A heat pump pulls heat out of outdoor air, which sounds impossible in January. But air at 0°F still holds heat; it's just harder to extract. The question is not whether a heat pump works in the cold, but how much heat it delivers when it's coldest, and what covers the gap.
Yes. Heat pumps keep heating well below freezing, but their output and efficiency fall as it gets colder. ENERGY STAR's cold-climate models must still deliver at least 70% of their rated capacity and a COP of at least 1.75 at 5°F, and they keep running below that, usually with backup heat for the coldest hours.
Why output falls in the cold
A heat pump moves heat instead of making it, which is why it can deliver more heat than the electricity it uses. As the outdoor air gets colder, there's a bigger temperature gap to pump heat across, so each unit of electricity moves less heat. Capacity drops, and so does efficiency.
Even so, a COP of 1.75 at 5°F still means 1.75 units of heat for each unit of electricity, 75% more than an electric resistance heater gets from the same power.
Standard vs. cold-climate heat pumps
Rated heating capacity is measured at 47°F. Our heat pump size calculator models how it falls:
| Outdoor temperature | Standard heat pump | Cold-climate heat pump |
|---|---|---|
| 47°F | 100% | 100% |
| 17°F | about 60% | about 79% |
| 5°F | about 40% | at least 70% |
The standard-unit figures are the calculator's assumptions. The cold-climate 70% at 5°F is ENERGY STAR's minimum, and 79% at 17°F falls on a straight line between the two.
Cold-climate models must also reach at least 8.1 HSPF2 for ducted systems or 8.5 for ductless. NEEP's cold-climate heat pump list publishes each model's measured capacity and efficiency at 5°F, 17°F and 47°F.
Defrost
In cold, damp weather, frost builds on the outdoor coil. The heat pump periodically reverses for a few minutes to melt it, and indoor air may feel cooler or backup heat may switch on briefly. That's normal.
Keep the outdoor unit raised above the snow line and make sure defrost water can drain away without refreezing.
Finding your design temperature
Your design temperature is the outdoor low your area stays above about 99% of the year, not the record low. Equipment is sized to this figure rather than the extreme, because the few hours a year below it are covered by backup or a brief indoor dip. Your local design temperature sets which capacity figure matters most.
The colder your design temperature, the more weight the 5°F capacity carries. Where winters bottom out near 5°F, a cold-climate model holding 70% there can often carry the house with little backup. Where the design low sits at −10°F or below, expect more backup hours even with a cold-climate unit, which is what the balance-point examples below show.
Capacity at 5°F is the single most useful spec to compare, because it is measured at a temperature most cold regions reach. A unit that holds a high fraction of its rating at 5°F holds more at milder lows too. Match the model's rated low-temperature limit to your design temperature before you size it.
Balance point and backup heat
The balance point is the outdoor temperature where the heat pump's output meets the house's heat loss. Below it, backup heat makes up the difference: electric strips in the air handler, or a gas furnace in a dual-fuel system.
For a 2,000 sq ft two-story home built between 1980 and 1999, with a cold-climate heat pump, the calculator estimates:
- Cold climate, 0°F design low: balance point about 9.5°F, about 2.7 kW of backup at the design low.
- Very cold climate, −10°F design low: balance point about 9°F, about 5.4 kW of backup.
In both cases the heat pump carries the house alone down to single digits, so backup heat runs only on the coldest days.
What it costs to run in the cold
Efficiency is a COP: heat delivered divided by electricity used. A 7.8 HSPF2 rating converts to a seasonal COP of 2.29 (7.8 ÷ 3.412), and the 1.75 COP floor at 5°F is the cold-climate minimum. Delivering one million BTU of heat takes 293 kWh of electricity (1,000,000 ÷ 3,412), divided by the COP.
At $0.18/kWh the cost to deliver one million BTU works out like this:
| Heat source | Efficiency | Cost per million BTU |
|---|---|---|
| Electric resistance (strip backup) | COP 1.0 | about $52.75 |
| Heat pump at the 5°F floor | COP 1.75 | about $30.14 |
| Heat pump, seasonal | COP 2.29 | about $23.03 |
| Heat pump, mild weather | COP 3.0 | about $17.58 |
| Gas furnace (assumed $1.35/therm) | 95% AFUE | about $14.21 |
Each electric figure is 293 kWh × $0.18 ÷ COP. The gas figure is ten therms per million BTU at an assumed $1.35 each, divided by 0.95 efficiency, so about $14.21; that gas price is an example, not a national rate.
This is why the backup choice matters. Electric strips run at a COP of 1.0, the most expensive row above, while a dual-fuel system burns gas on the coldest hours at well under a third of the strip cost per million BTU in this example.
The earlier backup figures turn into an hourly cost at $0.18/kWh. The 2.7 kW of strip backup at a 0°F design low costs about $0.49 an hour (2.7 × $0.18), and the 5.4 kW at a −10°F design low costs about $0.97 an hour (5.4 × $0.18). Those rates apply only during the coldest hours, when the heat pump alone cannot carry the load.
Sizing for cold climates
In cold climates the heating load usually decides the size. A cold-climate heat pump sized to the heating load, rather than the cooling load, keeps backup heat small. The heat pump size calculator sizes for both seasons and estimates the balance point and backup for your home.
Frequently asked questions
At what temperature do heat pumps stop working?
It depends on the model. ENERGY STAR notes heat pumps keep running below 5°F, and cold-climate models are designed for it. Check each model's rated low-temperature limit and its capacity at 5°F on NEEP's list.
Is a heat pump worth it in a cold climate?
With a cold-climate model sized to the heating load, usually yes. Whether it costs less to run than gas depends on local prices; see gas vs. electric heating cost.
Do heat pumps need backup heat?
In cold climates, usually a small amount for the coldest hours. A well-sized cold-climate heat pump keeps it to a few kilowatts in the examples above.
Do I need a cold-climate model in zone 4, 5 or 6?
It depends on your design temperature more than the zone label. A cold-climate model holds about 70% of its capacity at 5°F against roughly 40% for a standard unit, so the colder your winters, the more it pays off. Through the colder zones, where design lows reach the single digits or below, a cold-climate unit sized to the heating load keeps backup heat small.
Will my bills spike on the coldest days?
Running cost rises on the coldest days, because efficiency falls and backup heat may run. Strip backup runs at a COP of 1.0, so in the examples above 2.7 kW of it costs about $0.49 an hour and 5.4 kW about $0.97 an hour at $0.18/kWh. The spike is limited to the few hours below your balance point, and a dual-fuel system blunts it by burning gas instead.
Why does my heat pump blow cooler air in winter?
A heat pump delivers a steady stream of warm air rather than the short blasts of hot air a furnace gives, and during defrost the air can feel cool for a few minutes. Both are normal.
Sources & References
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