New York Times, Published: December 22, 2002
.........."John Thulin, president of Scandia Contractors, a geoexchange heating contractor in Southampton, N.Y., said there are three basic system designs for transferring heat between the inside of a house and the outside using a liquid. One method, Mr. Thulin said, is to dig two wells on the property - one that is used as a source of ground-temperature water supplied to the heat pump and the other that is used as a repository for water that has passed through the pump.
In most cases, he said, such an "open loop" system can be used only where there is a reliable source of water in the ground beneath the property. (That includes virtually every property that already has a well.)
When an open loop system cannot be used, Mr. Thulin said, a closed-loop system usually can.
With such a system, a continuous loop of plastic tubing, sometimes coiled like a Slinky, is buried underground below the frost line and connected to the input and output sides of the heat pump. Water mixed with antifreeze is then circulated through the tubing, and the water picks up or loses heat, depending on whether the system is heating or cooling. For homes that have plenty of property around them, Mr. Thulin said, the continuous loop can be installed horizontally, just a few feet below the surface.
For homes on small pieces of property - as well as those that have topography or geology that will not allow horizontal installation - it is possible to install a vertical loop by drilling several vertical holes in the ground into which the tubing can be installed.
In most cases, it is necessary to install several hundred feet of plastic tubing in a closed-loop system, with the amount of tubing determined by the heating and cooling requirements. And for that reason, Mr. Thulin said, closed-loop systems are generally more expensive than open-loop systems.
With an adequately sized system in place, Mr. Thulin said, a geothermal heat pump installed in the house can provide enough heat to run a radiant hot-water heating system or a forced-air heating system. In a house that already has a forced-air system in place, the heat pump can be connected to the existing ductwork.
In summer, Mr. Thulin said, the system uses an automatic reversing valve to provide cooling instead of heating. (That is somewhat like leaving the refrigerator door open to cool the inside of the house and venting the heat it produces out a window.) In addition, he said, the heat pump can be used to help provide hot water in the house - particularly in the summer when some of the heat being extracted from the house itself can be used to preheat water.
In most cases, Mr. Thulin said, an open-loop geoexchange system can be installed in a 2,500 square-foot house with existing ductwork for about $16,000. A closed loop system would cost considerably more because of the more extensive excavation or drilling needed.
Dr. Bose of Oklahoma State University said that while a geoexchange system costs more than a conventional system fueled by oil or natural gas, the cost of running a geothermal system is typically 30 to 40 percent less than the cost of a system that uses fossil fuel."......
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