<P> A vertical closed loop field is composed of pipes that run vertically in the ground . A hole is bored in the ground, typically 50 to 400 feet (15--122 m) deep . Pipe pairs in the hole are joined with a U-shaped cross connector at the bottom of the hole . The borehole is commonly filled with a bentonite grout surrounding the pipe to provide a thermal connection to the surrounding soil or rock to improve the heat transfer . Thermally enhanced grouts are available to improve this heat transfer . Grout also protects the ground water from contamination, and prevents artesian wells from flooding the property . Vertical loop fields are typically used when there is a limited area of land available . Bore holes are spaced at least 5--6 m apart and the depth depends on ground and building characteristics . For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need three boreholes 80 to 110 m (260 to 360 ft) deep . (A ton of heat is 12,000 British thermal units per hour (BTU / h) or 3.5 kilowatts .) During the cooling season, the local temperature rise in the bore field is influenced most by the moisture travel in the soil . Reliable heat transfer models have been developed through sample bore holes as well as other tests . </P> <P> A horizontal closed loop field is composed of pipes that run horizontally in the ground . A long horizontal trench, deeper than the frost line, is dug and U-shaped or slinky coils are placed horizontally inside the same trench . Excavation for shallow horizontal loop fields is about half the cost of vertical drilling, so this is the most common layout used wherever there is adequate land available . For illustration, a detached house needing 10 kW (3 ton) of heating capacity might need three loops 120 to 180 m (390 to 590 ft) long of NPS 3 / 4 (DN 20) or NPS 1.25 (DN 32) polyethylene tubing at a depth of 1 to 2 m (3.3 to 6.6 ft). </P> <P> The depth at which the loops are placed significantly influences the energy consumption of the heat pump in two opposite ways: shallow loops tend to indirectly absorb more heat from the sun, which is helpful, especially when the ground is still cold after a long winter . On the other hand, shallow loops are also cooled down much more readily by weather changes, especially during long cold winters, when heating demand peaks . Often, the second effect is much greater than the first one, leading to higher costs of operation for the more shallow ground loops . This problem can be reduced by increasing both the depth and the length of piping, thereby significantly increasing costs of installation . However, such expenses might be deemed feasible, as they may result in lower operating costs . Recent studies show that utilization of a non-homogeneous soil profile with a layer of low conductive material above the ground pipes can help mitigate the adverse effects of shallow pipe burial depth . The intermediate blanket with lower conductivity than the surrounding soil profile demonstrated the potential to increase the energy extraction rates from the ground to as high as 17% for a cold climate and about 5--6% for a relatively moderate climate . </P> <P> A slinky (also called coiled) closed loop field is a type of horizontal closed loop where the pipes overlay each other (not a recommended method). The easiest way of picturing a slinky field is to imagine holding a slinky on the top and bottom with your hands and then moving your hands in opposite directions . A slinky loop field is used if there is not adequate room for a true horizontal system, but it still allows for an easy installation . Rather than using straight pipe, slinky coils use overlapped loops of piping laid out horizontally along the bottom of a wide trench . Depending on soil, climate and the heat pump's run fraction, slinky coil trenches can be up to two thirds shorter than traditional horizontal loop trenches . Slinky coil ground loops are essentially a more economical and space efficient version of a horizontal ground loop . </P>

Where can ground source heat pumps be used