What temperature should a dual fuel heat pump switch over at?

Why the answer is a COP, not a temperature

Ask what temperature to switch a dual fuel system over at and you will get a number between 20F and 40F, offered confidently, usually with no reasoning attached. The number is not wrong so much as unanswerable as posed, and the reason is worth more than the answer would have been.

A furnace has a flat efficiency. A 95% AFUE furnace delivers 95% of the fuel's energy as heat at 40F and at minus 10F alike. A heat pump does not: its coefficient of performance falls as outdoor temperature falls, because there is less heat in the outdoor air and a larger temperature lift to work across.

So there is a temperature at which the two lines cross. Above it the heat pump delivers heat more cheaply; below it the furnace does. That crossing point depends on three things, and only one of them is the weather:

• What you pay for electricity, per kWh.
• What you pay for the other fuel, per therm or per gallon, and the efficiency of the machine burning it.
• Your heat pump's COP curve, which is a property of the specific outdoor unit and is not visible in its HSPF2 rating.

The first two combine into a single number: the COP the heat pump has to beat. Once you have that, the crossover temperature is just the point on your unit's published curve where it stops beating it. That is the number this page gives you.

A worked case: told 20F by one person and 30F by another

A New England household with a five zone Mitsubishi Hyper Heat and a furnace for backup opened a $700 December bill and went looking for the right crossover setting. The salesperson said switch to the furnace at 20F. The installer said 30F. Neither gave a reason. [r/heatpumps, crossover advice]

Ten degrees is a lot of winter. But the more useful observation is what neither person asked: what does the furnace burn?

Run their numbers three ways and the disagreement stops being the interesting part.

• If the furnace burns gas, the heat pump has to hold a COP between 3.4 and 4.7 across New England to break even. No cold climate heat pump on the market does that below roughly 40F, and most do not do it at any temperature. Both answers were far too cold, the furnace should have been doing nearly all of the winter, and a $700 December is the predictable result of it not doing so.
• If it burns propane, the target falls to between 1.7 and 2.2, which a Hyper Heat holds well into the teens. Now both answers are too warm, and switching at 30F means burning propane for weeks while the heat pump was still cheaper.
• If it burns oil, the target lands between 2.1 and 2.8, and the honest answer sits somewhere near the two numbers they were given. That is the only case where either piece of advice was accidentally reasonable.

The COP your heat pump has to beat, in all 51 states

Each figure is the coefficient of performance at which your heat pump costs exactly the same per delivered kWh as that fuel, at that state's December 2025 to February 2026 EIA prices. Above it the heat pump is cheaper. Below it the furnace is.

Six states have no gas column because EIA has not published a complete December to February residential gas season for them. Propane and heating oil are published by region rather than by state, so those columns carry the nearest regional figure; each state page names which region its number came from.

What the spread tells you

• Against natural gas, the target is out of reach almost everywhere. 44 of the 45 states with a published gas price require a COP above 2.0, which is roughly what a good cold climate unit manages at 17F. In those states the crossover on running cost alone is warmer than most people expect, and often warmer than the thermostat allows.
• Against propane it is reachable nearly everywhere. Only 12 of 51 states need more than 2.0. If your backup burns propane, a low crossover is usually correct and the common advice to switch at 30F or 35F is costing you money.
• Oil sits in between, with 12 of 51 states above 2.0.

Finding your own crossover in about ten minutes

1. Take your target COP from the table above, or compute it from your own two prices on the calculator if you have a bill in front of you. Your own prices beat a state average, sometimes by a lot.
2. Get the COP table for your outdoor unit. It is in the submittal or engineering data sheet, not the brochure, and it lists capacity and input power at 47F, 17F and 5F. COP is capacity in BTU/h divided by input watts, divided by 3.412. [AHRI 210/240-2023]
3. Read off the temperature where your unit's curve crosses the target. Interpolating between the published points is close enough for a thermostat setting.
4. Then move it warmer if your fuel price is volatile. Propane and oil move a lot within a single winter; electricity much less. A crossover set on a summer propane price will be wrong by January.

This will often not match what your installer set. The most common failure in these systems is a crossover left at a factory default with no relationship to what anybody pays for anything.

Who this page is for, and who it is not

It is for you if you already have a dual fuel or hybrid system, or a heat pump alongside a furnace that could be one, and you are deciding where to set the switchover.

It is not for you if your backup is electric resistance strips. There is no crossover to set: strips run at COP 1.0, the heat pump beats them at every temperature it operates at, and the number you actually want is your balance point rather than a crossover. That is a different page.

It is also not for you if you are deciding whether to install dual fuel in the first place. The comparison page covers the system; this one covers the setting.

How this was checked

Every figure in the table is computed from EIA state level residential prices for December 2025 to February 2026, using the same heating values and default efficiencies as the rest of this site: 100,000 BTU per therm at 95% AFUE for gas, 138,500 BTU per gallon at 83% for oil, 91,500 BTU per gallon at 95% for propane. [EIA units and calculators]

The arithmetic is one line and is stated in full above so it can be checked rather than trusted. The inputs and the outputs are both in the published dataset.

What this page does not cover

This page answers the running cost question only. It does not price the equipment, the installation, the control that switches between the two machines, or the cost of maintaining a gas connection you use for fewer hours.

It also does not model your house. The crossover depends on prices and on the unit's COP curve, not on your insulation, but the value of getting it right scales with how much heat you use below the crossover, and that does depend on your house.

Nor does it cover emissions. A crossover set purely on running cost will burn more fossil fuel than one set on carbon, and in a state with a clean grid those two settings point in opposite directions.

References

1. U.S. Energy Information Administration. Electricity Data Browser, retail sales and average price by state and sector, monthly. Series accessed through the EIA Open Data API v2.
2. U.S. Energy Information Administration. Natural Gas Prices, residential price by state, monthly, dollars per thousand cubic feet. Series accessed through the EIA Open Data API v2.
3. U.S. Energy Information Administration (2025). Electricity use is becoming more common for residential heating. Today in Energy, citing U.S. Census Bureau American Community Survey and the Residential Energy Consumption Survey.
4. U.S. Energy Information Administration. Units and calculators explained, energy conversion factors and fuel heat content.
5. Air-Conditioning, Heating, and Refrigeration Institute. AHRI Standard 210/240-2023: Performance Rating of Unitary Air-Conditioning and Air-Source Heat Pump Equipment. This standard defines the M1 test procedure and the HSPF2 metric.
6. Homeowner report, r/heatpumps. New England, five zone Mitsubishi Hyper Heat 2i on a dual fuel setup with a $700 December bill. The salesperson told the owner to switch to the furnace at 20F and the installer told them 30F, and neither gave a reason.
7. U.S. Department of Energy, Building Technologies Office (2021). Residential Cold Climate Heat Pump Technology Challenge Specification.