Five years ago, my husband and I walked out of what was left of our historic house after a propane explosion. As the house caught fire, the cat jumped out to safety, too. When we rebuilt, we wanted to avoid burning fossil fuels in our new home, and I remembered reading an article about an architect who drilled geothermal wells to heat and cool his Lower Manhattan town house.

Many people think geothermal energy means tapping the power of geyserlike hot springs from miles underground to turn turbines and generate electricity. They may also associate it with minor earthquakes like those that halted major geothermal deep-drilling projects in northern California and in Switzerland late in 2009.

There is, however, another way to make use of geothermal energy on a much smaller scale. Ground-source heat-pump geothermal systems take advantage of the earth’s constant temperature below the frost line to heat and cool buildings.

That line varies according to latitude, but ranges in the United States from about three to six feet. Below that depth the temperature stays around 50 degrees Fahrenheit, give or take a few degrees. That is why a subterranean cave feels warm in the winter and cool in the summer. Wells for this geothermal energy usually go down in the hundreds rather than thousands of feet.