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Index
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Reference
Fundamentals
Ground Water
(Open-Loop)
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Ground-Coupled
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Ground Temperatures
Solar energy is stored in the outer layers of the earth's crust. Energy is
transferred daily to and from the earth's surface by solar radiation, rainfall,
snow, ice, and wind.
Annual air temperature, moisture content, soil type, and vegetative cover all
have an effect on underground soil temperature. Typical mean ground water
temperatures are shown superimposed on this map of the United States. The mean ground water temperatures in shallow wells
closely approximate mean earth temperatures.
 As you would expect, the earth's temperature changes in response to weather
changes, but there is less change at greater depths. Less obvious, but important
to earth coil performance, is the time lag. The earth temperature several
feet deep reaches its coldest or warmest temperature several weeks after building
loads peak, with an annual swing ranging from 18 to 26°F for most sodded
surfaces.
At depths of twenty feet or more, there is no significant change from summer
to winter, and the mean ground temperature approaches the annual average air
temperature plus 2°F. down to about 200 feet. Thus, vertical loops generally
require much less pipe than horizontal loops closer to the surface.
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