On the eastern edge of Vienna, where the Danube canal meets the city’s vast Simmering sewage works, there is a building that does something most cities throw away. Every day, hundreds of thousands of cubic metres of treated wastewater leave the ebswien purification plant at a steady twelve to fifteen degrees — warm by the standards of a Central European winter, useless by the standards of anyone wanting a hot bath. Next door, since late 2023, a row of compressors the size of locomotives has been taking that lukewarm water and squeezing the warmth out of it, lifting it to a temperature high enough to feed the district-heating pipes that run under half the city. The water that used to flow uselessly into the river now heats homes.
This is a heat pump — the same machine, in principle, as the one a German homeowner now bolts to the side of a suburban house, only roughly ten thousand times larger. And it is the quiet story underneath the loud one. The loud story is about domestic heat pumps booming and then stalling. The quiet one is about cities discovering that the most consequential heat pump on the continent may not be the one in the garden at all, but the one in the power station.
The boom that wasn’t a beginning
In 2022, after Russia invaded Ukraine and European gas prices went vertical, the domestic heat pump looked like the answer to everything. The European Commission wrote them into REPowerEU, its plan to wean the continent off Russian fossil fuels. Sales jumped roughly 39 per cent in a single year. Factories were expanded, installers hired, order books filled. For a season, the heat pump was the most fashionable appliance in Europe.
It did not last. By the European Heat Pump Association’s own count, sales across fourteen tracked markets fell around 5 per cent in 2023 — from roughly 2.77 to 2.64 million units — and then fell off a cliff in 2024, dropping by something in the region of 21 to 22 per cent to about 2.31 million units across nineteen countries. The association reckons at least 4,000 jobs were cut in the sector that the energy crisis had so recently inflated. The reasons it gives are unglamorous: governments fiddled with subsidies and unsettled buyers; a cost-of-living squeeze made people defer big purchases; and, above all, gas got cheap again while electricity stayed dear.
That last point is the one that matters most, and it is structural rather than seasonal. A heat pump is three to four times more efficient than a gas boiler, but if a unit of electricity costs more than about three times a unit of gas — as it still does in much of Europe — the homeowner’s monthly bill barely moves. The machine is brilliant; the price signal is broken. Add a shortage of trained installers, long waits, and the genuine difficulty of fitting a low-temperature heating system into a draughty old building, and the 2024 slump starts to look less like a failure of the technology and more like a failure of everything around it.
The machine in the power station
While the domestic market wobbled, a different deployment was going on with almost no public attention. Across Europe’s district-heating cities — the places where a single municipal pipe network heats tens of thousands of buildings at once — utilities have been installing large heat pumps that draw warmth not from cold winter air but from steadier, more generous sources: rivers, the sea, treated sewage, the waste heat of industry. The physics is the same; the economics are completely different. A domestic unit fights the coldest air on the coldest day, exactly when it is least efficient. A wastewater pump runs on a source that is warm and constant all winter, and it spreads its enormous capital cost across an entire city.
Vienna’s machine is the headline example. Wien Energie, the city utility, describes the Simmering installation as the most powerful large-scale heat pump in Europe. Its first three units already supply district heating to around 56,000 households; when the full build-out reaches roughly 110 megawatts of output, the company expects to heat on the order of 112,000 homes. The electricity that drives the compressors comes from the Freudenau hydroelectric plant on the Danube a few kilometres upstream — so the warmth in the wastewater is lifted by the power in the river, and the whole loop, by the utility’s account, runs on renewable electricity.
The water that used to flow uselessly into the river now heats homes. The same machine as the one in the suburban garden — only ten thousand times larger.
Wien Energie, on the Simmering wastewater heat pump
From a Danish coal plant to the North Sea
If Vienna shows the scale, the Danish port of Esbjerg shows the substitution. For decades the city was heated by a large coal-fired power station on the harbour. In August 2024 that plant — a unit of several hundred thermal megawatts — was shut down. In its place, the local utility DIN Forsyning switched on a 70-megawatt heat pump that pulls warmth straight out of the North Sea. Built by the German engineering firm MAN Energy Solutions (now Everllence) and using carbon dioxide rather than a synthetic refrigerant, it is among the largest seawater heat pumps ever brought into operation. By the utility’s figures it should deliver around 280 gigawatt-hours of heat a year — enough for roughly 25,000 households — and cut something close to 120,000 tonnes of carbon dioxide annually. A coal plant on the quay became a machine that heats the city from the cold sea beside it.
None of this is wholly new. Stockholm worked it out forty years ago. When oil prices spiked in the early 1980s and Sweden had a surplus of cheap nuclear electricity, the city installed enormous seawater heat pumps at the Ropsten plant in the Värtan harbour — a facility of roughly 180 megawatts that has been quietly heating Stockholmers ever since, long before the rest of Europe thought of the idea. Helsinki followed a comparable logic underground: beneath Katri Vala park, the city utility Helen runs a plant that makes both district heat and district cooling from purified wastewater, with a heating capacity in the region of 160 megawatts. The Nordic cities treated the large heat pump as ordinary municipal plumbing decades before it became a climate headline.
Germany joins late, and big
The newer converts are German, and their projects read like an escalating bet. In Mannheim, the utility MVV connected a 20-megawatt heat pump drawing on the Rhine to its district-heating grid in October 2023 — modest, enough for some 3,500 households, but a deliberate proof of concept. The company has since announced a far larger river pump, in the order of 165 megawatts, intended to be among the biggest of its kind in the world when it comes online later this decade. In Hamburg, meanwhile, the city’s energy utility is building a 60-megawatt wastewater heat pump — four units of fifteen megawatts apiece — at the Dradenau treatment works, due to be running in 2026 and expected to heat up to 39,000 households while avoiding tens of thousands of tonnes of carbon a year.
The pattern is consistent across all of these. The heat source is a body of water or a flow of waste that no one previously valued. The output feeds an existing pipe network rather than an individual home. And the project is framed less as a green gesture than as a replacement: this is how the city will heat itself once the gas and the coal are gone. The large urban heat pump is, in effect, the district-heating equivalent of swapping the boiler — except the boiler heats a hundred thousand flats.
What still stands in the way
The barriers are real and they do not disappear at scale. The same broken electricity-to-gas price ratio that deters the homeowner also squeezes the utility’s operating margin; a large pump is only as cheap to run as the power it draws. The electricity grid is the harder constraint. A city that electrifies its heating — whether through a million domestic units or a handful of giant ones — places a load on the distribution network that the network was never built to carry, and reinforcing it is slow, expensive and frequently the thing that holds a project up. There is the shortage of engineers and installers, which the International Energy Agency expects to bite harder through the decade. And there is the awkward fact that the cities best placed to do this are the ones that already have a district-heating network — a Nordic, Austrian and German inheritance that much of southern and western Europe simply does not possess.
So the honest summary is not a triumph. Domestic heat pump sales are down, and may stay subdued until governments fix the price signal that makes the cleaner machine cost more to run than the dirtier one. But underneath that disappointing headline, a slower and more durable shift is underway: city utilities are quietly rebuilding the engine room of European heat, one river, one harbour and one sewage plant at a time. The machine in the garden gets the attention. The machine in the power station may, in the end, do more of the work.
Sources: European Heat Pump Association (EHPA) market reports and statistics, 2023–2025, and “Pump it down: why heat pump sales dropped in 2023” (April 2024); European Commission REPowerEU plan; International Energy Agency, The Future of Heat Pumps; Wien Energie and ebswien (Simmering wastewater heat pump); DIN Forsyning and MAN Energy Solutions/Everllence (Esbjerg seawater heat pump); Helen Oy (Katri Vala plant, Helsinki); Friotherm / Stockholm Exergi (Värtan Ropsten); MVV Energie AG (Mannheim Rhine heat pump); Hamburger Energiewerke and Hamburg Wasser (Dradenau wastewater heat pump); Johnson Controls.
