Heating Swimming Pools and Cities with Data Center Waste Heat

The Butte-aux-Cailles swimming pool in Paris maintains its water temperature using heat generated by computer servers, according to project data. Developed by start-up Stimergy, the system recovers waste heat from 3D animation rendering to reduce CO2 emissions by 65 tons annually, signaling a shift toward urban heat recovery in France.

The facility in Paris’s 13th arrondissement replaced its traditional heating methods with “digital boilers.” These server racks are submerged in a non-conductive oil bath that captures calories and transfers them to the pool water without the two circuits mixing.

The system serves a dual purpose. It cools the servers without requiring air conditioning while partially heating the pool basins. The servers are currently utilized by Team To, a 3D animation studio that uses the computing power to produce films.

Did You Know? The server racks at the Butte-aux-Cailles pool are housed in the building’s former coal boiler room, replacing 20th-century carbon combustion with digital processors.

How do servers heat a public swimming pool?

Six digital boilers provide 45 kW of power during full operation. This capacity maintains the pool water at approximately 27°C year-round, though it cannot heat the water from scratch after a full drainage.

According to the technical data, the system covers between 10% and 20% of the pool’s heating needs. The remaining heat is supplied by the CPCU network. The setup operates at a 95% efficiency rate, which exceeds the typical 90% efficiency of standard gas boilers.

The environmental impact is measured in two parts. The system avoids 45 tons of CO2 emissions related to water heating and an additional 20 tons that would have been emitted by server cooling systems. The City of Paris has established an eight-year partnership with Stimergy for the project.

Expert Insight: Samantha Carter notes that the transition from small-scale experiments like the Paris pool to massive infrastructure like the Saint-Denis network represents a critical shift. The stakes are now tied to regulatory compliance and the escalating thermal output of AI, which transforms waste heat from a novelty into a mandatory resource.

Where else is data center heat being repurposed?

The model is expanding across France. In Nantes, a digital boiler provides hot water for 40 social housing units at the Albert Londres residence. Similar systems serve Jean Moulin Lyon 3 University and a 110-unit student residence in Grenoble.

Larger infrastructure projects are also emerging. In Saint-Denis, the SMIREC network connects directly to an Equinix data center. This system uses over 90 km of piping to provide heat and hot water to the equivalent of 60,500 homes, supporting a zone (ZAC) that is 75% powered by renewable energy.

Why is waste heat recovery becoming mandatory in France?

Waste heat recovery is moving from optional to required. Under the DDADUE law of April 30, 2025, which transposes EU Directive 2023/1791, data centers exceeding 1 MW must recover and utilize their “fatal heat” by October 2025.

Non-compliant operators face formal notices and potential fines of up to €50,000. This regulation targets a significant energy leak; ADEME reports that France’s 352 active data centers consume 10 TWh of electricity annually and emit 3.6 TWh of waste heat.

According to ADEME, this waste heat is equivalent to the heating needs of 360,000 homes. Future projections suggest recovery potential could rise to between 4.09 and 12.94 TWh by 2035. This increase may be driven by the proliferation of data centers and the energy demands of generative AI.

The scale of AI’s thermal output is substantial. Training a model like GPT-5 requires up to 650 MWh, which is equivalent to the annual energy consumption of 60 French households. Consequently, cities may need to accelerate infrastructure development to capture this value before it is lost to cooling towers.

Frequently Asked Questions

How much CO2 is saved annually by the Butte-aux-Cailles system?
A total of 65 tons of CO2 are avoided: 45 tons from reduced water heating and 20 tons from eliminating the need for server air conditioning.

What are the penalties for data centers that fail to recover waste heat?
Data centers over 1 MW that do not comply by October 2025 face a formal notice followed by potential fines of up to €50,000, according to the DDADUE law.

How does the server heating process work without risking electrical shorts?
Servers are immersed in a non-conductive oil bath that captures heat and transfers it to the pool water through a fluid exchange system where the two circuits never mix.

How could your city integrate waste heat from digital infrastructure into public services?