District heating: an ancient concept for the modern world

The Romans knew a thing or two about staying cosy. They piped hot water beneath their homes, bathhouses and greenhouses, using early versions of centralised heating. Centuries later, the same core concept is transforming how we heat homes and businesses across whole cities.

District heating pops up in all sorts of places – from a (still-operating!) 14th-century system in France to the steam-fed setup at the US Naval Academy from the 1850s. But it was Birdsill Holly, an American engineer, who really sparked the modern movement. In 1877, he launched the first commercial district heating system in Lockport, New York – changing how cities could think about heat.

Fast forward to today and that ancient idea is being reimagined to form a vital part of our net-zero toolkit. At E.ON, we operate 70+ district heating sites around the UK, serving more than 50,000 customers with reliable, low-carbon heating and hot water – from Exeter to east London and from Southampton to Tyneside

According to the Heat Network Industry Council, district heating could deliver annual CO₂ savings of 15 million tonnes by 2050 – a 9% cut in the UK’s heating emissions. That’s why the Government wants to scale it rapidly, aiming for 20% of the nation’s heat to come from district systems (up from just 3% now). That’s a big shift, and one that works best in dense urban areas, where heat networks really shine.

How it works: the tech behind the heat

At its heart, a district heating network is simple: instead of each building running its own boiler, heat is produced at a central source and piped out to homes, offices, and public spaces via super-insulated networks. This centralised approach not only cuts carbon emissions, but it can also reduce energy bills and the air pollution connected to fossil fuel use.

But behind that simplicity lies smart, adaptable technology. We’re now in the fourth (and developing fifth!) generation of district heating systems, built with climate goals front and centre.

Today’s networks can run at lower temperatures (70°C and below), making them far more energy efficient. They can use a wide range of heat sources, from waste heat recovered from industrial processes and underground tube stations, to large scale heat pumps and geothermal systems. Crucially, they’re built to ‘flex’ – storing energy when demand falls or when generation is more plentiful, ready to deliver it when it’s needed most.

Inside a modern heat network

A fully functioning network typically includes:

• The heat source: This could be a biomass plant, energy from a waste plant where household rubbish is incinerated, a waste water treatment works, a CHP (combined heat and power) engine, a bank of heat pumps, or even a data centre giving off excess warmth
• The pipework: A network of insulated steel or plastic pipes that distribute hot water or steam across towns and cities
• The user connection: Each building is fitted with a heat interface unit and what’s called a ‘closed loop’ system that taps the heat from the network without needing to use the same water source.

As you move further from the heat source, the pipework gets smaller — a cleverly engineered system designed to maximise flow while minimising energy loss.

Heating the homes (and hearts) of our cities

At E.ON, we’re taking this ancient concept and giving it a modern twist – using digital tools, urban innovation and a laser focus on decarbonisation to bring district heating into the heart of our cities.

London and Sheffield are two cities where we are exploring exactly these opportunities – looking for ‘waste’ heat sources that we can capture and use in our district heat networks supplying thousands of customers.

In London, our Citigen network is a stand-out example. Hidden behind the grand façade of the old Port of London Authority building, we’ve built a hub of clean energy innovation. Heat pumps powered by geothermal energy from 200 metres below the city now feed into a 11km heating and cooling network, connecting 28 buildings including the Museum of London, Guildhall and the Barbican. It's a powerful showcase for how to deliver low-carbon heat in a densely populated, historic city – but also a logistical feat, requiring close collaboration with the City of London, Transport for London (TfL), and the GLA to lay pipe beneath some of the UK’s most congested streets.

In Sheffield, our Lower Don Valley network uses waste wood that would otherwise head to landfill. Plans are under way to extend our existing 8km network by another 10km, supplying more homes and businesses with low-carbon heat. We're also exploring industrial heat recovery – even tapping into potential from Yorkshire Water’s nearby Tinsley wastewater treatment works.

And in East London’s Royal Docks, we’re pioneering the UK’s first ectogrid™ – a 5th generation heat and cooling network at the Silvertown development that is based on the concept of shared energy. Unlike traditional systems, E.ON’s ectogrid™ moves low-grade energy across a shared loop, with each building using its own local heat pump to deliver the temperature it needs. It’s not just efficient, it’s transformative, and has the potential to reduce overall energy consumption by 70% - ideal for mixed-use, low-carbon neighbourhoods.

A network that cools too

District heating’s lesser-known cousin is district cooling – and it’s also gaining ground. By circulating chilled water from a central source (like a lake, a chiller, or recovered industrial cooling), we can provide efficient, low-carbon air conditioning without the waste heat of conventional units.

At Silvertown, our ectogrid™ technology does just that – balancing heating and cooling needs across buildings and storing excess chill for later. It’s a new frontier in energy efficiency, especially as cities grow hotter and more crowded.

A network for the future

More than 210,000 UK homes are already connected to a district heating network. But that’s just the start. With increased urbanisation and the urgent need to cut emissions, district heating is set to play a vital role in how we heat our buildings and our cities.

New schemes are springing up – including plans to capture waste heat from the London Underground – and E.ON is constantly testing new ideas, from tapping heat from rivers to seasonal heat storage to ultra-low temperature networks.

Across Europe, we’re also unlocking innovation at scale. In Berlin’s Schöneweide district, our subsidiary BTB uses river water from the Spree to heat over 80,000 homes and buildings. With AI-powered tools like Optiheat we can forecast demand up to five days ahead with 90% accuracy –  cutting emissions, reducing fuel costs and keeping cities warm, smart and sustainable.

Because while the Romans may have started it, it’s up to us to finish the job – building the networks that will carry our cities, our customers and our planet into a net-zero future.