Mine water heating is much what it sounds like. You use the warmth of water in an abandoned, flooded mine to help heat buildings.

When pumping stops, groundwater can fill the tunnels. Underground rock generally gets warmer with depth, and the water absorbs that natural heat. A mine that stopped producing coal, therefore, can become a useful reservoir of low-temperature heat.

That is what happened in Heerlen, in the Netherlands’ South Limburg region, although  decades after the mine closed. According to operator Mijnwater’s history of the project, Heerlen’s last coal mine closed in 1974. Pumping from the Limburg mines stopped in 1994, allowing the workings to fill gradually with groundwater.

The mine water project officially began in March 2004, exploring whether that underground resource could help heat and cool the city above.

How the Heerlen system works

The first plant went live in October 2008. A project case study hosted by Renewables Networking describes it as the world’s first mine water geothermal plant.

The early setup is easy to picture: water at two temperatures drawn from different depths. The EU-funded STORM project’s description records two deep wells in the north of the city supplying water at about 28 degrees Celsius. Two shallower wells in the south supplied water closer to 16 degrees.

Twenty-eight degrees is lukewarm. The system uses heat pumps to raise the temperature to the level buildings need for heating and hot water. That means using the mine water as a source of heat, rather than simply sending lukewarm water into conventional radiators. The cooler source can help remove heat from buildings that need cooling.

The individual components are familiar. Herman Eijdems, identified as Mijnwater’s innovation manager in a Construction21 article, put it plainly: “The funny thing is that we don’t actually use any exciting innovations at all.” The novelty lies in how heat pumps, pipes and storage work together.

From a heat supplier to a two-way grid

The original system supplied heating and cooling to a small group of customers through a largely one-way network. Expanding it meant finding a better way to use the available energy. The Renewables Networking case study also describes years of financial losses associated with design and commissioning problems.

Mijnwater says it began developing exchanges between groups of buildings from 2012.

Instead of treating the mine only as a source to draw from, the new approach also used it to store thermal energy. A building that needs cooling releases unwanted heat, which another building may be able to use. When supply and demand do not match, underground storage helps bridge the gap.

Mijnwater’s own explanation of the network gives a useful example: heat removed from a data centre can become a source for heating homes. It also makes clear that the mine workings still serve as both energy sources and thermal buffers. Heat exchange between customers does not make the mine redundant.

Eijdems described the approach as recombining established equipment: “Heat pumps, pipes and storage basements are techniques that have been around for 50 years or more,” he said. “But we have linked the various components together in a new way.”

What the results show

A shorter case study hosted by Renewables Networking reports a 65% reduction in carbon dioxide emissions associated with heating and cooling connected buildings.

Historical accounts also give snapshots of its size. Construction21 described the system as supplying 200,000 square metres of building space in 2018.

Still operating, and drilling deeper in 2026

The system remains active and is expanding. In August 2026, ThinkGeoEnergy reported that Mijnwater had six wells, three warm and three cold, and was developing a fourth warm-water source. Mijnwater’s own announcement of the drilling work identified the target as an old mine gallery about 634 metres underground, with expected water temperatures of roughly 28 degrees Celsius.

Where the idea can travel, and what it needs

The tempting conclusion is that any old mining town could do this. Eijdems has suggested that the approach “can also be used in other former mining areas to heat and cool buildings.” That is a possibility worth investigating, rather than a promise that every flooded mine will make a viable heating system.

Heerlen’s experience involved much more than finding warm water. It required wells, a pipe network, suitable buildings, funding and years of adjustments.

There is also a practical issue at the customer’s end. Mijnwater says most of its connected buildings are new construction. Existing buildings connected to the network have been upgraded to Dutch energy labels A or B to make them suitable for its heating and cooling service. Having a useful source underground does not remove the need to prepare the buildings above it.

What transfers is therefore an approach to using local energy, rather than a ready-made blueprint. The flooded mine provides a source and a place to store heat. The buildings provide demand and, when they need cooling, heat that can be recovered. The network brings those resources together.

Heerlen found a new use for the infrastructure left by an abandoned industry. Its achievement was not simply noticing that the water was warm. It was developing a system that could make that modest warmth useful, share energy between buildings and keep adapting as demand grew.