More than a hundred buildings in downtown Denver are still heated by a steam system first built in the 1800s — leaky, inefficient, and running on natural gas. Bills have more than doubled in a decade. And for buildings required by city ordinance to cut their emissions, staying on steam may make that impossible.
Inside a mothballed brick boiler house near downtown — a place Denver’s mayor describes as looking “like a good place for a rave or potentially a horror movie” — the city is betting on something different. The Cherokee Boiler House, cockroach carcasses and all, sits at the center of a plan to heat and cool downtown buildings with an energy source most people would never think to associate with clean power.
A Victorian-era steam system past its expiration date
Denver’s downtown steam network holds a genuinely unusual distinction: it is the world’s oldest continuously operating commercial steam system. When it launched in the late 1800s, newspapers called it a marvel of modern engineering. Today, that same system burns natural gas, leaks heat into the air, and drains the wallets of every building still connected to it.
Customer bills have more than doubled over the past decade, according to Denver’s climate office. Maintenance costs keep climbing. As customers quit the system one by one, the remaining users absorb an ever-larger share of the overhead — a spiral that only accelerates.
The pressure is not only financial. A 2021 city ordinance requires large buildings to cut their greenhouse gas emissions or face penalties. For customers locked into an aging, gas-fired steam system, meeting those targets may simply be out of reach. The steam network is not just outdated. It has become an obstacle.
The ‘ambient loop’: how sharing heat between buildings works
Denver’s answer is a thermal energy network it calls an “ambient loop.” The pilot will serve 11 city-owned buildings, connected by underground pipes filled with circulating water that moves between buildings continuously.
Each building gets fitted with water-source heat pumps — efficient appliances that can pull heat out of the circulating water or push heat into it, depending on what any given building needs at that moment. The real advantage is what happens when buildings interact.
If an art museum runs too warm, its heat pumps pull that excess heat out and dump it into the loop’s water. That warmer water flows to a nearby municipal building, where another heat pump draws on it to raise the indoor temperature. One building’s waste becomes another building’s fuel. The Cherokee Boiler House will serve as the central hub — the “brains and brawn” of the operation, in the words of Drew Halpern from the city’s climate office.
Tapping the Earth: geothermal boreholes beneath parking lots
As the loop expands, it will need a reliable thermal energy source to keep the circulating water at the right temperature. The city’s plan reaches underground for that supply.
Beneath downtown parking lots, Denver intends to drill hundreds of geothermal boreholes descending more than 1,000 feet. Pipes carry water down into those shafts, exchange energy with the Earth, and return that thermal energy to the loop above ground. Think of it as a battery embedded in the bedrock. Geothermal heat is essentially free once the infrastructure exists, though drilling carries significant upfront costs. Over time, the boreholes would give the network a stable, renewable foundation that does not depend on fuel prices or weather.
The unexpected ingredient: heat hiding in the city’s sewage
Denver’s wastewater — warm runoff from showers, laundry, and toilets across the city — carries enormous amounts of thermal energy. Right now, that heat flows through treatment facilities and drains into the South Platte River, still warm. That warmth is not harmless. Elevated water temperatures can harm river ecosystems, and Colorado’s environmental regulations will soon require Metro Water Recovery, Denver’s wastewater utility, to cool its discharge.
The scale of what is currently being discarded is striking. In certain weather conditions, Denver’s wastewater contains roughly four times the thermal energy used by the entire downtown steam system during peak winter demand, according to Dan Freedman, Metro’s director of technology and innovation. The proposed solution is a heat exchanger placed directly inside a major sewage line — siphoning thermal energy from the wastewater and feeding it into the ambient loop. The utility avoids chilling costs; the loop gains a powerful new energy source.
Costs, timeline, and what success could mean for the US
Building the full network will not be cheap. The city estimates a price tag of $280 million to $320 million over the next decade, funded initially through city dollars and a state grant, with bonds or private investment potentially following. Even so, a 2025 feasibility report found the loop could be up to 75% cheaper than other available decarbonization paths for those buildings — and cheaper than staying on steam.
The rollout is deliberately cautious. Within two years, just two buildings and a sidewalk snowmelt system will test a small version of the loop. Nine buildings are expected to be connected by 2030.
If that pilot succeeds, Mayor Mike Johnston believes the model could extend to thousands of natural gas customers near downtown and serve as a national blueprint. A city that once celebrated a Victorian steam network as the future of energy may be about to do it again — this time with water, the Earth, and the heat flowing beneath its streets.
