A data center can produce two useful outputs: computing and, in the right place, recoverable heat.
Imagine a cold winter morning.
Your apartment needs heat. A hospital nearby needs hot water. Schools and offices across the city are doing the same.
A few kilometers away, a giant AI data center is spending money to remove heat from thousands of servers.
That raises an obvious question:
If one system is paying to get rid of heat while another is paying to make heat, why not connect them?
In the right city, we can.
But that last phrase matters: in the right city.
The International Energy Agency says nearly all electricity used by data centers eventually becomes heat, and roughly 70–80% can potentially be recovered using heat pumps.[1]
That does not mean 70–80% will automatically end up warming homes.
For waste heat to become useful energy, five things have to line up:
Enough heat → useful temperature → nearby demand → timing match → workable contract
By the end of this article, you should be able to look at any “data center will heat the city” announcement and ask whether those five pieces are actually in place.
First, Where Does the Heat Come From?
Electricity enters a server.
The CPU, GPU, memory, storage, networking equipment, and power electronics use that electricity to perform work.
After the work is done, almost all of that energy eventually appears as heat.
That is why a data center needs a cooling system in the first place.
The normal energy path is:
Electricity → Compute → Heat → Outside Environment
Waste-heat reuse changes only the last step.
Electricity → Compute → Heat → Useful Heating
The data center still consumes electricity.
The servers still need cooling.
The difference is that someone else uses the heat instead of letting all of it disappear into the environment.
What Is District Heating?
The easiest place to reuse large amounts of server heat is usually a city that already has district heating.
District heating is a network of insulated pipes that moves hot water from heat sources to many buildings.
One network can serve homes, offices, schools, hospitals, shops, and other customers.
The heat does not have to come from one boiler. A modern network can combine several sources:
- power plants,
- industrial waste heat,
- wastewater,
- geothermal heat,
- large heat pumps,
- and data centers.
That shared pipe network is what makes a data center's waste heat useful at city scale.
Without a nearby customer or heat network, even a huge amount of recoverable heat may have very little economic value.
Why Can't We Send the Server Heat Directly Into Homes?
Because server heat is often not hot enough.
Modern cooling systems may collect heat in warm water, but an older district-heating network can require much hotter water.
The IEA notes that much waste heat below 100°C is still discharged, while modern lower-temperature district-heating networks can use low-temperature sources more easily. Heat pumps can raise the temperature when needed.[1]
A heat pump is best understood as a temperature elevator.
It does not create all of its delivered heat from electricity. It uses electricity to move existing heat from a lower temperature to a higher one.
The path can look like this:
Servers → warm cooling water → heat pump → hotter district-heating water → buildings
A 10 MW Example: What Does a Heat Pump Actually Add?
Let us use a simple teaching example.
Suppose a data center can provide 10 MW of low-temperature waste heat to a large heat pump.
Assume the heat pump has a COP of 4. COP, or coefficient of performance, means four units of useful heat are delivered for each unit of electricity used by the heat pump.
The energy balance is:
Delivered heat = recovered waste heat + heat-pump electricity
With a COP of 4:
10 MW waste heat + about 3.3 MW electricity ≈ 13.3 MW district heat
This is not a project-specific performance claim. Real COP changes with source temperature, district-heating temperature, weather, and equipment.
But the example teaches an important idea.
Recovered heat is not “free heating,” but the extra electricity can move much more heat than it consumes.
If 70–80% Is Recoverable, Why Isn't 70–80% Always Used?
This is one of the most important distinctions in the whole topic.
Recoverable heat means engineers can potentially capture it.
Useful heat means a real customer can take it at the right temperature, place, and time.
A city may not need much space heating in July.
The nearest district-heating pipe may be too far away.
The data-center cooling loop may be too cool for the local network without a large heat pump.
The pipe and heat-pump investment may be too expensive.
Or the utility and data-center operator may not agree on who pays and who carries the risk.
So a more useful chain is:
Waste heat → Recoverable heat → Deliverable heat → Useful heat → Economic value
Finland Shows What City-Scale Heat Recovery Really Requires
The Microsoft–Fortum project around Espoo and Kirkkonummi, Finland, is useful because it shows the scale of the infrastructure around the data center.
In May 2026, Fortum said large heat-pump plants at Hepokorpi and Kolabacken had started operating. Initially they use ambient air and electric boilers. Waste heat from Microsoft's data centers is scheduled to be integrated gradually starting in 2027 as the data centers are commissioned.[2]
Once fully implemented, Fortum expects the data-center waste heat to cover about 40% of the annual 2 TWh district-heating demand of roughly 250,000 heat users in the network area.[2]
Fortum also says it expects about 75% of the data centers' annual waste heat to be used for district heating. Summer demand is one reason the useful share is lower than the theoretical heat available.[3]
The supporting system is large in its own right.
Fortum's two sites include water-to-water and air-to-water heat pumps, up to 180 MW of district-heating output from the water-source heat pumps, 200 MW of electric boilers, and about 800 MWh of thermal storage.[2]
That is the key lesson.
The heat may come from the servers, but city-scale heat reuse requires city-scale infrastructure.
Stockholm Shows What Happens When Waste Heat Gets a Price
Stockholm offers a different lesson.
Stockholm Exergi operates Open District Heating, a marketplace where businesses such as data centers and supermarkets can deliver excess heat into the district-heating system.
The supplier can be paid for the heat.
Stockholm Exergi says the payment is based on the cost of producing equivalent heat in its own plants and varies with conditions such as outdoor temperature. Its current example says that delivering about 1 MW of excess heat for most of the year can generate roughly SEK 2 million annually.[4]
That creates a second product:
Electricity → Compute → Heat → Heat Sale
The data center still earns most of its money from computing.
But in the right heat market, a cost stream can become a revenue stream.
Why Doesn't Every Data Center Do This?
This is the question readers keep asking.
If the heat exists anyway, why throw it away?
The answer is not one technical obstacle. It is a chain of conditions.
Figure 1. Waste heat becomes valuable only when heat quantity, temperature, location, timing, and economics line up.
1. There must be enough heat
A small server room may not justify a major pipe connection. A large data center provides a more stable heat source.
2. The temperature has to match
Low-temperature server heat may need a heat pump before the city can use it.
3. The customer has to be close enough
Heat is physical. It needs pipes.
A 2026 engineering study found that long-distance recovery can be economically possible at very large scale—in its model, up to about 24 km for a 100 MW heat source—but that result depends heavily on system design, storage, and local economics.[7]
That is not a universal 24 km rule.
The simpler lesson remains: distance adds pipe, pumping, heat loss, and capital cost.
4. The heat demand has to exist at the same time
Servers produce heat all year.
A northern city needs far more space heating in winter than in summer.
Domestic hot water, thermal storage, swimming pools, greenhouses, or industrial heat can improve the match, but some heat may still have no buyer at certain times.
5. The contract has to work
Who pays for the heat pump?
Who pays for the connecting pipe?
Who guarantees the heat output?
What happens if the data center changes cooling technology or computing load?
Who benefits from lower cooling costs?
A technically elegant system can still fail economically if those questions are unresolved.
Can We Turn the Waste Heat Back Into Electricity Instead?
In principle, heat can be converted back into electricity.
But temperature matters.
Data-center waste heat is usually low-grade heat: useful thermal energy at a relatively low temperature.
Low temperature means there is less thermodynamic potential for efficient power generation.
That is why district heating is often a better match. Buildings need heat, not electricity, and a heat pump can upgrade the temperature if necessary.
The IEA specifically highlights district heating as one of the strongest ways to use low-temperature waste heat that would otherwise be discharged.[1]
Does Heat Reuse Make a Data Center “Green”?
No single metric can answer that.
Heat reuse does not erase the electricity used by the data center.
It does not tell us whether the electricity is low-carbon.
It does not tell us how much water the cooling system uses.
And it does not guarantee that the recovered heat displaces fossil fuel rather than another low-carbon heat source.
What heat reuse can do is improve the value extracted from energy that has already been used.
That is why it should be measured separately.
What Is Energy Reuse Factor?
The European Union's data-center reporting framework includes a metric called Energy Reuse Factor, or ERF.
The equation is simple:
ERF = Energy reused outside the data center ÷ Total data-center energy
The EU rules count energy only when it is actually reused outside the data-center boundary and substitutes for energy that would otherwise be needed there.[5]
This is useful because it forces an important distinction.
A data center can technically capture heat without anyone meaningfully using it.
ERF is about energy that crosses the boundary and becomes useful somewhere else.
A Simple Heat-Value Test
When you see a proposal saying a new data center will heat local homes, check five gates.
| Gate | Question | Why it matters |
|---|---|---|
| Quantity | How many MW of usable heat are available? | Sets the potential scale. |
| Temperature | What temperature leaves the cooling loop? | Determines heat-pump lift and efficiency. |
| Distance | How far is the nearest useful heat network or customer? | Drives pipe and pumping cost. |
| Timing | Does demand exist when the data center produces heat? | Separates recoverable heat from actually used heat. |
| Contract | Who pays, owns, guarantees, and buys? | Determines whether the project closes financially. |
If one of these gates is missing, “waste heat will warm the city” may still be a plan rather than a working energy system.
Could Heat Reuse Change Where Data Centers Are Built?
Yes, at least in some markets.
A traditional data-center site search focuses on electricity, grid capacity, fiber, land, cooling conditions, permitting, and risk.
Heat reuse adds another resource:
a nearby heat customer.
Fortum says the Microsoft sites in Espoo and Kirkkonummi were chosen with heat recovery in mind, including proximity to district heating, a large enough site, reliable electricity, and strong data connectivity.[3]
This changes the city's role.
The city is no longer only the place that supplies land, grid access, permits, and workers.
It can also become the customer for one of the data center's largest by-products.
So, Can Data Center Waste Heat Warm a City?
We started with two systems sitting near each other.
One was paying to remove heat.
The other was paying to produce heat.
Connecting them can make sense.
But the value does not come from heat alone.
It comes from matching the heat to a real local need.
Waste heat has value when the right heat meets the right customer at the right temperature, place, and time.
That is why Stockholm can buy heat from businesses.
It is why Fortum can build a city-scale system around Microsoft's future waste heat.
And it is why the same idea may make little sense for an isolated data center with no nearby heating network.
What to Watch Next
- Waste-heat temperature: warmer cooling water can reduce the heat-pump lift.
- Recoverable MW: how much heat is actually available at the handoff point?
- Annual useful share: what percentage is expected to find a customer across the full year?
- Heat-pump COP: how much extra electricity is needed to upgrade the heat?
- Connection distance: how much pipe and pumping are required?
- Thermal storage: can heat production and demand be shifted in time?
- ERF: how much energy is actually reused outside the data-center boundary?
- Commercial structure: who pays for the infrastructure and how is heat priced?
The next article brings the recent series together:
Cloud, Edge, or Orbit: Where Should AI Computing Actually Happen?
Key Terms
waste heat
Heat produced as a by-product of another process. In a data center, most electricity eventually appears as heat that must be removed.
district heating
A network that distributes hot water or steam from one or more heat sources to many buildings.
heat pump
A machine that uses electricity to move heat from a lower temperature to a higher, more useful temperature.
COP
Coefficient of performance. For a heating heat pump, useful heat delivered divided by electricity consumed by the heat pump.
low-grade heat
Heat available at a relatively low temperature. It may need a heat pump before it can serve a hotter heating network.
thermal storage
A system that stores heat so it can be used later, helping match steady heat production with changing demand.
Energy Reuse Factor (ERF)
Energy reused outside the data-center boundary divided by total data-center energy consumption.
Related Articles
- Smaller AI Models, More AI Use: Why Data Center Demand May Still Grow
- Will Edge AI Reduce the Need for Giant Data Centers?
- Space Is Cold. So Why Is Cooling a Data Center in Orbit So Hard?
- How Much Power Is 100 MW? An AI Data Center Compared with Entire Cities
Sources
- International Energy Agency — Opportunities for District Heating in the Changing Energy Landscape, 2025.
- Fortum — Heat production starts at two large data-centre sites in Finland, May 6, 2026.
- Fortum — Fortum and Microsoft's data-centre heat-recovery project.
- Stockholm Exergi — Open District Heating / Heat Recovery.
- European Union — Delegated Regulation (EU) 2024/1364: Data-centre sustainability indicators.
- Renewable and Sustainable Energy Reviews — Data Center Waste Heat for District Heating Networks: A Review, 2025.
- Energy — Large-scale long-distance data center waste heat district heating system, 2026.
Updated: October 3, 2026 · Sources checked through: October 3, 2026 · Heat-pump COP and the 10 MW example are teaching assumptions, not project-specific performance claims.