A dual-fuel system pairs a heat pump with a gas furnace, and a control decides which one runs when the weather gets cold enough that the heat pump stops being the right source. ACHR News coverage of dual-fuel thermostats reports that as heat pumps get more capable in cold weather, smarter thermostats are giving contractors "greater flexibility to optimize the handoff" between the two. That handoff is a setting, not a fixed fact of the equipment, and it is worth knowing where it sits in your house before the first hard freeze tests it.
What the Handoff Controls
Most dual-fuel setups switch based on outdoor temperature. Below a set point, often called a balance point or lockout temperature, the furnace takes the call. Some controls add cost logic that compares the price of electricity and gas, and some let you override the switch by hand. Your system may use one of these, a combination, or none of them, and the menu labels vary by brand. Start with the thermostat or control menu and find the outdoor temperature setting, any fuel-cost inputs, and any lockout or manual mode. If the installer left a commissioning sheet, it should list the values they chose. If you cannot find it, that is a question for your service company, not a guess.
Log the Switch Before You Change Anything
The fastest way to learn your system is a short log. Pick a cold stretch of several days and record four things: the outdoor temperature from a trusted reading, not a car parked on hot pavement or a sensor sitting in direct sun; the time the system changed sources; which equipment ran, based on the status indicator on the thermostat or the equipment's own display; and the setting you had at the time. Write it on paper or in a simple file and keep it with your service records. Do not change the setting until you have a few days of data, because a change made in the middle of a cold snap makes the log much harder to read.
When the log is done, compare it with what the control should do. A furnace running at temperatures where you expected the heat pump is a clue. So is a heat pump running well below the set point you believe you chose. Either one can point to a setting problem, a sensor problem, or an install that was never fully commissioned. The log tells the technician which one to chase, and it saves a visit spent guessing.
Check the Wiring and the Sensor
Dual-fuel logic depends on clean control signals. With the system running and your multimeter set to AC volts, carefully measure between the R and C terminals at the thermostat wiring, which should read roughly 24 VAC. A reading far from that, or one that sags sharply when equipment starts, points toward a wiring or transformer issue that a technician should look at. If your system uses a separate outdoor sensor, check its mounting so it reads ambient air. Keep it out of direct sun and away from the exhaust of the outdoor unit. A sensor mounted against a sunny wall can push the handoff in the wrong direction and make the log look like a control fault when it is a placement fault.
Stay Out of the Line Voltage and Gas Work
Everything in this check happens at low voltage, at the thermostat, the control board terminals, or the sensor. Do not open the furnace burner compartment, touch gas controls, or work inside the line voltage section of the outdoor unit. Those tasks belong to a licensed technician with the right tools and the manufacturer's data.
Questions to Bring to the Installer
Once your log is in hand, the service visit gets shorter and more useful. Ask what outdoor temperature the handoff is set to and why, which switching logic the control uses, whether the furnace can be locked out, where the outdoor sensor is mounted, and what the manufacturer's documentation says about those set points. Ask the technician to note the answers on the service sheet. A dual-fuel system only works as designed when the people who set it up and the people who maintain it agree on the numbers. Your log is the easiest way to make those numbers visible to both of them.