seemed to allow for the added cost of the tubing,its installation AND an additional $8,000 profit margin. There is little competition in installing geothermal, and the dealers charge what the traffic will bear. For me, it made much more sense to spent a small portion of the price difference on more insulation, better windows, etc. Geothermal should be feasible as a do it youself project. The only "technical" issue is insuring that the tubing doesn't leak. The material needs to be joined by heat fusing, and special heat fusion equipment is needed. (it is virtually the same tubing as used for natural gas except for the color.) I have several friends and relatives who have used geothermal heating for up to 20 years. The only minor complaint I have heard is that the system may seem more noisy because the entire system is installed indoors. I assume that you are defining "payout" as the ratio of cost of a new system to annual savings in energy costs. If you currently use a lot of propane and if can buy a geothermal system on the cheap, your payout may be good. If you currently don't use much propane and pay through the nose for a new system, the payout will be worse. However, before you invest in a new system of any kind, I suggest that you strongly consider whether you could lower your annual energy costs just as much by adding more insulation, weather stripping, insulated windows, etc.
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