Heat pumps reverse their cycle in winter to pull warmth from outside air
A heat pump in winter does the opposite of what it does in summer. Instead of moving heat out of your home, it moves heat in — even when the outside air is cold. The system uses a refrigerant that circulates through indoor and outdoor coils, absorbing warmth from the outside air and releasing it inside your home through your furnace or air handler.
The key is that cold air still contains heat energy. At 32°F outside, there is still warmth available to extract. A heat pump's compressor pressurizes the refrigerant to raise its temperature above the indoor air temperature, then releases that heat into your home. This process repeats continuously as long as the system runs.
Most air-source heat pumps work efficiently down to about 25°F to 35°F, depending on the model and manufacturer. Below that temperature, the system's output drops, and many heat pumps switch to electric resistance heating (a backup heater built into the system) to maintain comfort. This is why performance in winter varies by climate and outdoor temperature.
Key Takeaways
- Heat pumps extract warmth from outside air even in winter by reversing their refrigerant cycle and using a compressor to concentrate that heat.
- The system works most efficiently between 25°F and 50°F; below 25°F, most models set up electric resistance heating to supplement output.
- A four-way reversing valve switches the refrigerant flow direction, allowing the same equipment to heat in winter and cool in summer.
- Defrost cycles automatically melt frost buildup on the outdoor coil every 30 to 90 minutes during cold, humid conditions, temporarily reducing heating output.
- Ground-source heat pumps maintain higher winter efficiency than air-source models because soil temperature stays warmer than winter air.
The reversing valve switches heat flow direction
The reversing valve is the component that makes winter heating possible. In summer, refrigerant flows one direction through the coils. In winter, a solenoid-controlled valve flips the flow direction, so the outdoor coil becomes the evaporator (where heat is absorbed) and the indoor coil becomes the condenser (where heat is released).
This switch happens automatically when you set your thermostat to heating mode. The reversing valve responds to an electrical signal and changes position within seconds. If the valve sticks or fails, the system cannot heat properly — a common repair issue in older heat pumps.
Defrost cycles reduce heating output temporarily
When outdoor humidity is high and temperature is near freezing, frost accumulates on the outdoor coil. Frost acts as insulation and blocks airflow, reducing the system's ability to absorb heat. To clear this buildup, the heat pump automatically enters defrost mode every 30 to 90 minutes.
During defrost, the system reverses to cooling mode briefly, warming the outdoor coil with hot refrigerant until frost melts and drips away. This process typically lasts 5 to 15 minutes. While defrosting, your home loses heating output, and the backup electric heater may set up to maintain indoor temperature. You may notice warm air stop briefly or feel a slight temperature dip — this is normal.
Defrost cycles are most frequent in climates with wet winters (like the Pacific Northwest) and least frequent in dry cold climates (like Denver or Salt Lake City). Dirty outdoor coils trigger more frequent defrost cycles, so keeping the unit clear of leaves and debris improves winter efficiency.
Backup electric heating activates below the balance point
Every heat pump has a balance point — the outdoor temperature at which the heat pump's output equals your home's heating demand. Below that temperature, the system cannot keep up alone. At that point, electric resistance heating (also called auxiliary heat or emergency heat) turns on automatically.
Balance point varies by home insulation, size, and heat pump capacity. A well-insulated home with a larger heat pump may have a balance point around 20°F. A poorly insulated home with an undersized unit may have a balance point near 40°F. Electric resistance heating is expensive to run — it uses three to four times more electricity than the heat pump itself — so a low balance point saves money in winter.
You should never manually switch to "emergency heat" mode unless the heat pump fails. Emergency mode disables the heat pump entirely and runs only the electric heater, which costs significantly more to operate.
Ground-source heat pumps stay efficient in deep winter
Air-source heat pumps (the most common type) pull heat from outside air, which gets colder as winter deepens. Ground-source heat pumps, also called geothermal systems, pull heat from the earth 4 to 6 feet below the surface, where temperature stays between 45°F and 55°F year-round, depending on your location.
Because the ground is warmer than winter air, ground-source systems maintain high efficiency throughout winter and rarely need backup electric heating. They cost more to install — typically $15,000 to $30,000 — because they require underground loop installation. But in climates with long, cold winters, the energy savings over 15 to 20 years can offset the higher upfront cost.
Ground-source systems are most practical for homes with adequate land and no underground obstacles like bedrock or utility lines. Air-source heat pumps remain the standard choice for most homeowners because they cost less and require no excavation.
Winter performance depends on outdoor temperature and humidity
Heat pump output drops as outdoor temperature falls. At 47°F, a typical air-source heat pump delivers close to its rated capacity. At 32°F, output may drop to 60 to 75 percent of rated capacity. At 0°F, output may fall to 40 to 50 percent. This is why heating output ratings always include the outdoor temperature at which they were measured.
Humidity also affects winter performance. Frost forms more readily in humid cold (like coastal winters) than in dry cold (like mountain winters). More frequent defrost cycles mean more interruptions to heating, which slightly reduces overall seasonal efficiency. In very dry climates, defrost cycles may run only a few times per month. In wet climates, they may run every hour during certain weather patterns.
Manufacturers publish heating seasonal performance factor (HSPF) ratings that account for these variations across a typical winter. Higher HSPF numbers indicate better winter efficiency. Most modern air-source heat pumps have HSPF ratings between 8 and 12. Ground-source systems typically range from 3.5 to 5.0 HSPF (the scale is different because ground temperature is more stable).
Proper maintenance keeps winter heating reliable
Heat pump winter performance depends on regular maintenance. Dirty outdoor coils reduce heat absorption and trigger more frequent defrost cycles. Clear leaves, snow, and debris from around the outdoor unit. If snow piles up against the unit, it blocks airflow and can damage the coil — keep at least 2 feet of clearance on all sides.
Indoor air filter replacement every 1 to 3 months keeps airflow high and prevents the indoor coil from freezing. A frozen indoor coil stops heat transfer and forces the system to shut down. Check your filter monthly during winter and replace it when it looks dirty.
Have a technician service the system annually, ideally in fall before heating season. They will check refrigerant charge, test the reversing valve, inspect coils, and verify that defrost cycles are working correctly. A system running low on refrigerant will not heat properly and will waste electricity trying.
Frequently Asked Questions
Why does my heat pump sometimes blow cold air in winter?
Cold air during defrost cycles is normal. The system reverses to cooling mode to melt frost off the outdoor coil, which temporarily reduces heating output. Defrost typically lasts 5 to 15 minutes. If cold air blows for longer than that, or if the system never switches back to heating, the reversing valve may be stuck and needs service.
Should I turn on emergency heat mode when it gets very cold?
No. Emergency heat mode disables the heat pump and runs only electric resistance heating, which costs three to four times more to operate. The heat pump is designed to work in cold weather and will automatically set up backup electric heating when needed. Switch to emergency mode only if the heat pump fails.
Can a heat pump heat a home in freezing weather?
Yes, but with reduced output. Most air-source heat pumps work down to 25°F to 35°F. Below that, electric resistance heating supplements the heat pump. In climates with sustained temperatures below 0°F, a heat pump alone may not keep up, and you may need a backup heating system like a furnace.
Why is my heating bill higher in winter than summer cooling?
Heating demand in winter is usually higher than cooling demand in summer, especially in cold climates. Additionally, when outdoor temperature drops below the balance point, expensive electric resistance heating activates. A heat pump's winter efficiency also depends on defrost cycles and outdoor humidity, both of which reduce output compared to summer cooling.
How often should the outdoor coil be cleaned?
Check the outdoor unit monthly during winter and clear away leaves, snow, and debris. If the coil itself looks dirty (dark or discolored), have a technician clean it during your annual fall service. A dirty coil reduces heat absorption and increases defrost frequency, which wastes energy.