The Earth Connection once installed, the loop in a geothermal heat pump system remains out of sight beneath the Earth~`s surface while it works unobtrusively to tap the heating and cooling nature provides. The loop is made of a material that is extraordinarily durable but which allows heat to pass through efficiently. This is important so it doesn~`t retard the exchange of heat between the Earth and the fluid in the loop. Loop manufacturers typically use high-density polyethylene, a tough plastic. When installers connect sections of pipe, they heat fuse the joints. This makes the connections stronger than the pipe itself. Some loop manufacturers offer up to 50-year warranties. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system.

Another type of geothermal heating and cooling is Direct geothermal heat pump (DX) systems, which utilize copper piping placed underground. As refrigerant is pumped through the loop, heat is transferred directly through the copper to the earth.

To ensure good results, the piping should be installed by professionals who follow procedures established by the International Ground Source Heat Pump Association (IGSHPA) or local government authorities.

The length of the loop depends upon a number of factors, including the type of loop configuration used; a home?s heating and air conditioning load; soil conditions; local climate; and landscaping. Larger homes/building~`s with larger space conditioning requirements generally need larger loops than smaller ones. Homes/buildings in climates where temperatures are extreme also generally require larger loops. A heat loss/heat gain analysis should be conducted before the loop is installed.

Types of Loops. Most loops for residential geothermal heat pump systems are installed either horizontally or vertically in the ground, or submersed in water in a pond or lake. In most cases, the fluid runs through the loop in a closed system, but open-loop systems may be used where local codes permit. Each type of loop configuration has its own, unique advantages and disadvantages, as explained below:

Horizontal Ground Closed Loops. This configuration is usually the most cost effective when adequate yard space is available and trenches are easy to dig. Workers use trenchers or backhoes to dig the trenches one to 2 meters below the ground, then lay a series of parallel plastic pipes. They backfill the trench, taking care not to allow sharp rocks or debris to damage the pipes. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 120 to 180 meters long per ton of heating and cooling capacity. The pipe may be curled into a slinky shape in order to fit more of it into shorter trenches, but while this reduces the amount of land space needed it may require more pipe. Horizontal ground loops are easiest to install while a home is under construction. However, new types of digging equipment that allow horizontal boring are making it possible to retrofit geothermal heat pump systems into existing homes with minimal disturbance to lawns. Horizontal boring machines can even allow loops to be installed under existing buildings or driveways.

Vertical Ground Closed Loops. This type of loop configuration is ideal for homes/buildings where yard space or usable land are insufficient to permit horizontal buildings with large heating and cooling loads, when the Earth is rocky close to the surface, or for retrofit applications where minimum disruption of the landscaping is desired. Contractors bore vertical holes in the ground 50 to 138 meters deep. Each hole contains a single loop of pipe with a U-bend at the bottom. After the pipe is inserted, the hole is backfilled or grouted. Each vertical pipe is then connected to a horizontal pipe, which is also concealed underground. The horizontal pipe then carries fluid in a closed system to and from the geothermal heat pump system. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth deeper down is alternatingly cooler in summer and warmer in winter.

Pond Closed Loops. If a home is near a body of surface water, such as a pond or lake, this type of loop design may be the most economical. The fluid circulates through polyethylene piping in a closed system, just as it does in the ground loops. Typically, workers run the pipe to the water, then submerge long sections under water. The pipe may be coiled in a slinky shape to fit more of it into a given amount of space. Geothermal heat pump experts recommend using a pond loop only if the water level never drops below 2 to 2.5 meters at its lowest level to assure sufficient heat-transfer capability. Pond loops used in a closed system result in no adverse impacts on the aquatic system.

Open Loop System. This type of loop configuration is used less frequently, but may be employed cost-effectively if ground water is plentiful. Open loop systems, in fact, are the simplest to install and have been used successfully for decades in areas where local codes permit. In this type of system, ground water from an aquifer is piped directly from the well to the building, where it transfers its heat to a heat pump. After it leaves the building, the water is pumped back into the same aquifer via a second well--called a discharge well--located at a suitable distance from the first. Local environmental officials should be consulted whenever an open loop system is being considered.

Standing Column Well System. Standing column wells, also called turbulent wells or Energy WellsTM, have become an established technology in some regions, especially the northeastern United States. Standing wells are typically 15 cm in diameter and may be as deep as 450 meters. Temperate water from the bottom of the well is withdrawn, circulated through the heat pump?s heat exchanger, and returned to the top of the water column in the same well. Usually, the well also serves to provide potable water. However, ground water must be plentiful for a standing well system to operate effectively. If the standing well is installed where the water table is too deep, pumping would be prohibitively costly. Under normal circumstances, the water diverted for building (potable) use is replaced by constant-temperature ground water, which makes the system act like a true open-loop system. If the well-water temperature climbs too high or drops too low, water can be "bled" from the system to allow ground water to restore the well-water temperature to the normal operating range. Permitting conditions for discharging the bleed water vary from locality to locality, but are eased by the fact that the quantities are small and the water is never treated with chemicals.

Other loop designs are also being used. In a few places, for example, home builders have installed large community loops, which are shared by all of the homes in a housing development.