Two Balance Points, and Why Installers Usually Set Only One
A dual-fuel system pairs a heat pump with a gas, propane or oil furnace and hands the heating over at an outdoor temperature you choose. There are two different reasons to pick that temperature, and they rarely land in the same place.
- Thermal balance point: the outdoor temperature where the heat pump's output falls to the house's heat loss. Below it, the heat pump alone can't keep up.
- Economic balance point: the outdoor temperature where heat from the heat pump starts costing more than heat from the furnace. Below it, the furnace is the cheaper way to keep warm, even if the heat pump could keep up.
The setting that saves the most is whichever of the two is higher. Many systems are left at a factory default or set by rule of thumb. The Northwest Energy Efficiency Alliance's 2026 modeling for a Portland home found that raising the switchover from 30°F to 40°F increased the gas burned from 105 to 309 therms a year and the total heating bill by about $90 — and that the right answer flipped entirely with local prices.
The Break-Even COP
A heat pump's efficiency is its coefficient of performance (COP): units of heat out per unit of electricity in. The heat pump is cheaper whenever its COP is above:
Break-even COP = electricity $/kWh × 293 × furnace AFUE ÷ fuel $ per million BTU
Natural gas at $1.70 per therm is $17 per million BTU. With electricity at 18¢ and a 95% furnace, the break-even COP is 0.18 × 293 × 0.95 ÷ 17 ≈ 2.95. A typical single-stage heat pump drops below that around 32°F, so that's where gas should take over. An inverter heat pump holds its efficiency better and stays cheaper down to about 18–20°F.
An older 80% furnace wastes a fifth of its fuel, which lowers the break-even COP to about 2.5 and lets the heat pump run about 10°F colder before gas takes over.
Propane and Oil: Let the Heat Pump Run Much Colder
Delivered fuels cost far more per BTU than piped gas. At $2.50 a gallon, propane is about $27 per million BTU, and the break-even COP with 18¢ electricity falls to about 1.8. Even a single-stage heat pump stays cheaper than a propane furnace down to around 6°F; an inverter unit stays cheaper to about 0°F. Heating oil at $3.80 is similar. NEEA's Missoula propane case reached the same conclusion: the lowest bill came from letting the heat pump carry the load down to 20°F or below.
In practice, with propane or oil the switchover is set by the thermal balance point — how cold it gets before the heat pump runs out of capacity — not by price.
Where to Find Your Heat Pump's Real Numbers
The presets above are typical rated values, and units vary widely. For your exact model:
- AHRI certificate. Search the model number in the AHRI directory; heating capacity is listed at 47°F and 17°F.
- NEEP cold-climate heat pump list. For listed units it gives capacity and COP at 47°F, 17°F and 5°F, at minimum and maximum speed.
- Manufacturer's extended performance tables in the installation or engineering manual.
ENERGY STAR's cold-climate certification requires a COP of at least 1.75 at 5°F, so a cold-climate unit should never fall below that at 5°F.
Setting It on Your Thermostat
The setting goes by different names: balance point, compressor lockout, dual-fuel changeover, or breakpoint. Look in the installer or advanced menu. Most thermostats also have a separate auxiliary-heat lockout; on a dual-fuel system the "aux" stage is the furnace, so set it to match. If your system also has electric strip heat, see the aux heat cost calculator — strips cost far more than either the heat pump or the furnace.
Revisit the setting when prices change. A gas rate increase or a cheaper winter electric plan can move the economic balance point by 10°F or more.