But the company will soon be putting its literal hot air — the waste heat pumped out by one of its data centers — to good environmental use. And every bit of waste heat recycled into energy saves some fuel — often fossil fuels — from doing the same job. And that’s not even considering any higher-tech solutions for capturing and using waste heat, many of which are now in the offing. For more than a century, waste heat has been used for its most obvious application: heat (think of your car, which uses waste heat from its engine to heat your interior). In London, the heat from the underground rail system is being channelled to heat homes in Islington.