The Basic Setup: Two Systems, One Goal
A heat pump paired with a furnace is called a dual-fuel system or hybrid system. The heat pump handles heating and cooling most of the year because it moves heat efficiently. When outdoor temperatures drop too low for the heat pump to work well—usually below 35°F to 40°F depending on your model—the furnace automatically kicks in to provide backup heat. You do not switch between them manually; the system decides which one to use based on the outdoor temperature and how much heating your home needs.
This setup costs more upfront than either system alone, but it saves money on energy bills because heat pumps use less electricity than furnaces for most heating days. The furnace only runs on the coldest days, so it sits idle for months at a time in milder climates.
Key Takeaways
- A heat pump and furnace work together automatically—the heat pump heats your home in fall and spring, and the furnace takes over when outdoor temperatures drop below a set point, usually 35°F to 40°F.
- The thermostat controls which system runs by monitoring outdoor temperature; you set one temperature and the system switches fuels without your input.
- Heat pumps move existing heat from outside air or ground into your home, while furnaces burn fuel or use electric resistance to create new heat.
- A backup heating element inside the heat pump itself can also provide heat between the heat pump's limit and the furnace's startup point, reducing furnace runtime.
- Maintenance requires annual service for both the heat pump and furnace, including filter changes, refrigerant checks, and fuel line inspection.
How the Heat Pump Operates in Mild Weather
A heat pump is essentially an air conditioner running in reverse. It pulls heat from outdoor air (or from the ground, if it is a ground-source model) and moves that heat indoors using a refrigerant fluid that circulates through coils. Even when the air outside feels cold, heat energy exists in it—the heat pump extracts and concentrates that energy to warm your home.
This process is highly efficient because the heat pump does not create heat; it relocates it. For every unit of electricity it uses, it can move three to four units of heat into your home. That efficiency advantage disappears when outdoor temperatures fall too far. Below roughly 35°F to 40°F, the outdoor coil gets too cold to pull useful amounts of heat, and the system would have to work so hard that it becomes less efficient than straightforward burning fuel.
When the Furnace Takes Over
Your thermostat monitors the outdoor temperature continuously. When it drops below a set point—often called the balance point or switchover temperature—the thermostat signals the furnace to start instead of the heat pump. The furnace burns natural gas, propane, or oil, or uses electric resistance heating to generate warmth directly. It then pushes that warm air through the same ductwork the heat pump uses, so your home receives heat from whichever system is running.
The switchover happens automatically and you will not notice it. If your furnace is gas-fired, you may hear it ignite, but the air coming from your vents stays at the same temperature. The system is designed so that the transition is seamless—your home stays warm without any gap in heating.
The Backup Heat Element Between Systems
Many heat pump systems include an electric resistance heating element inside the indoor unit, positioned between the heat pump's lower temperature limit and the furnace's startup point. This element acts as a bridge. When outdoor temperatures drop but have not yet reached the furnace switchover point, this backup element can provide supplemental heat alongside the heat pump, reducing the workload on both systems.
This backup element is not as efficient as the heat pump—it converts electricity directly to heat at a 1:1 ratio rather than moving heat at a 3:1 or 4:1 ratio—but it is more efficient than running the furnace. Some systems use it only during defrost cycles, when the heat pump briefly reverses to shed ice from the outdoor coil. Others use it continuously in the transition zone. Your installer sets the temperature at which this element activates based on your climate and equipment.
How the Thermostat Controls Both Systems
A dual-fuel thermostat has a sensor that reads outdoor temperature and logic that decides which heating source to use. You set your desired indoor temperature once, and the thermostat manages the rest. Most systems allow you to set the balance point—the outdoor temperature at which the furnace takes over—though many homeowners leave it at the manufacturer default, which is usually 35°F to 40°F.
Some thermostats let you override this and force the furnace on manually, or lock the heat pump on even in cold weather, but automatic mode is the standard. The system is programmed to minimize energy use, so it will run the heat pump as long as it can meet your heating demand, then switch to the furnace only when necessary. If you set the balance point too high, the furnace will run too often and waste money. If you set it too low, the heat pump will struggle and cycle on and off inefficiently.
Ductwork and Air Distribution
Both the heat pump and furnace connect to the same ductwork system in your home. The heat pump's indoor unit and the furnace share the same blower fan and return-air ducts. When the heat pump is running, it sends warm air through the ducts. When the furnace takes over, it uses the same blower to push its warm air through the same ducts. A damper or valve inside the system may direct airflow, but most homes straightforward have both units connected in parallel, with only one running at a time.
This shared ductwork is why a dual-fuel system is practical to install in existing homes. If you already have a furnace and ductwork, adding a heat pump usually means installing the heat pump's outdoor unit and indoor coil, then connecting them to your existing ducts. The furnace stays in place as backup.
Maintenance and Seasonal Checks
A dual-fuel system requires maintenance for both components. The heat pump needs annual service in spring before cooling season—a technician will check refrigerant levels, clean coils, and inspect the compressor. The furnace needs annual service in fall before heating season—a technician will inspect the burner, check the heat exchanger for cracks, test the ignition system, and replace or clean the filter.
You should change the furnace filter every one to three months during heating season, and check the heat pump's filter monthly during cooling season. If either system is not heating or cooling as it should, or if you hear unusual noises, contact a technician. Do not attempt to add refrigerant or adjust the balance point yourself; both require specialized tools and knowledge.
Frequently Asked Questions
Will my furnace run all winter if I have a heat pump?
No. The heat pump handles most of the heating season. The furnace runs only on the coldest days—typically 10 to 30 days per year depending on your climate. In mild climates, the furnace may run only a handful of times. In very cold climates, it may run for weeks at a time during deep freezes.
Can I turn off the furnace and use only the heat pump?
You can, but it is not recommended. If you disable the furnace and outdoor temperatures drop below the heat pump's effective range, your home will not heat adequately. Most systems allow you to lock the furnace off manually, but doing so removes your backup protection. It is safer to leave both systems active and let the thermostat choose.
What temperature should I set the balance point at?
The manufacturer default—usually 35°F to 40°F—works for most homes. Setting it lower saves money by running the heat pump longer, but the heat pump may cycle inefficiently in very cold weather. Setting it higher runs the furnace more often and costs more. A technician can help you find the right balance for your climate and equipment.
Does the heat pump still cool my home in summer?
Yes. In cooling season, the furnace is not used at all. The heat pump reverses and pulls heat from inside your home, moving it outdoors. The furnace sits idle until fall, when heating season begins again.
What happens during a power outage?
If you have a gas or oil furnace, it may have a battery-backed ignition system that allows it to start without electricity, though the blower fan will not run. If you have an electric furnace, neither system will work without power. A backup generator can restore power to both systems, but most homes without generators will lose heating during an outage.