Air source heat pumps have been used for many years, but due to the constant temperature of ground water, water source heat pumps became popular in the 1970's. This was promoted in part by the National Water Well Association of Dublin, Ohio. Due to cheap drilling methods, earth coupled closed loop heat pump systems gained in popularity in the 1980's. The latter consist of either vertical or horizontal installations of a closed loop heat exchanger. The vertical installation normally uses a 4-inch diameter hole, in which two one-inch diameter-plastic pipes are placed and grouted. Drilling and piping costs are around $2.50 per foot. A rule of thumb is to use 100 to 150 foot of hole for each 12,000 Btu/hr peak load required. Since most residences require around 3 tons of cooling or heating load, up to 450 feet may be required. The national average is 150 to 200 feet per bore. Water with ethylene glycol is circulated in the system. In areas where drilling is difficult or expensive, horizontal closed loops are used; buried four to five feet below the surface. These loops are generally longer than the vertical loops, and use extensive surface land area. In some states, ponds are used as the heat source or heat sink. The pipes are placed on the bottom of the pond, which are generally at least eight feet deep. The International Ground Source Heat Pump Association, located on the Oklahoma State University Campus at Stillwater, is the main promoter of earth coupled systems