A hybrid system contains both a heat pump and a gas furnace. During most of the year, when outdoor temperatures are relatively mild, the heat pump efficiently provides the home's heat.
At a certain point, when outdoor temperatures get very cold, the gas furnace becomes more efficient at heating than the heat pump. At this temperature, the thermostat senses the cold outdoor temperature and automatically switches on the gas furnace and switches off the heat pump, maximizing the efficiency of your heating system.