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 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 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 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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