Homes set to be heated by sewage plants in future
Warmth may also be sucked out rivers and the sea - and from old coalmines - using heat pumps which work like fridges in reverse.
These are grids of pipes laid under city streets to convey warm water generated at a centralised location by low-carbon technology.
It's part of a heating revolution being forced ahead by the UK's commitment to combat climate change by ending the burning of gas for heat.
So far, debate has focused on the battle between individual air source heat pumps or hydrogen heating for people's homes.
But Chris Stark from the government's advisory Climate Change Committee told BBC News: "It's really important to get district heating into the discussion. It's so appealing in population-dense cities.
"And it's the best answer for conservation areas, because it offers a low-carbon solution for housing where it would be difficult or expensive to upgrade the fabric of the building itself."
