Denver will pilot a thermal energy network to heat and cool a cluster of downtown buildings using water, geothermal energy and heat recovered from wastewater. The system will be centered on the Cherokee Boiler House near downtown and is projected by city officials to help the city reduce greenhouse gas emissions to zero by 2040.

Buildings are Denver's largest source of greenhouse gas emissions, and heating and cooling skyscrapers in the city currently requires burning fossil fuels. More than 100 buildings downtown are heated by the world's oldest continuously operating commercial steam system, a network first built in the late 1800s that requires burning natural gas. According to the city's climate office, customers' steam bills have more than doubled in the past decade due to increased maintenance costs, rising fossil fuel prices and a steady loss of customers.

A 2021 city ordinance requires large buildings in Denver to cut their greenhouse gas emissions or face potential penalties starting in a few years. Over the next decade, the city plans to repurpose parts of its old steam system to create a new heating and cooling network for 11 city-owned buildings, called the "ambient loop."

The ambient loop will use underground pipes filled with water, and each connected building will be outfitted with water-source heat pumps. When a building is too hot, its heat pumps will transfer heat from interior air into the circulating water; when a building is too cold, the pumps will extract heat from the water to raise the indoor temperature. Buildings on the loop will share thermal energy by transferring excess heat between one another. "Basically, heat pumps can move heat wherever you need it," said Elizabeth Babcock, executive director of Denver's Office of Climate Action, Sustainability and Resiliency.

Denver also plans to drill hundreds of geothermal boreholes more than 1,000 feet deep beneath downtown parking lots, with water-filled pipes circulating through the boreholes to exchange energy with the Earth. The city plans to tap excess heat from wastewater as an additional energy source for the network. Similar thermal networks already exist on campuses and in some cities around the world. Drew Halpern, a climate office official, said the Cherokee Boiler House will serve as the "brains and brawn" of the new thermal network.

The city estimates building out the network will cost approximately $280 million to $320 million over the next decade. The pilot phase is funded by a combination of city funds and a state grant, and the city may issue bonds or seek private investment to fund further expansion. A 2025 feasibility report found the ambient loop could be up to 75% cheaper than other methods of decarbonizing the pilot buildings.

"We think we are standing in what can be the future of energy in Denver, which is both pollution free and affordable," Mayor Mike Johnston said.