Dual Fuel Heat Pumps: When to Switch to the Furnace

A dual fuel system pairs a heat pump with a furnace. The heat pump heats efficiently in mild weather; the furnace takes over when it gets cold enough that the heat pump can't keep up, or when burning fuel becomes cheaper than running the compressor. The temperature where that handoff happens is the switchover (or changeover, or lockout) temperature.

Set it too high and you burn fuel you didn't need to. Set it too low and the house falls behind on cold mornings, or you pay more for heat than you had to. Two balance points decide it.

1. The thermal balance point

As it gets colder, the house needs more heat and the heat pump makes less. The thermal balance point is where the two lines cross. Above it the heat pump carries the house alone; below it, something has to help.

To find it you need the house's design heating load (from a Manual J or at least a heat loss calculation) and the heat pump's maximum heating capacity at 47, 17 and, ideally, 5°F from the manufacturer's extended performance data. Draw the load as a straight line from zero (at roughly 60–65°F) up to the design load at the design temperature, draw the capacity curve, and read off the crossing. The balance point calculator does exactly that.

Because most dual fuel controls run the furnace instead of the heat pump, not alongside it, the switchover can't sensibly be below the thermal balance point. Below it, the heat pump alone would let the house cool.

2. The economic balance point

The other question is cost. Heat from a heat pump costs the electricity price divided by its COP, and the COP falls as it gets colder. Heat from a furnace costs the fuel price divided by its efficiency, and stays about the same. Somewhere they cross.

The cleanest way to compare them is the cost of a million BTU of delivered heat:

• Electricity: $/kWh × 293.07 ÷ COP (293.07 kWh is one million BTU)
• Natural gas: $/therm × 10 ÷ AFUE (10 therms per million BTU)
• Propane: $/gallon × 10.93 ÷ AFUE (about 91,500 BTU per gallon)
• Heating oil: $/gallon × 7.22 ÷ efficiency (about 138,500 BTU per gallon)

Set the two equal and solve for the COP:

Break-even COP = $/kWh × 293.07 ÷ fuel cost per million BTU delivered

The economic balance point is the outdoor temperature where the heat pump's COP drops to that break-even value.

Worked example: natural gas

Electricity at $0.15/kWh, gas at $1.20/therm, a 95% furnace:

• Gas heat: 1.20 × 10 ÷ 0.95 = $12.63 per million BTU
• Break-even COP: 0.15 × 293.07 ÷ 12.63 = 3.48

A heat pump with a COP of 3.5 at 47°F and 2.3 at 17°F crosses 3.48 at about 46°F. With cheap gas and average electricity, the furnace is the cheaper heat for most of the winter. That's why dual fuel is popular where gas is inexpensive, and why the switchover is often set by cost rather than capacity.

Worked example: propane

Same electricity, propane at $2.80/gallon, 95% furnace:

• Propane heat: 2.80 × 10.93 ÷ 0.95 = $32.23 per million BTU
• Break-even COP: 0.15 × 293.07 ÷ 32.23 = 1.36

Few heat pumps fall that low until it's very cold, so with propane the heat pump is the cheaper heat almost all the time. The switchover should sit at the thermal balance point, not above it.

3. Set the switchover at the warmer of the two

• If the economic balance point is warmer, switch there: above it the heat pump is cheaper, below it the furnace is.
• If the thermal balance point is warmer, switch there: below it the heat pump can't keep the house warm on its own.

Prices move, so it's worth re-running the numbers when rates change, and using the delivered price from a real bill, including delivery charges, not just the commodity rate.

4. Then adjust for the house and the controls

• Comfort. Heat pump supply air is cooler than furnace air. Some homeowners prefer a slightly higher switchover for warmer air at the registers.
• Defrost. In cold, humid weather the heat pump spends time defrosting, which lowers real output below the published curve.
• Rate structures. Time-of-use or demand charges can make the economic balance point change by hour.
• Control options. Many thermostats and outdoor controls allow a heat pump lockout temperature and a separate furnace lockout above which the furnace never runs. Set both deliberately.
• Equipment limits. Some heat pumps stop running below a cutoff temperature regardless of settings. The switchover must be above it.

5. Size the furnace for the whole load

Because the furnace runs alone below the switchover, it has to carry the full design heating load on its own, not just the shortfall. That's different from electric backup, which only has to cover the gap between the load and the heat pump's output at design.

Try it with your numbers

The heat pump balance point calculator takes the design load, the heat pump's capacity and COP at 47/17/5°F and your energy prices, and returns the thermal and economic balance points, a suggested switchover, and the hours a typical year spends below each. For the bigger picture, read heat pump sizing, step by step.

Where calc.blue fits

calc.blue compares heat pumps and furnaces against the house at its own design temperatures, including dual fuel, starting from a room-by-room load built on ACCA Manual J 8th edition procedures. Join the waitlist.

ACCA and Manual J are trademarks of the Air Conditioning Contractors of America. calc.blue is not affiliated with or approved by ACCA.