A heat pump has no single temperature where it stops working. On the maker sheets and pages we read, stated limits run from about 5°F to -23°F, and one ordinary unit’s sheet states none. That limit is rarely what goes wrong first. Many heat pumps fall short of the house at a much warmer temperature, and at some point, set by prices and not by the machine, a heat pump costs more per unit of heat than the fuel beside it.
So there are three temperatures to find for your own house. This guide takes them in order and points to the page that works each one through. The Last Furnace sells and installs nothing, and on some of these numbers the right answer is to keep your furnace. Maker figures are the makers’ claims, and models are named as examples of how to read a sheet, not as picks.
First: where the compressor stops
Published limits vary widely. Trane’s page gives as low as 5°F for one variable-speed model. Carrier states -23°F for its 27VNA1. A 2023 Mitsubishi sheet for the MUZ-FS12NA is rated to -13°F, cuts out at -18°F and restarts at -14°F. The Rheem WP14AY sheet, for an ordinary unit, gives no heating limit that we found, and its performance data stops at 17°F. Past that point its figures are unknown rather than low.
Set those beside your design temperature, a cold-night planning figure and not a record low. The ENERGY STAR guide’s 2015 table lists 12°F for Boston’s county, 2°F for Chicago’s, -8°F for Minneapolis’s and -17°F for Aroostook County, Maine. Treat these as a starting point to check with an installer.
Only Aroostook falls below that Mitsubishi’s -13°F rating, but all four sit below the 17°F where the ordinary sheet ends. In much of the country, the question isn’t whether the compressor will quit. It’s how much of the heat it can still supply on those nights. Start with what temperature heat pumps stop working, which has the fuller table, field results and what to do if yours stops in the cold.
Second: where it stops keeping up
The balance point is the outdoor temperature where the heat pump’s output equals the home’s heat loss. Above it, the heat pump carries the house alone. Below it, the heat pump keeps running and something else fills the gap. A unit can still be operating and fall well short: the Mitsubishi lists 11,000 Btu/h at -13°F, about half of its 21,000 at 47°F.
Take an invented house losing 40,000 Btu/h at 5°F. The ordinary 3-ton Rheem falls behind near 26°F, on our straight-line arithmetic. A 4.5-ton cold-climate Carrier holds on to near -3°F, but that machine is 50% larger by nominal tonnage, so size matters as well as design.
Labels won’t settle this. The ENERGY STAR cold-climate test asks for a COP of at least 1.75 and at least 70% of 47°F capacity, both at 5°F. The federal minimum sets no bar at 5°F. A label says a machine passed a test, not that it carries your house at its size.
Next, read what really happens to a heat pump in the cold, with three federal-lab field studies and a full day’s cost at 5°F. A load estimate from your bills is a rough start, and the heat pump size calculator estimates the balance point. Neither replaces a load calculation by a qualified person.
Falling short isn’t automatically a mistake. The National Renewable Energy Laboratory (NREL) concluded in 2023 that sizing to the 99% design temperature may not be needed in every home.
Third: where it stops being the cheaper heat
COP is heat delivered per unit of electricity, and electric resistance heat is 1. It falls as the air cools. On three makers’ sheets it runs about 3.1 to 4.2 at 47°F and about 2.5 to 3 at 17°F, and on the two that go that cold, about 2.2 to 2.4 near 5°F. Real homes can trail the sheet. In the Department of Energy’s field test of 22 new ducted prototypes, the median COP was 1.9 at 0 to 5°F, with defrost and backup heat left out. Don’t treat any of these as what your home will get. Our heat pump efficiency chart and capacity check shows the rows side by side and reads your own model’s sheet at your design temperature.
Whether that COP is good enough depends on prices. This table shows the COP where a heat pump matches each fuel. Above it, the heat pump costs less to run. Below it, the fuel does.
| Place | Against 95% gas furnace | Against 80% gas furnace | Against 85% oil | Against 90% propane |
|---|---|---|---|---|
| United States | 3.55 | 2.99 | 1.01 | 1.60 |
| Massachusetts | 3.84 | 3.24 | 1.68 | 1.98 |
| Minnesota | 3.98 | 3.35 | 1.00 | 2.02 |
| Georgia | 2.69 | 2.26 | Not published | 1.17 |
This is our arithmetic, and the efficiencies are illustrative. It uses EIA electricity prices for August 2026, EIA winter gas prices for Q1 2026 by Census division, EIA weekly oil and propane prices for 26 October 2026 before tax, and EIA’s Btu factors. Your own bills beat every figure here.
At the prototypes’ 1.9, a gas furnace of either efficiency costs less to run on every row, and the heat pump stays ahead of oil on the rows with a price. Propane depends on the state. Most heating hours aren’t 5°F, though, so one cold day doesn’t decide the winter. NREL’s seasonal figures on older equipment, with backup counted, were 1.5 to 2.9. A 2020 British Columbia study estimated 2.4 to 3.3 by system type, backup counted. Neither reaches the US figure against a 95% furnace.
Backup heat sets the bill
Below the balance point, ducted systems often lean on electric strips. At the US average of 18.16 cents per kWh, our arithmetic puts 5 kW at about $0.91 an hour and 15 kW at about $2.72 an hour. In the DOE test, median backup use was about 11% of heating energy at 5°F. In NREL’s homes, it topped 40% of compressor energy at 5 of 11 all-electric sites. One control or sensor fault left a home on strip heat alone from 16 February to 11 April without the homeowner noticing.
If strip heat seems to run on mild days, have it checked. And ask your installer how your thermostat handles recovery, because a heat pump ramps slowly and a morning warm-up can call the strips. Emergency heat is a different thing: a setting that runs the strips and locks out the compressor, so it isn’t a way to live with a fault.
What’s normal in deep cold
A heat pump doesn’t behave like a furnace. Near-constant running at low temperatures is normal, and NREL found supply air on ducted systems rarely above 100°F, with around 80°F not uncommon. In humid cold, frost forms on the outdoor coil and the system briefly reverses to melt it, so cooler air and steam from the outdoor unit may just be defrost.
Don’t chip ice, pour hot water on the unit or open it up. If thick ice stays after defrost or the unit stops, follow the manual and go to our guide on a heat pump freezing up in winter.
When a furnace is the better call
A furnace makes sense when:
- You have gas at the prices above and electricity at or above your state’s average.
- Your load is well above what any quoted unit makes at your design temperature, and the gap would fall to strip heat.
- You have no cooling to replace and no failing equipment forcing the decision.
- You don’t want a house that depends on one system through a week below zero.
The heat pump vs gas furnace calculator does the yearly math from your bills, and the savings calculator for oil, propane or electric heat does the same for those fuels, where the case usually runs the other way against oil and electric strips.
The middle road is dual fuel: a heat pump paired with a furnace, switching at the temperature where the fuel gets cheaper. It’s a cost switch, not the balance point, and our dual fuel switchover tool estimates it. Efficiency Maine’s rebates exclude dual fuel.
What to get in writing
Before you sign for a heat pump in a cold area, ask the installer to write down:
- The design temperature used and where it came from, plus a room-by-room load calculation.
- The exact outdoor and indoor model numbers, with the maker’s capacity and COP for that pair at your design temperature.
- The expected balance point, and how many kW of backup heat are included and in what stages.
- What heats the house if the compressor fails in January. If the furnace or boiler is coming out, ask about frozen pipes: Efficiency Maine requires its vendors to assess that risk.
General information, not professional, tax or legal advice. Follow the manual for your own equipment, which overrides anything here.
Sources checked on 4 November 2026.
