Waste heat: how data centers are becoming district heating plants

Heat, an unavoidable by-product

A server turns almost all of its electricity into heat. For a 100 MW campus, that is the equivalent of the heating capacity of tens of thousands of homes, available all year round, day and night. This "waste" heat, produced without being sought, is currently removed by chillers and dry coolers.

Opponents of the Fouju AI campus, for example, estimate that the site will produce the equivalent of the heat of 200,000 households. Used well, this heat can instead become an argument in favour of projects.

What the law says

The French law known as DDADUE 5 introduced an obligation for data centers with an installed capacity of at least 1 MW. They must reuse their waste heat, in particular through a heating or cooling network. The obligation is considered met if the energy reuse factor (ERF) reaches at least 0.20. This threshold is meant to rise to 0.40 as technologies and outlets develop.

For data centers whose permit was filed before 1 January 2026 and which do not reuse their heat, the operator must carry out a cost-benefit analysis. Exemptions exist when technical and economic conditions do not allow the threshold to be reached, conditions that the government has yet to specify.

The European framework points the same way with the Energy Efficiency Directive, and France's energy plan aims to multiply by seven the recovered heat delivered into networks between 2022 and 2035.

Why so little heat is recovered today

Three obstacles explain the gap between potential and reality.

Temperature. In an air-cooled data center, water leaves at around 30 °C, too cold for a conventional district heating network. Heat pumps are then needed to raise the temperature, which costs energy and investment. With liquid cooling, heat can leave at around 60 °C, which changes the picture (see Liquid cooling, immersion: the innovations changing how data centers are built).

Location. Many data centers were built far from heat networks. In Seine-Saint-Denis, for example, many sites are not located close to existing networks. Building a new network is expensive, and municipalities do not always have the means.

Mismatched demand. A data center produces heat continuously, whereas heating needs vary with the seasons. Complementary uses (domestic hot water, swimming pools, greenhouses, industry) or thermal storage are therefore needed.

Examples that show the way

In Saint-Denis, an Equinix data center reuses part of its heat, notably to heat a swimming pool. In Stockholm, heat recovered from data centers supplies the equivalent of 10,000 homes through district heating. The Nordic countries, though colder, recover more because their heat networks are highly developed and data centers connect to them from the design stage.

Support exists in France: energy savings certificates (standard sheet RES-CH-108) can fund the recovery of waste heat into a heat network or a third-party site.

What this changes for projects

Heat reuse is not added at the end of a project. It is decided when the site is chosen and during design: cooling technology, temperature level, location of the heat exchangers, routing to users, agreement with a heat network operator. It is an engineering subject at the crossroads of building services, energy and urban planning.