Boise is the capital of Idaho, and the Idaho Statehouse is the only one in the United States to use geothermal heat. Geothermal heating is possible in Boise because of fault lines that expose its groundwater to hot rocks. The underground water supply in Boise is heated to around 170 degrees Fahrenheit. It is the basis of the largest municipally-run geothermal system in the country.
People in Boise began using this natural resource to supply heat to buildings in the 1890s by drilling wells into aquifers that yielded hundreds of thousands of gallons of piping hot water a day. The water heated pools and public baths, a Victorian mansion, and, eventually, hundreds of homes in what was called the Boise Warm Springs Water District.
The number of buildings that the city of Boise heats with its geothermal resource has grown more than sixfold over the past 40 years. The water is drawn from wells in the nearby foothills and pumped into a closed-loop network of pipes that reach into buildings. In each building, the geothermal heat is transferred to water in separate adjoining pipes that distribute the heat throughout the building. The well water goes back to the aquifer to be heated again.
Today, there are four separately run geothermal water systems in Boise. One is run by the city, another by the Boise Warm Springs District, and two others that serve the Capitol and Veterans Affairs Buildings.
In 2024, city officials calculated that using geothermal heat reduced annual carbon dioxide emissions by 7,000 tons, the equivalent of removing 1,500 gas-powered cars from the road each year.
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They’re in Hot Water in Idaho. Here’s Why That’s a Good Thing.
Photo, posted May 26, 2010, courtesy of Jason W via Flickr.
Earth Wise is a production of WAMC Northeast Public Radio