District Heat

The pipe is the product

A heat pump moves heat rather than makes it, and at city scale the physics is the same — compressor, refrigerant, condenser — but the delivery mechanism changes entirely. Instead of a box on a wall serving one building, a district heating network runs insulated pipes under streets, distributing hot water to thousands of homes and commercial buildings from a single plant. The compressor gets bigger; the economics change in ways that cut both directions.

A heat pump unit on a bracket against a house wall with insulated pipework and a condensate drain, daylight — In this sectionA heat pump does not make heat, it moves it, which is why its output can exceed the energy put in without breaking anything. Three or four units for one

The numbers already in the ground are substantial. Stockholm's district heating network, operated by Stockholm Exergi and covering roughly 80 percent of the city's heated floor area, is among the largest in Europe. Helsinki, Copenhagen, and Warsaw each run networks serving the majority of their urban building stock. In China, district heating covered approximately 10.5 billion square metres of floor space by the early 2020s, the largest system in the world by area, though most of it is still fed by coal-fired boilers.

The central plant can use heat sources that are impossible at household scale.

The central plant can use heat sources that are impossible at household scale. Seawater, river water, treated sewage effluent, and industrial waste heat all sit at temperatures low enough to be useless to a single building but workable for a large heat pump with long enough pipes to carry the output. Stockholm Exergi draws from Lake Mälaren; a planned expansion would use large electric heat pumps drawing on Baltic seawater. Geothermal is another input: the Hellisheidi geothermal plant in Iceland feeds Reykjavik's district system, which covers close to 90 percent of the city's space heating through roughly 3,000 kilometres of pipe.

The economics are not automatically favorable. A district network requires enormous upfront capital in civil works — trenching, insulated pipework, substations — most of which depreciates over forty to sixty years. Connection rates must be high enough to justify that investment; a network serving only a third of buildings along a street does not pencil. Low-density suburbs are generally uneconomical to connect. The model works best where buildings are dense, old enough to have high heat demand, and where the municipality or a long-tenured utility can commit capital over decades.

An outdoor heat pump unit with frost on its fins in winter light — Performance falls as the outside gets colder, which is exactly when the heat is wanted, and that trade is the whole engineering argument. The cold end

Operating temperature matters as well. Older networks run at 80–90 °C and require conventional heat sources to reach that; newer fourth-generation designs target 50–60 °C, which is within reach of large heat pumps running at reasonable efficiency. Retrofitting the building side — radiators, controls — to accept lower supply temperatures is often the binding constraint, not the plant.

The Boden industrial cluster in northern Sweden, developed partly around low-cost renewable electricity, illustrates the logic in reverse: waste heat from electrolysis and data centres is a potential district heat source, not a problem to be discharged.