While everyone’s been focused on accelerating the adoption of electric vehicles to cut carbon emissions, a technological hero has been rapidly ascending under the radar: the heat pump. Instead of burning natural gas or coal to produce heat, this fully electric device extracts warmth from outdoor air—even when it’s freezing outside—and pumps it inside to warm a structure. After years of stealthy, steady growth, heat pumps are now outselling gas furnaces in the United States, whereas EVs accounted...