What Outdoor Temp Should Your Dual-Fuel System Switch?
Switch where the heat pump and furnace cost the same per MMBtu — the balance point for your gas price, electricity rate and a NEEP COP curve, solved below.
· Source: EIA Electric Power Monthly Table 5.6.A (July 2026 data) · next EIA update ~Oct 23
On this page: CalculatorTableHow we calculatedSources
Jump to calculator ↓— optional preset from the Sept 2026 STEO (residential, Oct–Mar simple average): electricity 18.2¢/kWh, natural gas ≈$1.36/therm, propane $2.44/gal, heating oil $5.30/gal. Your own bill beats any national average.
- What it finds
- Economic balance point
- COP data
- NEEP ccASHP, 1,418 units
- Rating points
- 47F / 17F / 5F
- Fuels
- Gas, propane, oil, resistance
- Also shown
- Thermal balance point
- Federal credit
- $25C ended 31 Dec 2025
Why this number
293.07 x electricity price / COP, and a furnace costs 10 x gas price per therm / AFUE. Set them equal and the heat pump wins above the temperature where its COP exceeds 29.307 x electricity price x AFUE / gas price. This tool solves that against real capacity-COP curves from the NEEP cold-climate database, not a single seasonal average.A dual-fuel system has one setting that decides most of what it costs to run: the outdoor temperature at which it stops using the heat pump and starts using the furnace. Set it too high and you burn expensive fuel while the heat pump was still winning. Set it too low and you run the heat pump into weather where gas was cheaper.
That crossover is not a fixed number. It moves with your fuel prices, and it depends on how your particular heat pump behaves in the cold — which is why a single seasonal efficiency figure cannot answer it.
The arithmetic, in full
Both sides are converted to the same unit — dollars per million Btu actually delivered into the house — which is the only way to compare a fuel burned at 95% efficiency against electricity used at a COP of 2.
Heat pump. One kWh is 3,412 Btu, so a million Btu is about 293.07 kWh of heat. The pump delivers that from 293.07 / COP kWh of electricity:
Heat pump $/MMBtu = 293.07 × (your $/kWh) ÷ COP(T)
Furnace. One therm is 100,000 Btu, so a million Btu is 10 therms of fuel, of which only the AFUE fraction reaches the house:
Furnace $/MMBtu = 10 × (your $/therm) ÷ AFUE
Setting those equal and solving for COP gives the threshold the heat pump has to beat:
Break-even COP = 29.307 × (your $/kWh) × AFUE ÷ (your $/therm)
The switchover temperature is simply where the unit’s COP curve crosses that line. Nothing about it is proprietary, and the same formula is what a contractor’s commissioning guide uses.
Other fuels convert to a therm-equivalent price first, using heat contents published by the EIA: natural gas sold by the CCF is about 103,800 Btu per unit (divide by 1.038), propane is 91,452 Btu per gallon, and heating oil is about 138,500 Btu per gallon.
One thing to be careful about: use only the marginal unit price. The fixed monthly customer charge on a gas bill is paid whether the furnace runs or not, so including it makes gas look worse than it is for this decision.
Economic versus thermal balance point
These are two different questions, and confusing them is the most common error in dual-fuel setup.
The thermal balance point is a matter of physics. Your house loses heat roughly in proportion to the indoor–outdoor temperature difference:
Load(T) = UA × (65 − T), where UA = design heat load ÷ (70 − design temperature)
The heat pump’s maximum output falls as it gets colder at the same time as the load rises. Where the two lines cross, the heat pump can no longer keep up, and backup heat becomes physically necessary regardless of what anything costs. For the 3-ton ducted class above, maximum output drops from roughly 36,000 Btu/h at 47°F to about 21,000 at 5°F — while the load at 5°F is far higher than at 47°F.
The economic balance point — what this calculator solves — is a matter of price. It is where the two fuels cost the same per delivered MMBtu. It moves whenever gas or electricity prices move, and it is completely independent of capacity.
The correct switchover is the higher of the two. In a genuine dual-fuel system the furnace is sized to carry the whole house on its own, so capacity is not the constraint and the economic point is normally the one that binds. In a heat-pump-with-resistance-backup system it is the other way round, and the thermal point matters more.
Why resistance backup changes everything
Electric resistance heat has a COP of exactly 1.0 — every watt becomes one watt of heat, no more. That puts it at 293.07 × your electricity price per MMBtu: about $52.75/MMBtu at 18¢/kWh, which is higher than natural gas at almost any price this decade.
This is the whole reason dual-fuel exists. A heat pump running at COP 2 costs half what resistance does; the moment resistance strips engage, the economics invert. If your system uses resistance backup, the useful figure is the blended efficiency:
Effective COP = Load ÷ [ min(Load, HP capacity) ÷ COP_HP + max(0, Load − HP capacity) ]
The second term in that denominator is resistance heat at COP 1, and it drags the blended figure down fast. Sizing the heat pump so that term stays near zero is worth more than any efficiency upgrade.
Where these COP figures come from
Every efficiency number in the calculator is a median of real listed products, not a manufacturer headline or a rule of thumb.
- Source: the NEEP cold-climate air-source heat pump database , filtered to ENERGY STAR Cold Climate products.
- Deduplication: the same outdoor unit is listed once for every indoor unit it can be paired with, so a raw read of the database heavily over-weights whichever manufacturer lists the most combinations. Records were deduplicated on outdoor unit number, leaving 1,418 distinct outdoor units.
- Sampling: pages were sampled evenly across the entire filtered listing rather than reading the first few pages, because the listing clusters by manufacturer. The split came out at 607 ducted and 432 ductless units with a usable nominal tonnage.
- Operating point: figures use maximum-compressor-speed COP, which is what the unit actually does when it is cold and working hard, at the 47°F / 17°F / 5°F points the database requires.
- What is not shown: the −20°F point is optional in the database and several classes had only one or two units at that temperature. Publishing a “median” of one unit would imply precision that does not exist, so those points are suppressed and the calculator extrapolates below 5°F on the 5–17°F slope instead, which it flags by never returning a figure below −30°F.
Independent cross-checks. The ENERGY STAR v6.2 Cold Climate specification requires COP ≥ 1.75 at 5°F; every class here sits between 1.90 and 2.00, comfortably above the floor, as a filtered set should. Field measurements published by PNNL (DOE Cold Climate Heat Pump Challenge field validation, PNNL-37127 ) found a median COP near 1.9 in the 0–5°F range — effectively identical to the 1.98 median across these classes, which is reassuring given that one is a laboratory rating and the other is measured in real houses.
The data is used here in aggregate with attribution and is not republished in bulk.
What this deliberately leaves out
- Defrost cycles, which reduce real output in humid cold weather and are not in the rated figures.
- Distribution differences between a ducted system and a mini-split serving one zone.
- Fixed charges, deliberately, as explained above.
- Comfort, which is not a number. Heat-pump supply air is cooler than furnace air, and some households switch over earlier than the economics require for that reason alone. That is a legitimate choice; it just is not a cost optimisation.
The 2026 tax position
State and utility rebates are entirely unaffected by that change and are frequently worth more than the federal credit ever was — check DSIRE for your state and your own utility’s program list. See the 2026 energy tax credit guide for the full federal position. Nothing here is tax advice.
Related
Compare whole-season operating costs with the heat pump vs furnace calculator , see what electricity costs in your state , or work out what everything else in the house costs with the electricity cost calculator .
Frequently asked questions
What is the difference between the economic and thermal balance point?
They are different questions and are constantly confused. The thermal balance point is where the heat pump’s maximum output equals the house’s heat loss — below it, backup heat is physically necessary no matter what energy costs. The economic balance point is where the two fuels cost the same per unit of delivered heat; it moves every time prices move and has nothing to do with capacity. In a dual-fuel system the furnace can meet the whole load on its own, so the economic point is normally the one that binds.
Why not just use one seasonal COP figure?
Because a heat pump’s efficiency falls as it gets colder, and the whole question is about cold weather. A unit rated around COP 3.2 at 47F typically delivers about 1.9 at 5F. Using a single seasonal average hides exactly the range where the switchover decision is made, and it systematically overstates savings in northern climates.
Where does the COP data come from?
The NEEP cold-climate air-source heat pump database, filtered to ENERGY STAR Cold Climate products. We sampled across the whole listing and deduplicated on outdoor unit number, because the same outdoor unit appears once for each indoor unit it is matched with. That left 1,418 distinct outdoor units, and each figure here is the median for its class with the sample size shown.
Does electric resistance backup change the answer?
Enormously, and for the worse. Resistance heat has a COP of 1.0, so it costs 293.07 times your electricity price per MMBtu — about $52.75/MMBtu at 18 cents. That is higher than almost any gas price. The entire point of a dual-fuel setup is to avoid ever running resistance backup; if yours engages, you have usually lost the comparison against gas in any price scenario.
Is there still a federal tax credit for a heat pump?
No. The 25C energy-efficient home improvement credit, which covered heat pumps at up to $2,000 a year, ended for property placed in service after 31 December 2025. State and utility rebates are unaffected and are often substantial — check DSIRE. Nothing here is tax advice.
How we calculated this
Heat pump $/MMBtu = 293.07 × $/kWh ÷ COP(T); furnace $/MMBtu = 10 × $/therm-equivalent ÷ AFUE. COP(T) is interpolated between the NEEP class-median ratings at 47°F, 17°F and 5°F. The economic balance point is the temperature where the two are equal, solved by inverting the COP curve; the thermal balance point is where the unit’s capacity curve meets your entered heat loss.Sources: U.S. Energy Information Administration (EIA) electricity rates · DSIRE incentive records · public IRS/OBBBA guidance. Figures are modeled estimates, not quotes or tax advice. See our methodology.
Where to go next
- Winter heating cost: gas vs propane vs oil vs electric — All five heating options per MMBtu at EIA’s September outlook, plus 20 cold-state pages from their own prices.
- Gas vs heat pump heating cost — The COP a heat pump needs to beat your gas price, and which side each state falls on.
- Oil vs heat pump cost — For oil-backup systems: the electricity price ceiling under which the heat pump wins.
- Heat pump crossover temperature — Named NEEP units against your state’s EIA rates — the monthly view of where each winter month lands.
- Heat pump vs furnace cost calculator — Whole-season dollars from your annual heating spend.