In previous episodes, we’ve talked about geothermal energy—the heat of the earth.

Where extreme heat is near the surface, it can be used to produce steam to drive electric generators. Where this heat warms groundwater, it makes hot springs.

But there’s another type of geothermal energy that’s more widely available: not extreme heat, but constant, low-temperature heat.

Anywhere in temperate latitudes, if you dig 10 feet or more below the surface, the ground is a near-constant mild temperature all year long.

You may have experienced this in a cave. Caves are warmer than the surface in winter and cooler in summer.

Low-temperature geothermal energy can work the same way in buildings, in place of conventional heat and air conditioning.

The core of the system is a network of pipes laid in shallow horizontal trenches or somewhat deeper vertical wells.

Through these, a fluid circulates, taking on the temperature of the earth—between 45 and 75 degrees Fahrenheit in the continental U.S., depending on latitude.

That liquid then passes through a heat exchanger in the building. Air blows over the tubing, takes on the temperature of the fluid, then blows through the house, office, or school.

These systems, supplemented as needed, can keep buildings a mild, constant temperature year-round.

The network of pipes is more expensive to install than conventional systems, but the energy savings can pay off the difference in several years.

Maybe that’s why 50,000 of these systems are installed in the U.S. each year.