A dual-fuel system, also called a hybrid system, pairs two heat sources: an electric heat pump and a gas furnace. They share the same ductwork and indoor blower, and the system automatically picks whichever is cheaper to run at the moment. The heat pump handles mild weather; the furnace takes over in deep cold.
The hand-off happens at a setting called the switchover or balance-point temperature. A heat pump pulls heat from outdoor air, and the colder it gets, the less heat there is to pull, so its efficiency falls. Below a chosen outdoor temperature — commonly set around 30°F to 40°F — the controls shut off the heat pump and light the furnace instead. Above that temperature, the heat pump runs because it is the cheaper option.
The right switchover point is not a fixed number. It depends on your local electricity and gas prices and on how well your specific heat pump holds up in the cold. Your installer should calculate it for your home and fuel costs, and a smart thermostat can fine-tune it over a season. Setting it too high wastes gas; setting it too low runs the heat pump when gas would be cheaper.
For a buyer, a dual-fuel system makes the most sense in a climate with real winters and access to natural gas. You get the low running cost of a heat pump most of the year and the strong, fast heat of a furnace on the coldest days. The trade-off is buying and maintaining two pieces of heating equipment instead of one.