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Everything it takes to heat all four floors of Boise City Hall through the winter is contained in a small, blue box tucked in the corner of a basement mechanical room.

A short section of insulated lime green pipe comes into the device and loops back out again, carrying with it thousands of gallons of high-pressure water sourced from a fault line at the base of the foothills.

Water topping out at 170 degrees snakes through pipes beneath the streets of downtown Boise, heating 6 million square feet of buildings during the winter.

Started in 1983, Boise’s geothermal system is the largest of its kind in the United States and sixth largest worldwide, according to city officials, and they want to see it expand. This year, the city received permission from the Idaho Department of Water Resources to increase the amount of water it uses. With more water, the city is hoping to connect more downtown buildings.

According to Public Works Director Steve Burgos, the city plans to grow the system by encouraging buildings that are already near the system to join, as opposed to digging lines farther out of the downtown core.

“Early on, it was a new system, and it was anyone who was able to connect we wanted them to connect,” Burgos said. “Now we have the ability to be strategic and forward-thinking about, ‘What buildings can we connect and how can we plan this system out over 20 years?’”

About a third of downtown Boise is heated by geothermal energy, including some of the largest buildings such as Boise Centre, JUMP and the Idaho Independent Bank and Banner Bank buildings. For all of the 92 buildings and 6 million square feet on the system, it costs the city approximately $1,000 per month total to heat, resulting in large energy savings for buildings that opt in to the system. Customers on the geothermal system pay for power costs as well as for each gallon of water used by their heat exchanger to warm the building.


Contrary to what some might think, the water in the geothermal system is not heated by volcanic activity beneath the city.

Instead, the water is heated by the decay of a large mass of rock formed from cooled magma beneath the mountains of central Idaho called a batholith.

Snowmelt and rainwater seep into the rocks from the mountains and come in contact with the rock, which is slowly giving off radioactive isotopes as it breaks down. The particles’ splitting off the atoms in the rock results in a high temperature beneath the surface. There has been no indications of decreasing water temperature over the last 125 years and the water is not expected to stop being heated to the current temperature for centuries.

Along the Boise foothills near Military Reserve, there is an old fault line where a crack in the earth allowing water heated by the rock to come to the surface.

There, the city, as well as the other three geothermal districts in Boise, have wells where the water is drawn to the surface and sent to heat offices and homes.

The other districts are operated by the state, powering the state Capitol and a small network run by the Veterans Administration. The third system, which has been in place since 1892, provides hot water to approximately 300 customers along Warm Springs Avenue in the East End. According to the Boise Warm Springs Water District website, it is the oldest continuously operating geothermal system in the world.

The water flows into the buildings and then passes through a heat exchanger, which is a small device that transfers the heat from the water through a series of thin metal plates to then be released into the rooms of the structure. Once the water passes through, it is immediately returned to the system and pumped back into the aquifer beneath the city. On an annual basis, the city pumps between 240 and 290 million gallons of water to heat all 92 buildings on its system.

Geothermal coordinator Jon Gunnerson said his favorite part about the system is its simplicity.

“We pump the water up, it goes into the buildings, the heat is exchanged and it goes right back into the ground,” Gunnerson said, standing beneath the geothermal pipes in the City Hall basement. “There’s very little maintenance for users, which is one of the biggest draws for using geothermal.”

The system has its own enterprise fund, which means user fees go directly back into paying for improvements and upkeep. Revenues for the geothermal fund are projected to be roughly $789,000 for fiscal year 2019, which represents a net surplus of $33,000. There are no current discussions of increasing costs for consumers.

Even during the summer months when the system is not actively heating anything, between 50 and 200 gallons per minute continues to flow through the system. Flows are low now, but at its peak during the winter, the city can pump 1,500 to 2,000 gallons per minute through the system in order to keep up with the demand. In comparison, the Boise River runs at 2.7 million gallons per minute at its peak in the spring.

Expansion is available anywhere in the downtown core. While the city has the options to lay more pipes in the streets to expand the reach of the system, its current strategy is to encourage customers located near the lines to connect if they have not already.

Currently the city is permitted to pump 310 million gallons annually, but with the newly approved water rights, it will be able to use an additional 15 million gallons per year until it has added an additional capacity of 75 million gallons per year. For context, City Hall uses roughly 2 million gallons a year, and Boise Centre uses between 3 million and 5 million gallons annually.

Even during the most extreme weather, the city still has never hit the maximum amount of water it is allowed to use. Two winters ago, when Boise was hit with record-breaking snowfall, the city still used only 285 million gallons during fiscal year 2017. The pumps are set up so when more water is used on colder days, the decrease in pressure automatically pulls more water from underground to meet the demand.


When the city first started using the geothermal system, the water was pumped out of the ground, piped through the buildings and then dumped into the Boise River. This method of operation worked as a way to heat the buildings, but the water levels in the aquifer below the city began to steadily drop over the next five years.

Gunnerson said the city was then slapped with several lawsuits from other geothermal districts in the city and groups concerned about the depletion of the water levels, so the city had to find a way to replenish the water supply, or it might have been forced to end the program.

As a solution, the city drilled an injection well in 1999 on the edge of Julia Davis Park to return all of the water back into the aquifer. There were fears that pumping the water back into the ground would cause fissures in the ground and increase seismic activity in the area, but that has not occurred.

Once the the city began pumping the water back down into the ground, the water level in the aquifer began to slowly rise every year. The water levels hit their pre-1983 levels in 2013, and the levels have continued to incrementally climb since.

“Right now the rate of recharge is faster than people are pumping it out, and because every gallon we pump out we are reinjecting, this is what makes the system truly sustainable,” Gunnerson said. “That first five years, that was not sustainable, but because of the success of the reinjection, that’s how this utility is surviving and growing.”


In order to have enough energy to heat buildings, the water running into the heat exchangers needs to be approximately 170 degrees. After it is used, the temperature drops down to around 120 degrees.

This water cannot be used for heating buildings again, but the city recently began selling it to customers at the end of the geothermal line in the Central Addition a second time for heating water and a variety of other purposes. Several buildings, including a few downtown hotels use the cooled water to pre-heat their hot water for guests. It is also used for laundry facilities, as well as to heat the pools and showers of the Downtown Boise YMCA.

This practice of maximizing geothermal energy’s use is a central pillar in the city’s LIV District initiative in the Central Addition, which seeks to promote the neighborhood and grow development in the area on the edge of downtown Boise. The city touts “green turnkey development,” which means businesses are able to easily connect to a series of environmentally friendly infrastructure if they choose to set up shop in the area, which includes geothermal.

Although city officials say no development has occurred in the area specifically because of the ease of geothermal connectivity, it has generated interest. Boise’s Environmental Division Senior Manager Haley Falconer said the promotion of geothermal connectivity in the Central Addition and the move to encourage more businesses to connect will help the city reach its goal of increasing geothermal energy’s portion of total energy in the city from two percent to a greater number.

“Right now 2 percent of our community energy is geothermal,” Falconer said. “So we start there and we only have the possibility of increasing that, especially as we decrease the other number and expand the renewable energy of the others.”

Margaret Carmel covers the city of Boise. Follow her on Twitter @mlcarmel or reach her by phone at 757-705-8066.

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