Dual-Fuel Switchover Calculator

There is no universal answer, and that is the whole point. The economic switchover is the outdoor temperature where the heat pump stops being cheaper to run than the furnace, and it moves with your electricity rate, your fuel price, and your furnace efficiency. In the worked example on this page (0.16 $/kWh, 1.40 $/therm gas, 95 percent furnace), the break-even COP is 3.18 and the crossover lands near 39 F with the defrost correction. Change any one of those three prices and the number moves. Enter your own rates and your unit's rated numbers to get yours.

Both, and they are different temperatures. The economic crossover is where the furnace becomes cheaper to run. The thermal balance point is where the heat pump can no longer meet the building's heat loss at all, no matter the cost. If you set the furnace to take over at the economic crossover but that is colder than the thermal balance point, the heat pump cannot carry the building in the gap and a cold snap leaves the home under-heated. Set the changeover at or above the thermal balance point. Comfort and safety outrank the last few dollars.

The logic inverts. Electric resistance strips run at a COP of 1.0, so the heat pump is cheaper than the strips wherever its COP, defrost included, is above 1.0. Down to that point there is no economic reason to hand hours to the strips: run the compressor at least that low and use the strips only for capacity the heat pump cannot meet. The calculator shows where your unit's corrected COP falls under 1.0, if it does in its rated range. Setting an economic compressor lockout on a strip-backup system is the most common and most expensive mistake in this whole subject.

Because two published standards disagree about defrost. A heat pump loses capacity and spends energy running defrost cycles in cold, damp weather. RESNET/ICC 301 Addendum 82 models that penalty and applies it down to the coldest temperatures; AHRI and DOE apply no defrost penalty below 17 F. The correction is worth several degrees on the crossover, so we show it both ways rather than hide the disagreement behind one number. The RESNET number is the more conservative of the two.

Break-even COP equals your electricity rate times your furnace efficiency times 100,000, divided by your fuel price times 3,412. Those two constants are the FTC EnergyGuide figures: 1 kWh delivers 3,412 Btu and 1 therm is 100,000 Btu. Above the break-even COP the heat pump delivers heat for less per Btu than the furnace; below it the furnace wins. Because the break-even COP depends on all three of your inputs, it spans a six-to-one range across ordinary US rates, so this tool never shows a break-even COP without the three numbers that produced it.

For an accurate crossover, yes: enter the capacity and COP at 47 F and 17 F from the unit's AHRI certificate, and the 5 F point too if it is a certified cold-climate unit. The tool interpolates capacity and power separately and divides for COP, rather than interpolating COP directly, because COP is a curved ratio and interpolating it straight is optimistic in the cold. If you enter a physically impossible set of numbers, such as more capacity at 17 F than at 47 F, the tool refuses rather than fitting a curve to it. About 3 percent of the manufacturer-submitted rows in the public ENERGY STAR data have exactly that impossible shape.

No. The interpolation model this tool uses (ANSI/RESNET/ICC 301 Addendum 82) explicitly excludes ductless and multi-split systems, so running it on a mini-split would be applying a model outside its scope. This tool is for ducted central heat pumps. The reference units listed on this page are all ducted split systems for the same reason.