How do I set the switchover temperature on a dual-fuel system?

A dual-fuel system pairs a heat pump with a gas or propane furnace. The thermostat or controller runs the heat pump in milder weather and hands over to the furnace below a set outdoor temperature. Where to set that temperature is a question with two separate answers, and the switchover belongs at whichever is higher.

What is the capacity limit?

A heat pump delivers less heat as the outdoor air gets colder, while the house loses more. The outdoor temperature where the two meet is the balance point. Below it, the heat pump alone cannot hold the indoor temperature. DOE’s Building America guidance shows how widely it varies with sizing: from around 40 °F (4 °C) for a heat pump sized to the cooling load, to −15 °F (−26 °C) or colder for a cold-climate unit sized to the full heating load.

The balance point comes from two things you can get in writing: the home’s heat loss line from a load calculation, and the heat pump’s maximum capacity at each outdoor temperature from the manufacturer’s extended performance data.

What is the cost limit?

Even above the balance point, the furnace may be the cheaper way to make heat. The heat pump is cheaper while its coefficient of performance at the current outdoor temperature is above a break-even value set by prices:

break-even COP = electricity price per kWh × 29.307 × furnace AFUE ÷ gas price per therm

The constant converts units: a therm is 100,000 Btu and a kWh is 3,412 Btu, so one therm equals 29.307 kWh of energy. AFUE is written as a fraction.

At EIA’s U.S. average residential prices for January 2026, 17.45 cents per kWh and $13.96 per thousand cubic feet of gas ($1.346 per therm):

Furnace	Break-even COP
80% AFUE	about 3.0
95% AFUE	about 3.6

A heat pump whose COP is above those values delivers heat for less than the furnace. Heat pump COP falls as it gets colder: ENERGY STAR’s cold-climate threshold is a COP of 1.75 at 5 °F (−15 °C). At national average prices, then, a heat pump needed a COP of 3.0 to 3.6 to beat the furnace, well above that cold-climate threshold; the outdoor temperature where a given model reaches it is in its performance data. With cheaper electricity or dearer gas, the break-even COP drops and the heat pump stays cheaper to colder temperatures.

How do I find my own switchover?

1. Work out your break-even COP with the formula, using the full price per kWh and per therm from winter bills.
2. Find the temperature where your heat pump’s COP falls to that value. Extended performance tables list COP, or capacity and power input, at several outdoor temperatures; interpolate between them.
3. Find your balance point from the load calculation and the same tables.
4. Set the switchover at the higher of the two temperatures. Above it, the heat pump both keeps up and costs less.

Recheck after a significant change in energy prices, because the cost limit moves with them while the balance point does not.

What about propane or electric backup?

The same formula works for propane with its price per gallon converted to a price per therm: EIA’s factor is 91,452 Btu per gallon, or 0.915 therm. If the backup is electric resistance auxiliary heat rather than a furnace, there is no cost limit to find: resistance heat has a COP of 1, so the heat pump is cheaper at any temperature where it runs, and the switchover is set by capacity alone.