In short
- 1 MW of IT running all year gives about 8,760 MWh of heat.
- Server heat leaves at 25–55 °C; most district networks run at 65–90 °C. A heat pump closes the gap.
- Liquid cooling makes the heat warmer and easier to use: a 55 °C source into a 70 °C network reaches a COP near 11.
- Demand drops in summer, so even good schemes reuse about three quarters of annual waste heat.
- EU and German rules are turning heat reuse from an option into a duty.
Where the heat is, and how warm
| Cooling type | Source temperature | Typical capture |
|---|---|---|
| Air cooled (CRAH return air) | ~28 °C | Low-grade heat from the return air or chilled-water return |
| Rear-door heat exchanger | ~35 °C | Water from the door coils |
| Cold plate, W32 | ~40 °C | Facility water return from the CDUs |
| Cold plate, W45 | ~55 °C | Hot enough for low-temperature networks almost directly |
Heat is taken from the cooling water loop through a heat exchanger. The warmer the loop, the less work is left for the heat pump. That is why liquid-cooled AI halls are the best heat sources yet. More detail on the temperatures section.
Why a heat pump is usually needed
A heat pump uses electricity to move heat from a lower to a higher temperature. Its efficiency, the COP, falls as the lift grows. With a realistic 50 % of the Carnot limit:
| Source → network | COP | Electricity per MWh of heat |
|---|---|---|
| 28 °C → 70 °C (air cooled) | 4.1 | 0.24 MWh |
| 40 °C → 70 °C (cold plate W32) | 5.7 | 0.17 MWh |
| 55 °C → 70 °C (cold plate W45) | 11.4 | 0.09 MWh |
If the source is hotter than the network, a heat exchanger alone is enough. The calculation is explained in the heat reuse calculation guide.
District heating takes the heat
- 3rd generation networks run at 80–100 °C: common in older city systems; need a large lift.
- 4th generation at 50–70 °C: the sweet spot for data center heat.
- 5th generation near ambient, with heat pumps at each user: can take even air-cooled heat directly.
Distance matters as much as temperature. A network within a few kilometres makes a scheme work; one 20 km away rarely does. Germany's draft amendment uses 5 km as the test.
The summer problem
A data center makes heat all year; homes need it mostly in winter. In summer part of the heat is still rejected through dry coolers. Hot water demand, seasonal storage, swimming pools, greenhouses and industrial process heat raise the share. Espoo's plan with Fortum aims to reuse about 75 % of annual waste heat.
Who owns what
- Operator-owned: the data center sells heat at the fence at network temperature.
- Utility-owned: the heat company takes low-grade heat and runs the heat pumps, often on the data center's land.
- Joint venture: shared investment and revenue.
Whatever the model, a long-term offtake agreement and a reachable network are what make the scheme bankable. Some heat is given free; Equinix PA10 supplies its heat free of charge for 15 years.
Case studies
| Scheme | Scale | Temperatures |
|---|---|---|
| Meta, Odense · since 2019 | 215,000 MWh/yr, 12,000+ homes, ~45 MW heat-pump output | to 70–75 °C |
| Microsoft with Fortum, Espoo & Kirkkonummi · planned | ~40 % of the area's district heat | ~75 % of annual waste heat reused |
| Equinix PA10, Saint-Denis · since 2024 | 10,000 MWh/yr, 1,000+ homes equivalent | 28 → 65 °C, three heat pumps |
Sources are listed in the case studies section.
Questions about heat reuse
How much heat does a data center produce?
About as much as the electricity it uses. 1 MW of IT running all year is 8,760 MWh of heat, enough for several hundred homes after the heat pump.
Does heat reuse lower PUE?
No. PUE ignores where heat goes. Reuse is reported as ERF, the energy reuse factor.
Can air-cooled data centers reuse heat?
Yes, but the source is only about 28 °C, so the heat pump works harder (COP around 4 into a 70 °C network). Low-temperature networks help.
Is data center heat green?
It replaces heat that would otherwise come from boilers. The CO₂ saving depends on what it replaces and how clean the heat pump's electricity is.
Sources
- Ramboll (Odense), Eurelectric on Fortum and Microsoft (Espoo), Data Center Knowledge (Equinix PA10).
- COP = η · Tsupply ÷ (Tsupply − Tsource), η = 0.5, temperatures in kelvin.
- Temperatures and network generations are typical engineering values.