The Butte-aux-Cailles swimming pool in the 13th arrondissement of Paris has been the first pool in the world whose water is kept at temperature thanks to computer servers since 2017. No solar panels on the roof, no deep geothermal energy: server racks housed in the pool's former coal-fired boiler room, which calculate continuously and, like any computer, give off heat that nothing obliges to waste.
Key points
- A Parisian pool transforms servers into a digital boiler — but how does it really work?
- 45 tonnes of CO2 saved per year — what is the secret of this 95% efficiency?
- From Paris to Saint-Denis: waste heat from data centers now powers thousands of homes
A boiler room that doesn't look like one
The pool in the 13th arrondissement is testing digital boilers developed by the Grenoble-based start-up Stimergy. The principle? Server racks are immersed in a fluid that allows heat exchange with the pool water. The servers are plunged into a non-conductive oil bath that recovers the calories they emit to heat the water. This fluid then circulates the heat to the pools, without the two circuits ever mixing. Direct result: the pool water is partially heated at low cost, while the servers are cooled without using air conditioning.
About a hundred computer servers have been installed in the pool's former coal boiler room — a powerful symbol. Where coal was burned in the 20th century, processors now run calculations for a 3D animation studio. The company Team To, a 3D animation studio, notably benefits from this system: Stimergy hosts its Paris branch, which develops its computing capacity to produce films. Every time the graphic designers of this infographic studio run a 3D render, they unknowingly heat the water of a Parisian pool.
A few degrees that change everything
With 45 kW of power at full operation, the six digital boilers keep the pool water at around 27°C throughout the year, but they are insufficient to heat the water after a drain, for example. Overall, 10 to 20% of the needs could be covered by this system, with the rest of the heat supplied by the CPCU network. So it is not a total solution, but a complementary one that makes sense. Due to very low energy loss, this data center's efficiency is estimated at 95%. For comparison, a conventional gas boiler rarely exceeds 90% efficiency under real conditions.
The environmental impact is doubly positive. 45 tonnes of CO2 emissions are thus avoided each year for water heating, and an additional 20 tonnes that would have been emitted if the servers had needed to be air-conditioned. The City of Paris has entered into an eight-year partnership with Stimergy for this system. The original coal boiler is now covered with server racks. The building's history continues, but in a different direction.
From Paris to Nantes: digital heat spreads
Butte-aux-Cailles was just a starting point. In Nantes, one of these digital boilers has been installed to supply hot water to 40 social housing units at the Albert Londres residence. The solution also equips Jean Moulin Lyon 3 University and a student residence of 110 units in Grenoble, which can hope to save half of its hot water consumption. The model also works for collective housing: the digital boiler is inserted upstream of the main boiler to cover the baseline consumption, i.e., the constant hot water needs.
Even more ambitious, the heat from data centers is beginning to power urban networks on a large scale. In Saint-Denis, a project with the SMIREC network is emblematic: directly connected to the Equinix data center, the network recovers the residual heat emitted by the servers to supply a development zone with 75% renewable energy. This network extends over more than 90 km of pipes and supplies heat and domestic hot water equivalent to 60,500 homes. This is no longer an experimental project. It is infrastructure.
A legal obligation, colossal potential
What was a pioneering initiative in 2017 has become a regulatory requirement. Since October 2025, data centers over 1 MW must recover and valorize their waste heat. The DDADUE law of April 30, 2025 transposes European directive 2023/1791 into French law and introduces specific obligations for data centers. In case of non-compliance, managers face a formal notice, then a fine of up to 50,000 euros.
This framework is not trivial. ADEME lists 352 data centers in operation in France, with a total electricity consumption of 10 terawatt-hours per year. Of the 117.9 TWh of waste heat emitted each year in France by industry, data centers alone account for 3.6 TWh, equivalent to the heating consumption of 360,000 homes. Energy that until now went into air conditioners and cooling towers.
The future potential far exceeds the pool in the 13th arrondissement by several orders of magnitude. ADEME's forward-looking scenarios anticipate a recovery potential of between 4.09 and 12.94 TWh by 2035, driven by the combined effect of the multiplication of data centers and the increase in consumption induced by artificial intelligence. Every request made to a generative AI tool, every trained model, produces heat. Training a model like GPT-5 requires up to 650 MWh, equivalent to the consumption of 60 French households for a year. The question is no longer whether this heat can be useful, but whether cities will be able to organize quickly enough to capture its value before it evaporates.
Sources: engie-solutions.com | calculcee.fr
