Material Insights · Understanding the Material World

District heating and cooling networks: How it works

Four generations of networks, each running cooler than the last

What this tab covers How the energy actually moves, step by step: the science that makes it work and the engineering that makes it practical.

A district heating network makes heat at a few central plants and pumps it as hot water or steam through buried insulated pipes to many buildings, then brings the cooled water back to be reheated. District cooling does the same with chilled water; industrial steam lines do it over shorter distances at higher pressure.

Step by step

  1. Make the heatHeat enters the water at a plant: a boiler, a combined heat and power station using heat a power plant would otherwise throw away, or a large heat pump. Esbjerg in Denmark runs a 70 MW seawater heat pump using CO2 as its working fluid, supplying about 280 GWh a year (enough for an estimated 25,000 homes), alongside a 60 MW wood-chip boiler and a 40 MW electric boiler for peaks and backup.
  2. Choose a temperatureEarly networks (1880 to 1930) sent steam below 200 C; the second generation used pressurised water above 100 C; the third, from 1980, uses factory-made pre-insulated pipes below 100 C; the fourth targets 50 to 60 C. Cooler water leaks less heat and lets cheaper low-grade sources join.
  3. Pump it through insulated pipesThe standard pipe is a steel carrier inside polyurethane foam and a plastic jacket. Traditional networks are built for 120 C for 30 years and up to 25 bar; flexible plastic (PEX) pipes work below about 80 C at 6 to 10 bar.
  4. Hand over at the buildingA substation in each building passes heat through a heat exchanger into the building's own radiators and taps, without mixing the water, and a meter records the energy.
  5. Return the cooled waterThe cooler the water comes back, the more heat each litre has delivered and the less must be pumped. In a worked EU example, lowering the return temperature by 5 C cut pipe losses by about 5% and pumping electricity by about 36%.
  6. Steam lines and coolingIndustrial steam mains run from about 15 to 600 psig; steam gives up heat as it condenses, and traps drain the condensate back to the boiler. Manhattan's Con Edison system has about 105 miles (about 169 km, calculated) of pipe at 125 to 180 psi. District cooling plants chill water centrally and pipe it out instead.

The science

Heat leaks out of a pipe by conduction through the insulation, at a rate roughly proportional to the temperature difference between the water and the surrounding ground. Halving that difference roughly halves the leak, which is why each generation runs cooler. The heat delivered equals the water flow multiplied by the drop in temperature across the customer (and the heat capacity of water), so a wide gap between supply and return lets a smaller flow carry the same heat. In a given pipe the pressure drop rises with the square of flow and pump power roughly with the cube, so a small cut in flow saves a lot of electricity. Steam carries much more energy per kilogram because it releases its latent heat when it condenses, but it needs higher pressures, traps to remove condensate, and careful operation: condensate pooling in a steam main can cause violent water hammer.

The engineering

Pre-insulated pipes come in 12 m or 16 m lengths with welded joints, foam-filled couplers and alarm wires that detect water entering the insulation. Twin pipes put supply and return in one casing. Networks are sized by heat density: in a Swiss model network the cost-optimal pipe was DN 80 (80 mm bore). Large networks add thermal stores, booster pumps and control software. Steam systems need traps, expansion loops and condensate return lines. Cooling networks need larger pipes because chilled water runs with a small temperature difference, so each litre carries less energy.

Key numbers

European networks about 17,000 networks, nearly 190,000 km of pipe, about 300 GW of heat capacity, about 70 million people served (Euroheat & Power, 2023 outlook)4 Renewables and waste heat in European district heat 44.1% in 2023, up from 202220 4th-generation supply temperature 50 to 60 C (up to 70 C)1 Traditional pipe design rating 120 C for 30 years, peaks to 140 C, up to 25 bar2 Industrial steam pressures 15 to 600 psig (about 1 to 41 bar gauge, calculated)7 Manhattan steam system about 105 miles of pipe (about 169 km, calculated); about 1,555 customers; largest district steam system in the US824 Largest district cooling provider (by its own account) Empower (Dubai), 1.7 million refrigeration tons connected at end 2025 (about 6 GW of cooling, calculated)10

Sources

  1. Progression of District Heating: 1st to 4th generation Aalborg University (Thorsen, Lund, Mathiesen), 2018
  2. Distribution technologies for low-temperature district heating (LTDH) European Commission, Smart Cities Marketplace, 2019
  3. District heating and cooling is instrumental to decarbonisation (summary of Euroheat & Power DHC Market Outlook 2023) International District Energy Association, 2023
  4. Steam Tip Sheet 2: Insulate Steam Distribution and Condensate Return Lines US Department of Energy, 2012
  5. Con Edison Steam: Future Overview (presentation to NYISO) Con Edison / NYISO, 2022
  6. Empower's connected capacity has increased by 315 times since its establishment International District Energy Association, 2026
  7. DHC Market Outlook 2025: The heat transition in action Euroheat & Power, 2025
  8. Corporate facts Con Edison, 2026