What if your heating and cooling worked during power outages, and you could count on it becoming more efficient and climate-friendly over time? That's the promise of district heating and cooling or district energy, where entire communities share a heating and cooling system. That allows them to tap into many low-carbon energy sources they couldn't before, more efficiently than any individual building could. As governments look to tackle carbon emissions from buildings the third largest source in Canada behind the oil and gas industry and transportation " it's an idea that more of them are considering and implementing. Peter Ronson is chief operating officer for Markham District Energy (MDE), a network in a Toronto suburb that heats and cools 14 million square feet in more than 200 buildings, including condo buildings up to 44 storeys tall, a hospital, data centres, hotels and two schools. Some, like the data centres, produce a lot of heat, while others may use lots of hot water. "We got all sorts of stuff on the system," Ronson said. "When [some are] throwing away energy, I can recover and give it to somebody else." That kind of heat-sharing is efficient. Excess heat in the system can also be stored (underground in water or rock, for example) ... and much more cheaply than electricity. All that means converting buildings that burn fossil fuels to district energy can potentially put less strain on the grid than electrifying individual buildings using heat pumps or baseboard heating.