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Earth Connections

Closed Loops
Horizontal Closed Loops
Vertical Closed Loops
Pond Closed Loops

Open Loops


Introduction

GeoSource heating and cooling systems use the earth as an energy source and heat sink. A series of pipes, commonly called a "loop," are used to connect the GeoSource system's heat pump to the earth. In a few installations, refrigerant from the heat pump is circulated through the ground in a closed loop. However, the more common loops discussed here use only water or a water and antifreeze mixture.

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Closed Loops

Closed loop systems are becoming the most common. When properly installed, they are economical, efficient, and reliable. Water (or a water and antifreeze solution) is circulated through a continuous buried pipe. The length of loop piping varies depending on ground temperature, thermal conductivity of the ground, soil moisture, and system design.

The most commonly used type of pipe in GHP installations is high quality, high density polyethylene. All below-grade connections must be made by heat fusing, which yields connections stronger than the pipe itself.

All installations must be purged to remove construction debris, flushed to remove air, and pressure tested before backfilling or grouting. Proper installation is a key to success. Dealing with experienced professionals who size and install ground loops is essential. It is critical that the installation prescriptions of the International Ground Source Heat Pump Association (IGSHPA) are followed. Installers should be IGSHPA certified or show equivalent training by manufacturers or other recognized authorities.

Modern GeoSource heat pumps work efficiently even with large seasonal swings in the ground loop water temperature. Letting the ground around the loop freeze releases substantial amounts of heat and generally improves the performance of the ground loop. Thus, a mixture of water and antifreeze may sometimes be used to improve performance while protecting the equipment and loop from freezing.

Acceptable antifreezes vary among jurisdictions, but many cost-effective and relatively benign options are available. The most restrictive regulations require food-grade antifreeze, such as propylene glycol. The primary concerns regarding the use of propylene glycol are its expense and that its viscosity increases as the temperature decreases--causing pumping energy to rise substantially. Potassium acetate, methanol, denatured ethanol, and inorganic salts such as calcium chloride are commonly acceptable. Ethylene glycol (automotive antifreeze) is not allowed in many parts of the United States, but is often used in other countries. Experienced professionals can recommend the most beneficial antifreeze mixtures.

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Horizontal Closed Loops

Horizontal closed loop installations are generally most cost-effective for small installations, particularly for new construction where sufficient land area is available. These installations involve burying pipe in trenches dug with back-hoes or chain trenchers. Up to six pipes, usually in parallel connections, are buried in each trench, with minimum separations of a foot between pipes and ten to fifteen feet between trenches. Often "SlinkyTM" coils -- overlapping coils of polyethylene pipe -- are used to increase the heat exchange per foot of trench, but require more pipe per ton of capacity. Two-pipe systems may require 200 to 300 feet of trench per ton of nominal heat exchange capacity. The trench length decreases as the number of pipes in the trench increases -- or as SlinkyTM coil overlap increases.

Improved drilling technologies and loop configurations available in some areas. For example, several vendors are using a horizontal drilling apparatus for residential retrofit applications. This equipment allows for minimum disruption of existing landscaping, and it can even allow for the installation of heat exchange loops under buildings.

A ground loop is only as good as its connection to the ground. Therefore, horizontal installations require careful trench backfilling. The backfill must be free of sharp rocks that could harm the pipe, and the pipe needs maximum contact with the ground. Some installers use large amounts of water to break up soil clumps and slurry (i.e., a solid transported by a liquid, e.g.-mud) the fill to assure optimal contact. This procedure reuses the removed soil of the trench. Flowable fill brought to the site in concrete mixers is preferred by some installers. The fill contains sand, fly ash, and a small amount of cement, which offers proper contact and high thermal conductivity that assures a good connection between the loop and the ground. In cases where the soil is dry or cooling loads are very high, some installers add a drip line that adds small amounts of water to the earth near the loop. This assures optimum heat exchange through moist soil.

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Vertical Closed Loops

Vertical closed loops are preferred in many situations. For example, most large commercial buildings and schools use vertical loops because the land area required for horizontal loops would be prohibitive. Vertical loops are also used where the soil is too shallow for trenching. Vertical loops also minimize the disturbance to existing landscaping.

For vertical closed loop systems, a U-tube (more rarely, two U-tubes) is installed in a well drilled 100 to 400 feet deep. Because conditions in the ground may vary greatly, loop lengths can range from 130 to 300 feet per ton of heat exchange. Multiple drill holes are required for most installations, where the pipes are generally joined in parallel or series-parallel configurations.

Installation costs depend on geological conditions and local drilling industry experience. In some areas, competition among drillers has led to widespread use of modified water well drilling equipment with bores in the 4" diameter range, and prices are modest. Elsewhere, the combination of minimal competition, difficult drilling conditions, and peculiarities of local environmental regulations (such as requirements for boreholes more than 6" in diameter) has kept costs much higher.

One item of concern is that the vertical borehole may allow downwash of contaminated surface waters into potable water aquifers, or cross contamination among aquifers. Careful backfilling, as prescribed by IGSHPA and local regulations, is important for GeoSource installations. Usually, high solids bentonite grout, installed continuously from the bottom of the boreholes, is required and acceptable, but some jurisdictions prefer cement grout under special conditions.

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Contact SoundGT

Ground Source
Heat Pump
Technology

Economics of a
Geothermal Heat
Pump

Operating Cost
Estimator

Energy Recovery
Ventilation"ERV"
and
Heat Recovery
Ventilation"HRV"

Practical GeoExchange Applications

IGSHPA
Certification
and Training

Other Sites and
More Information
about
Water Source
Heat Pumps

Key Personnel

Dealerships

SoundGT.com
Home

Pond Closed Loops

Pond closed loops are a special kind of closed loop system. Where there is a pond or stream that is deep enough and with enough flow, closed loop coils can be placed on the pond bottom. Fluid is pumped just as for a conventional closed loop ground system where conditions are suitable, the economics are very attractive, and no aquatic system impacts have been shown.

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Open Loops

Open loop systems are the simplest. Used successfully for decades, ground water is drawn from an aquifer through one well, passes through the heat pump's heat exchanger, and is discharged to the same aquifer through a second well at a distance from the first. Generally, two to three gallons per minute per ton of capacity are necessary for effective heat exchange. Since the temperature of ground water is nearly constant throughout the year, open loops are a popular option in areas where they are permitted. Open loop systems do have some associated challenges: (1) Some local ground water chemical conditions can lead to fouling the heat pump's heat exchanger. Such situations may require precautions to keep carbon dioxide and other gases in solution in the water. Other options include the use of cupronickel heat exchangers and heat exchangers that can be cleaned without introducing chemicals into the groundwater. (2) Increasing environmental concerns mean that local officials must be consulted to assure compliance with regulations concerning water use and acceptable water discharge methods. For example, discharge to a sanitary sewer system is rarely acceptable.

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Open Loops - Standing Wells

Standing wells, also called turbulent wells or Energy WellsTM, are an established technology in the Northeast and some other areas. Standing wells, which may be as deep as 1500 feet, withdraw water from the bottom of the well, circulate it through the heat pump's heat exchanger, and return it to the top of the water column in the same well. Usually, the well also serves to provide potable water. Careful design assures good heat exchange, which means relatively short loops: some designers claim much less than 100' of well per ton of nominal capacity. 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 goes outside a specified temperature range, water can be "bled" from the system, to allow ground water to restore temperatures to the normal operating range.

Permitting conditions for discharging the bleed water will vary from locality to locality, but are eased by the fact that the quantities are small and the water is never treated with any chemicals. Clearly, a standing well could not be installed where the water table is very deep, because pumping requirements would become prohibitive.

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Conclusion

GeoSource heating and cooling systems can be connected to the earth in a variety of ways -- all thoroughly field proven. However, high performance requires the use of experienced professionals who understand local conditions. To contact Sound Geothermal, Corporation call 970-240-6018 (Montrose, CO), or 801-722-5877 (Roosevelt, UT).

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