Søren Freiesleben has lived in Odense his entire life. He likes the historic Danish city for its size. It’s not too big—just 200,000 people live there—and he never feels like he’s drowning in crowds. So far so normal. But there is something unusual about Odense: Its homes are heated by the social giant Meta.

Since 2020, Meta’s hyperscale data center—spanning 50,000 square meters on an industrial estate on the edge of the city—has been pushing warm air generated by its servers into the district heating network under Odense. That heat is then dispersed through 100,000 households hooked up to the system, with Meta providing enough heat to cover roughly 11,000.

Freiesleben, a local councilor, has a few gripes about sharing such a small city with one of the world’s biggest tech companies. Meta should pay more taxes in Denmark, he argues. Locals have also complained about the bright lights positioned around the data center’s perimeter, he says. But those concerns are trumped by the benefits of the heating system. “It’s a really good idea to use the heat that would otherwise just vanish into thin air,” Freiesleben says. Would he accept more data centers in his city if they were also hooked up to the heating system? “The simple answer,” he says, “is yes.”

Odense is the first place in the world where Meta has tried to pipe excess heat from a data center directly into people’s homes—but it’s not the only tech company doing so. In Ireland, an Amazon data center already helps heat TU Dublin, a university, while Microsoft is building what is expected to be the world’s largest data center heating system in Espoo, a city in southern Finland.

It’s a trend that’s expected to grow. The current AI boom has been accompanied by a data center construction rush. Companies like Microsoft, Google, Meta, and Amazon will have to invest about $1 trillion on infrastructure to handle computing demand for AI, according to research firm Dell’Oro Group.

All those data centers produce a lot of heat. If that heat is redirected to local homes, it can help Big Tech firms meet their climate pledges, and governments can say they struck a deal that made Big Tech give something back to the local community.

“Data centers are like giant refrigerators,” says David Lunts, CEO of Old Oak and Park Royal Development Corporation, a UK property developer. In November, the company announced a new development in London which will be heated by a nearby data center.

A lot of the time, the heat these data centers generate is simply released into the atmosphere. And that, Lunts claims, makes using it to heat nearby buildings a no-brainer. “It’s a win for the data centers because they’re not expelling their surplus heat into the atmosphere, which is not great for their green credentials,” he says. It’s also great for developers, he says, who get a source of “cheap heat.”

The heat pumped out by data centers isn’t hot enough to be plugged into people’s houses directly, and an energy company has to both boost the temperature and handle billing. No one in Odense is getting an energy bill straight from Meta. In Denmark, that process is managed by Fjernvarme Fyn. The utility company captures the heat from Meta at around 80 degrees Fahrenheit (around 26.6 degrees Celsius), before boosting it to the 170 degrees Fahrenheit (76.6 degrees Celsius) needed for district heating, says Palle Grøndahl, Fjernvarme Fyn’s acting head of development.

For Big Tech, there are few better places to experiment with data center heating than in the Nordics. This idea works best when data centers can be connected to preexisting district heating systems, where a group of buildings share a common heating system instead of each having their own. These communal systems are commonplace in countries like Denmark, Finland, and Sweden—and tech isn’t the first industry to experiment with connecting to them.

For the past 20 years, Patrik Öhlund’s home in the northern Swedish city of Luleå has been partly heated by the waste heat from a nearby steel plant. Now, Öhlund, who is director of Energy Markets at Microsoft, is working on recreating this system in the Finnish city of Espoo. But this time it’s Microsoft that’s being hooked up to the local district heating network as part of a project that will eventually heat 100,000 households. Once completed, it’s expected to be the largest data center heating system in the world.

Microsoft’s project in Espoo will generate slightly hotter water—90 degrees Fahrenheit (32.2 degrees Celsius)—than Meta’s Danish system, partly because the Finnish data center will also have the capacity to power AI systems. Finnish energy company Fortum will then boost the heat to between 180 and 250 degrees Fahrenheit (82.2 and 121.1 degrees Celsius), before it enters people’s homes—which should happen sometime after 2025. Heat extracted from data centers that power AI tends to be hotter because they often have a higher-density setup of server racks, says Tom Glover, head of data center transactions at real estate consultancy JLL. “You’re provided with a higher quality of heat, which can be used better within district heating grids,” he adds.

When Microsoft’s Espoo system is switched on, energy prices won’t necessarily be cheaper, according to Teemu Nieminen, who leads the data center heat recovery project for Fortum. Neither company will disclose how much Microsoft is charging for the heat, but they do confirm it’s part of a commercial arrangement. It might not be cheaper, but prices should be more stable, says Nieminen, “compared to fossil fuels, where prices fluctuate very wildly.”

Microsoft also hopes this stability will help make data centers on this scale more welcome in local communities, some of whom take issue with Big Tech sucking up huge amounts of renewable power. “It will keep the prices stable, and with people living nearby knowing this … they are also more positive to our data centers,” says Öhlund.

In Ireland, where data centers have been labeled “energy vampires” by environmentalists and were responsible for 19 percent of the country’s total energy use last year, Amazon is giving its data center’s excess heat away for free to the local university and local government offices, according to John O’Shea, senior energy systems analyst at Codema, Dublin’s energy agency. Amazon’s donation is not an entirely selfless act. “We provide free cooling to them as a byproduct of taking their heat,” O’Shea adds.

So far, the arrangement is working. The project is being expanded and the city is in the process of connecting the Amazon data center to more buildings, as well as 135 apartments currently under construction. But O’Shea is wary of endorsing the data centers. Despite the heating project, there are still concerns in the region of County Dublin, where there are between 65 and 70 data centers, about the amount of energy they use. “The development of data centers themselves is something to be discussed,” O’Shea says. “But if they are being developed, we think it makes sense to use this waste source that is otherwise just being pumped into the air or into waterways.”

There’s a similar attitude in Denmark. Data centers will be responsible for 14 percent of the country’s total energy consumption by 2030, according to a forecast by the Danish Energy Agency. Big Tech data centers might source their energy from renewable sources, but a data center hooked up to local homes still requires the energy of more wind turbines than the homes would if they were heated by the turbines directly, says Henrik Lund, professor of energy planning at Denmark’s Aalborg University. “The data centers themselves are putting pressure on the green transition,” he adds.