Heat from London Underground tunnels will be used to keep homes in Islington warm under a new scheme, Transport for London (TfL) has said.
Warm air from a disused Northern Line station will support a heat network supplying up to 1,000 homes and businesses by the end of the year.
The scheme is a joint project between Islington Council, TfL and engineering firm Ramboll.
TfL said it could be a "significant low-carbon energy source".
A spokesman said: "We are also carrying out further research to identify opportunities for similar projects across the network."
A heat pump will capture "waste heat" from a ventilation shaft on City Road, which currently pumps out air at 18C to 28C (64F to 82F).
This will then be used to supply heat and hot water to properties.
The project is the second phase of Islington's new Bunhill Energy Centre which already keeps about 700 new homes warm.
Enough heat is wasted in London to meet 38% of the city's heating demand, according to the Greater London Authority.
With the expansion of district heating networks it is though this could rise to 63% of demand by 2050.
Lily Frencham, head of operations at the Association of Decentralised Energy, said: "Using surplus heat rather than wasting it is a great way to ensure that we cut carbon emissions whilst helping people stay warm at an affordable cost."