Do Heat Pumps Work in Cold Weather? What the Data Says

Yes. A cold-climate air-source heat pump keeps heating well below freezing. To earn ENERGY STAR’s cold-climate label, it must still deliver at least 70% of its rated capacity at 5°F, at a coefficient of performance (COP) of 1.75 or better: 1.75 units of heat for every unit of electricity.⁵ In a Department of Energy field test in 22 homes, the median COP between 0°F and 5°F was 1.9.⁶ What changes in the cold is how much heat the unit can make, how often it defrosts, and when backup heat has to help.

Whether that matters depends on your winters. In NOAA’s 1991–2020 climate normals, the coldest month’s average low is below 17°F in about one in four ZIP codes in the contiguous US, and below 5°F in about one in 23.¹ Below are those numbers for 21 cities, and what monthly averages can’t tell you.

What cold weather does to a heat pump

A heat pump moves heat from outdoor air into the house. The colder that air, the harder the job, so both output and efficiency fall. Pacific Northwest National Laboratory (PNNL) notes that a conventional heat pump’s capacity drops below its rating once it is colder than 40°F, and only a fraction is left in the teens.⁶ That is where the idea that heat pumps “don’t work in the cold” comes from.

Everything below is about air-source heat pumps. Ground-source (geothermal) systems draw heat through underground pipes instead, and NYS Clean Heat notes that the ground 10 feet down stays at a constant temperature however cold the air gets.¹⁰

Cold-climate models close most of that gap with variable-capacity (inverter) compressors. The Northeast Energy Efficiency Partnerships (NEEP) requires one for its cold-climate list,² and NYS Clean Heat credits inverter compressors and better defrost controls for today’s performance.¹⁰

The three temperatures on a spec sheet: 47°F, 17°F and 5°F

NEEP’s list reports capacity and COP at 47°F, 17°F and 5°F, plus the lowest temperature the manufacturer publishes data for.² 47°F is a mild day; 17°F is close to a normal January night in Chicago (19°F) or Albany (16°F); 5°F is deep cold. ENERGY STAR tests cold-climate units down to 5°F and notes that they keep running below it.⁴

What makes a heat pump “cold-climate”

NEEP’s specification is meant as a minimum for efficiency-program qualification, and NYS Clean Heat requires it for rebates.¹⁰ DOE’s Cold Climate Heat Pump Challenge set a stricter target that eight manufacturers built prototypes for.⁸

Sources: PNNL’s summary of the ENERGY STAR criteria,⁵ NEEP² and DOE.⁷ Our glossary explains what HSPF2 measures. PNNL says a properly sized cold-climate heat pump

can meet 100% of a home’s heating load in nearly any location

To check a model, look it up on NEEP’s cold-climate heat pump product list³ or ask whether it is ENERGY STAR cold-climate certified.

How cold it gets where you live

A 5°F rating matters only if your winters get near 5°F. We found the coldest month for every contiguous-US ZIP code in NOAA’s 1991–2020 normals, using the nearest weather station, and grouped ZIP codes by that month’s average daily low.¹

About 26% of ZIP codes have a coldest-month average low below 17°F. Only 4% are below 5°F, mostly in Minnesota, the Dakotas and Wisconsin, with pockets in northern New England and the Rockies. Nights below zero are more widespread: half of all ZIP codes average at least one a year, and 19% average ten or more.¹

For 21 cities, here is the coldest month next to the design temperature that load calculations use. Those come from ENERGY STAR’s Design Temperature Limit Reference Guide, which gives each county a 99% heating design temperature from ASHRAE and ACCA Manual J data.⁹

Nights at or below 0°F and 10°F are NOAA’s average days a year with a low at or below that mark, at the same station. Where a county has several stations, ENERGY STAR uses the coldest.⁹

What monthly normals can and can’t tell you

Monthly normals are 30-year averages: good for comparing places, not for choosing equipment.

• An average hides cold nights. Minneapolis averages an 8°F January low, yet 23 nights a year below zero.
• A daily low is one moment. It doesn’t say how many hours stay that cold. In PNNL’s field test, only 8 of 22 homes logged 10 or more hours of heating below 0°F.⁶
• Design temperatures are colder. In every city in Table 2, the 99% design temperature is roughly 9 to 18°F below the coldest month’s average low: 8.5°F lower in Atlanta, 17.6°F in Denver. Equipment selection starts from the design temperature.⁹

Valleys also run colder than hilltops, and the nearest station can be miles away; our methodology page explains how ZIP codes are matched to stations.

How cold-climate units did in real homes

From 2022 to 2024, PNNL monitored DOE Challenge prototypes in 22 occupied homes. All were centrally ducted, most were 3-ton units, and all had staged electric backup heat.⁶ Leaving out defrost and backup periods, median COPs below 30°F ran from 1.6 to 2.7, and 1.9 between 0°F and 5°F.

observed COP of most of the units in the field is also higher than 1.75

Between −10°F and 0°F, backup elements were on about 25% of the time; backup energy fell from a median of about 11% of heating energy at 5°F to 0% in milder weather. Heating satisfaction rose from 3.61 to 3.88 out of 5. Many sites had milder winters than normal, so the coldest bins hold less data.⁶ New York’s energy agency (NYSERDA) likewise found that properly sized cold-climate units gave adequate heat and comfort in New York homes.¹⁰ Read the full PNNL field report (PNNL-37127).

Defrost cycles: why the outdoor unit ices up

In cold, humid weather, moisture freezes on the outdoor coil, and the frost cuts how much heat the unit can pull from the air. To clear it, the heat pump briefly reverses into cooling mode, moving heat to the outdoor coil to melt the frost.⁶

The air from your vents can turn cooler meanwhile. PNNL found most units kept the fan running with the first stage of backup heat, about 5 kW; others ran the fan alone or switched it off.⁶ A short pause or cooler air during defrost is normal; defrost can use significant energy when backup heat runs.

Balance point and backup heat

As it gets colder, your home needs more heat while the heat pump makes less. Where those lines cross is the balance point; below it, PNNL explains, the heat pump cannot meet the home’s needs without backup.⁵

Where it lands is a design choice. In PNNL’s Minneapolis example, four sizing approaches put it at 40°F, 30°F, 0°F or −15°F, from a unit sized for cooling to one sized for all the heat.⁵ Backup is electric resistance strips or a furnace (dual fuel). Resistance heat has a COP of 1, so hours on backup cost more than heat pump heat. Whether gas backup is cheaper for you is a rates question, covered in our heat pump vs. furnace running-cost guide.

The switchover temperature, below which backup may run, is programmed into the thermostat. In the field it varied from home to home, even for the same manufacturer, which PNNL attributes most likely to thermostat programming and occupant behavior calling for backup in warmer weather.⁶ If bills spike on mild days, ask the installer to check it.

How to get good performance in a cold winter

1. Choose a cold-climate model: ENERGY STAR cold-climate certified or on NEEP’s list.
2. Ask for capacity at your design temperature. PNNL says equipment should not be selected on nameplate capacity.⁵
3. Seal and insulate first. NYS Clean Heat recommends an energy assessment and air sealing before sizing.¹⁰
4. Keep the outdoor unit above the snow. NYS Clean Heat advises mounting it at least 18 inches above the ground, on a gabled side of the house.¹⁰
5. Ask how backup is set up: its type, stages and switchover temperature.
6. Plan for outages. A heat pump stops when the power does; our generator size calculator sizes a backup unit for your elevation.

NEEP’s specification puts the second point plainly: consumers, contractors and designers should

review building loads, equipment capacities at design temperatures, and other important factors

Next step

Compare your city’s design temperature in Table 2 with the 5°F figure on any quote, then check running costs in our heat pump vs. furnace guide. More: heating and cooling guides, home energy, climate guides.