If you've travelled on the London Underground you'll know how hot it can get. But what if the stifling air could be put to good use?

At Islington's derelict City Road station, a huge fan extracts warm air from the Northern Line tunnels and sends it to an energy centre above ground.

It is used to heat water that's pumped to 1,350 homes, a school and two leisure centres.

Transport for London claims the Bunhill 2 Energy Centre is the first of its kind in the world, and is assessing whether 56 other ventilation shafts have potential for exporting waste heat.

Video by Jamie Moreland