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.