Dual fuel and hybrid heat pump systems

A dual fuel (or hybrid) system is an electric heat pump paired with a fossil-fuel furnace, usually natural gas or propane. DOE's definition: "Dual fuel systems use both electric and gas heating."^{[Source 1]} The heat pump handles cooling and most heating, and the furnace takes over when it is too cold for the heat pump to keep up or too expensive to run it.

How the switchover works

There are two common control approaches, both described in DOE training material:^{[Source 1]}

• Changeover at a set outdoor temperature. The controls switch from the heat pump to the furnace at a programmed outdoor temperature. DOE says this is "programmed in the controls for automatic changeover at 5°F to 35°F depending on system capabilities," and that "only one system is on at a time."^{[Source 1]}
• Droop (both can run). The furnace's heating setpoint is set lower than the heat pump's, so the furnace only comes on if the heat pump cannot hold temperature. DOE's example sets the backup "3°F colder than the heat pump system's setpoint."^{[Source 1]}

DOE adds that, "depending on the system, user needs, and fuel prices, the heat pump can be disabled at higher loads, or the gas system can simply supplement the heat pump."^{[Source 1]}

On many thermostats the changeover is a compressor lockout setting based on outdoor temperature. See Heat pump backup heat and lockout temperatures.

Two temperatures that matter

• Balance point. The outdoor temperature where the heat pump's output equals the home's heating need. Below it, the heat pump alone cannot keep up.^{[Source 2]} DOE says to find it where the load line from a load calculation crosses the heat pump's capacity curve, using the manufacturer's extended performance data or the NEEP cold-climate product list.^{[Source 1]}
• Switchover temperature. The outdoor temperature at which the system actually changes from heat pump to furnace.^{[Source 2]} It can be the same as the balance point or higher, if the furnace is cheaper to run at that temperature.

Thermal and economic balance points covers the difference between the thermal and the economic balance point in more detail.

Picking a switchover temperature

The cost comparison comes down to one number: the heat pump COP (coefficient of performance) at which electric heat costs the same as furnace heat. A 2026 analysis for the Northwest Energy Efficiency Alliance (NEEA) uses this method.^{[Source 2]} It works out to:

Breakeven COP = (electricity cost per million Btu ÷ fuel cost per million Btu) × furnace efficiency

To put prices on the same basis, use EIA's conversions: 1 kWh = 3,412 Btu, 1 therm = 100,000 Btu, and 1 gallon of propane = 91,452 Btu.^{[Source 3]} That makes:

• Electricity cost per million Btu = price per kWh × 293
• Natural gas cost per million Btu = price per therm × 10
• Propane cost per million Btu = price per gallon × 10.93

Furnace efficiency is its AFUE as a decimal. ENERGY STAR defines AFUE as "the percentage of the heat in the incoming fuel which is converted to space heat instead of being lost."^{[Source 4]}

When the heat pump's COP at a given outdoor temperature is above the breakeven COP, the heat pump is cheaper to run at that temperature. COP falls as the outdoor temperature drops, so there is a temperature below which the furnace is cheaper. That temperature depends on your prices and your specific heat pump's published performance, so it differs from house to house.

Worked example from the NEEA analysis. Using 2023 EIA average rates for Washington and Oregon ($0.1157 per kWh, $1.507 per therm) and a 95% AFUE furnace, NEEA calculated a breakeven COP of 2.1 for natural gas. For propane in Idaho and Montana ($0.1161 per kWh, $2.21 per gallon), the breakeven COP was 1.3.^{[Source 2]} A lower breakeven COP means the heat pump stays cheaper down to colder temperatures. Your numbers will differ; EIA publishes current electricity and natural gas prices by state.^{[Source 5][Source 6]}

What the NEEA analysis found

The NEEA study modeled dual fuel controls in Northwest homes. Its main findings, in its own words:^{[Source 2]}

• "Switchover temperature is the largest lever."
• "Set switchover close to the heat pump balance point," and "a few degrees above balance point."
• "Size to ~30F balance point."
• For propane, "go to 20F balance point or lower."

In its Portland example with a single-speed heat pump, the modeled annual operating cost was $821 with a 30°F switchover, $842 at 35°F and $912 at 40°F, against $1,093 for the furnace alone.^{[Source 2]} These figures come from that study's prices and house model and are a pattern, not a prediction for any one home.

When dual fuel is considered

Dual fuel is common where a working furnace is already in place, where gas is inexpensive relative to electricity, or where the heat pump chosen does not have enough capacity at the coldest design temperature. The heat pump still has to be matched to the furnace's air handler and ductwork, and the AHRI certificate should list the outdoor unit, indoor coil and furnace together. For the all-electric alternative, see Heat pumps in cold weather.