An answer appears on a screen. In a sealed building somewhere behind it, electricity has crossed millions of microscopic switches and emerged in its final physical form: heat.
Today, most of that heat is treated as a problem. Fans, pumps and cooling towers carry it away from processors that must remain within a narrow temperature range. The computation travels onwards as language, an image or a prediction. The warmth is released into the air.
France is preparing a second destination for it. Water will collect heat close to the chips. A heat pump will lift its temperature. Insulated pipes will deliver it beneath streets to apartments, offices, hospitals, schools and swimming pools. The same electricity will perform two useful jobs in sequence: first computation, then heating.
This is already happening in Saint-Denis. Servers inside an Equinix data centre help warm the Olympic Aquatics Centre and the surrounding Plaine Saulnier development. France's next AI campuses will be vastly larger, their processors more densely packed and their liquid-cooling systems better suited to capturing useful heat. National law now requires large data centres to recover that heat whenever the economics work. France is simultaneously planning to double or triple the energy carried by its district-heating networks.
ParallaxSee forecasts that by the end of 2035, heat-recovery systems attached to French data centres will deliver at least 2.5 terawatt-hours of useful heat in a single year—the annual heat demand of 250,000 average French homes. House confidence: 76%.
The household number is an energy equivalent. Some of the heat will warm homes directly; some will serve offices, pools, hospitals, hot-water systems, greenhouses and other nearby customers. The measurable forecast is the heat: 2.5 TWh delivered beyond the computing halls during 2035.
AI will live in the cloud. Its exhaust will run beneath the pavement.
01 — Every computation ends as heat.
A processor appears to deal in abstractions. Physically, it moves electrical charge. Billions of transistors switch between states; wires resist the current passing through them; microscopic structures store and release charge. The useful result may be a sentence, but the energy leaves the chip as random molecular motion. That is heat.
The arithmetic is unusually direct. A computing hall drawing one megawatt for its processors is also producing approximately one megawatt of continuous heat inside the hall. Electricity used by power supplies, networking and cooling eventually joins it. Unlike the warm air from a house, this flow is concentrated, measurable and available through every hour of the year.
The servers must still be protected. Bruno Lafitte, an information-technology expert at France's ecological-transition agency, ADEME, calls cooling, in translation from French, “one of a data centre's vital functions.” Traditional rooms hold the air near 25°C and spend a significant share of their electricity moving heat away. ADEME's French report from inside the data-centre industry
The efficiency measure is called PUE, or power usage effectiveness. A perfect score of 1 would mean every watt entering the building reaches computing equipment. ADEME modelled representative French systems and found PUE values around 1.36 to 1.39 for air-cooled chillers and 1.24 to 1.27 for water-condensed systems using free cooling. Direct liquid cooling performed better again. ADEME's French study of data-centre cooling
Those figures describe a new urban resource. The processor consumes high-value electricity, performs high-value computation and releases a steady stream of low-temperature energy. France will build the machinery that gives that final stream a customer.
02 — AI makes the heat easier to capture.
Older data centres cool a room. Fans push cold air through the fronts of server cabinets; hot air leaves through the backs; chillers then extract heat from a large volume of moving air. The method protects ordinary equipment, but it gathers heat far from its source and at a modest temperature.
AI changes the geometry. Accelerators used to train and operate large models crowd enormous electrical power into each rack. Air struggles to remove that intensity. New systems bring liquid directly to cold plates mounted on processors or immerse components in a dielectric fluid. Water carries far more heat than the same volume of air, and collecting it millimetres from a chip preserves a higher, more useful temperature.
ADEME's 2026 French forecast explicitly identifies liquid cooling as the technology that makes server heat easier to recover at a valuable temperature. Its separate cooling study found direct liquid systems produced the lowest operating overhead among the configurations examined. ADEME's French data-centre outlook to 2060, ADEME's French cooling comparison
Academic modelling points towards the next step. A 2024 Applied Energy study found that direct-to-chip liquid cooling can produce water warm enough for suitable building systems to use through a heat exchanger, sometimes without a heat pump. The modelled arrangements recovered heat while reducing the electricity required for cooling. Applied Energy study of liquid-cooled “data furnaces”
The density of AI therefore creates its own remedy. More computing power produces more heat within a smaller footprint. Liquid captures it closer to the silicon. Warmer water improves the performance of the heat pump—or allows a future low-temperature building to bypass that pump altogether.
The AI rack is becoming a boiler whose principal product happens to be intelligence.
03 — France is assembling the heat source now.
ADEME counted 352 active data centres in France consuming approximately 10 TWh of electricity a year. RTE, the national transmission operator, expects data-centre consumption to reach 15 to 20 TWh in 2030 and 23 to 28 TWh in 2035. ADEME's French 2026 data-centre assessment, RTE's French data-centre figures
RTE's central forecast already assumes that only part of the connection queue reaches operation. By May 2026, the operator had signed reservations for almost 18 GW across roughly 80 projects, up from about 5 GW at the end of 2024. Around ten proposals requested more than 400 MW apiece. Five fast-track sites can accept between 400 MW and more than one gigawatt with limited new overhead transmission construction.
The attraction is electrical as much as digital. France generated more electricity than it consumed for a second consecutive year, exported one-fifth of production and produced power that was more than 95% decarbonised in 2025. EDF is offering former industrial sites with strong grid connections to data-centre operators. Large AI workloads can also sit farther from Paris than latency-sensitive trading and cloud services.
Every completed megawatt adds an almost continuous thermal source. ADEME estimates that France's net recoverable data-centre heat could rise from 1.77 TWh in 2024 to between 4.09 and 12.94 TWh in 2035, depending on the path taken by digital demand and public policy. Its net figure already adjusts the gross heat for improving recovery technology and the availability of possible customers. The agency also stresses that the economic and local constraints must be solved project by project. ADEME's French prospective study
ParallaxSee's 2.5-TWh forecast uses 61% of the bottom of that range. France will convert that share into pipes, contracts and metered deliveries. The processors will supply the heat automatically. The coming decade belongs to the connections.
04 — Saint-Denis has proved the complete circuit.
At Equinix's PA10 data centre in Saint-Denis, the cooling loop captures water at approximately 28°C. That is warm enough to contain valuable energy and too cool for the older radiators connected to a conventional municipal network. Three heat pumps perform the lift. They compress a refrigerant, concentrating the captured energy until outgoing water reaches approximately 65°C.
The hot water then enters the Plaine Commune Énergie network. It travels through a 2.61-kilometre extension and ten new substations to the Plaine Saulnier development and the Olympic Aquatics Centre. The machines that once needed only a cooling outlet have become a municipal heat source. Plaine Commune's French explanation of the project
The regional project accounts make the physics visible. The installation is designed to deliver 10,364 MWh of heat each year, including 7,873 MWh classified as renewable and recovered heat from the data centre. The difference is principally the electricity used by the heat-pump system. Those figures imply a coefficient of performance close to four: roughly four units of useful heat leave for each unit of electricity consumed by the pump.
The same system covers 68% of the development's annual heat demand, supplies 1,083 average-home equivalents and avoids a projected 1,533 tonnes of carbon dioxide each year. The official comparison defines an average home as 10 MWh of annual heat—the conversion used in this forecast. Île-de-France's French project and financing report
The benefits have already widened. ADEME reports that the urban network heats the aquatics centre, approximately 1,600 surrounding dwellings and a rooftop farm that produces tomatoes and herbs for a local solidarity grocery. Different official counts describe different boundaries, which is why ParallaxSee uses metered energy rather than the number of connected addresses as its resolution test. ADEME's French report from Saint-Denis
Saint-Denis supplies the template: capture, lift, pipe, meter. The next projects will repeat it at a much larger scale.
05 — The first project's economics already work.
The Saint-Denis recovery system and network extension cost €3.52 million. Heat-capture equipment, exchangers, pumps and distribution represented €2.15 million; civil works and the heat-pump connection represented €1.36 million. ADEME and the Île-de-France region each supplied about €507,000. Equity and borrowing covered the rest. Île-de-France's French financing report
That is approximately €3,250 of capital for each average-home equivalent served. Equinix agreed to provide the captured heat without charge for fifteen years. The network operator pays to upgrade and distribute it, while customers gain a source insulated from the international price of natural gas.
Mechanical scaling gives the national forecast an order of magnitude. Repeating the Saint-Denis ratio until France delivers 2.5 TWh would require roughly €850 million of heat infrastructure and avoid around 370,000 tonnes of carbon dioxide each year. National costs will differ because a long pipe through a dense street costs more than a short connection beside new construction, while a large campus can share equipment across many more megawatts. The calculation still exposes the scale: a programme measured around one billion euros can unlock the thermal by-product of a digital build-out associated with more than €90 billion of announced investment.
The proposed Equinix PA15x campus at Meudon shows the coming unit size. Its environmental assessment describes two recovery installations totalling 28.8 MW of thermal output. Together they could supply roughly 1.73 million square metres of offices—the equivalent of about 8,600 homes—while recovering close to 60% of the site's computing power as useful heat. French environmental authority assessment of PA15x
Twenty-nine installations of that scale equal the headline's 250,000 homes. France already has around 80 projects in RTE's connection pipeline. The large urban campuses closest to customers will make heat recovery a normal part of data-centre infrastructure.
06 — France is building the network that can receive it.
Useful heat needs a road. France had 1,041 district-heating networks extending across 7,944 kilometres in 2024. They delivered 28.3 TWh to more than 52,000 buildings, including 14.3 TWh to the residential sector. Renewable sources supplied 46% of network energy, while recovered energy grew 30% in a single year. French government statistics for heat networks in 2024
France's third Multiannual Energy Programme makes these pipes central to the national transition. By 2035, the government intends to double or triple the energy delivered by heat networks and raise the renewable-and-recovered share to 80%. Its building pathway calls for roughly 360,000 additional homes to join heat networks each year, reaching 5.8 million connected homes by 2035. France's PPE3 energy programme, in French, France's draft low-carbon building pathway, in French
A 2.5-TWh data-centre contribution would represent approximately 9% of the heat delivered by today's networks. After the planned expansion, its share falls to roughly 3–4%. The national target therefore supplies ample room for server heat alongside geothermal energy, biomass, recovered industrial heat and energy from waste.
France is also mapping supply and demand. The public EnRezo and France Chaleur Urbaine services place data centres, industrial heat sources, existing pipes and dense buildings on the same map. That changes heat recovery from an environmental promise added late in planning into a criterion for choosing the site itself. Cerema's French map of recoverable heat, France Chaleur Urbaine's French network data
Electric grids brought power to the twentieth-century city. Heat maps will determine where the computational city grows next.
07 — French rules will make the hot-water pipe part of the data centre.
The regulatory machinery is now catching up with the thermodynamics. Data-centre operators with at least 500 kW of installed computing power must report their annual electricity, water and reused-heat performance. Centres with at least 1 MW of total building power must recover their waste heat unless a technical-economic analysis demonstrates that recovery is unreasonable. French Ministry for Ecological Transition guidance
That test matters because it moves heat into the design phase. A developer seeking land and a high-voltage connection must also identify nearby demand, model pipe length and agree who will finance the heat pumps. Liquid cooling, exchange equipment and space for pumps can be designed into the building before concrete fixes the layout.
Île-de-France contains 44% of French data-centre capacity and has adopted an even clearer doctrine. Its 2026–2028 strategy calls for heat recovery to become systematic, cites three regionally supported projects at Saint-Denis and La Courneuve and proposes that grid access reflect each project's territorial value. Rural sites are directed towards greenhouses, aquaculture, crop drying and low-temperature industry when housing networks are unavailable. Île-de-France's French data-centre strategy
At Meudon, Equinix and ENGIE Solutions have already signed the partnership for a second local heat network. Thierry Landais, ENGIE Solutions' regional network director, described the model in French. “It is a virtuous circle between industry and local communities, accelerating decarbonisation on both sides,” he said in translation. The original French announcement and quotation
By the end of this decade, a large French data centre that releases all its heat into the open air will look incomplete. The high-voltage cable brings its energy in. The hot-water connection will carry part of that energy onwards.
08 — Constant computation will serve a changing city.
A data centre produces heat through summer and winter. Homes demand much more of it on a January night than on a July afternoon. France can turn that difference into a portfolio of customers. Domestic hot water, hospitals, swimming pools, laundries and industrial processes require heat throughout the year. Greenhouses and food production extend demand beyond the residential season. Large water tanks can store hours or days of production, while geothermal plants and other network sources cover the coldest peaks.
This consistency gives server heat special value. Academic analysis using real district-heating demand and data-centre operating profiles found that recovered heat remained usable for more than 95% of operating hours across its scenarios. Thermal-storage research shows that water tanks can also absorb the mismatch between steady computing and changing demand while shaving winter peaks. Open-access district-heating study in Energy, Open-access study of data-centre heat storage
Temperature will improve too. Existing French networks often carry water hot enough for older buildings, which is why Saint-Denis lifts 28°C water to 65°C. Newer buildings need less heat and can operate with lower-temperature loops. Direct liquid cooling will meanwhile collect warmer water from AI accelerators. Those two trends move towards each other: server output rises while building-network temperature falls.
Distance becomes a planning variable. A pipe crossing ten kilometres of established city can erase the advantage of free heat. A data centre beside a new neighbourhood, hospital campus or existing network can unlock it. France's fast-growing clusters around Paris, Marseille and the Hauts-de-France combine dense demand, existing infrastructure and large proposed computing loads.
The winning sites will sell computation and place their warmth locally. The city will supply the missing half of the machine.
09 — The path to 250,000 homes is already visible.
The forecast's 2.5 TWh equals 61% of ADEME's most restrained 4.09-TWh case and 19% of its 12.94-TWh trend case. Measured against the French government's expanding heat-network programme, it becomes a modest but valuable national source.
The conversion is transparent. Île-de-France's project accounts use 10 MWh as the annual heat demand of an average dwelling. Divide 2.5 million MWh by ten and the result is 250,000 French-home equivalents. Metered heat preserves the comparison as the mix of homes, offices and public buildings changes.
Between 2026 and 2029, reporting rules and cost-benefit studies will expose which campuses have a viable customer nearby. Saint-Denis and Meudon will turn recovery from a novelty into a procurement template. AI's shift towards liquid cooling will raise the quality of the available heat.
Between 2029 and 2032, the first wave of France's newly connected high-power campuses will enter service. Municipalities will extend networks towards the strongest sources; developers will place new neighbourhood loops beside stable computing loads; contracts will divide the costs and benefits among data-centre operators, heat-network companies and local authorities.
Between 2032 and 2035, several tens of large recoveries and a wider field of small installations will cross the threshold. Twenty-nine Meudon-scale systems would be enough. Hundreds of embedded computing boilers in apartment blocks and commercial buildings can supply the remainder.
The forecast resolves true if qualifying recovery systems attached to data centres physically located in France deliver at least 2.5 TWh of useful heat during calendar year 2035 to customers or uses beyond the computing process. Announcements, theoretical potential and heat released into the atmosphere do not count.
The answer on the screen will disappear when the tab closes. Its warmth will remain in the building long afterwards.
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