Why heat pumps lose efficiency when it gets cold

Heat pumps pull warmth from outside air even in winter, but as the temperature drops, there is less heat available to extract. Below about 32°F, a heat pump has to work much harder to move that small amount of heat indoors. Most heat pumps also switch on a backup electric heating element when outdoor temperatures fall below a certain point—often around 35°F to 40°F depending on your model—and that electric heat costs more to run than the heat pump itself.

The colder it gets, the more your system relies on that expensive backup heat. A heat pump running at 0°F might be pulling 30 to 50 percent of its heating from the electric element rather than from the outdoor air. You cannot change the weather, but you can reduce how hard your heat pump has to work and how often it switches to backup heat.

Key Takeaways

  • Seal air leaks around windows, doors, and ducts so your heat pump does not have to reheat air that escaped outside.
  • Keep the outdoor unit clear of snow, ice, and debris so it can pull in air freely without the system overworking.
  • Set your thermostat to one steady temperature rather than raising it suddenly, which triggers the expensive backup heating element.
  • Close off rooms you are not using and lower the temperature in those spaces so the heat pump heats only the square footage you occupy.
  • Have a technician check your refrigerant charge and defrost cycle in fall, because both problems force the system to run longer in cold weather.

Seal leaks in your home's envelope

Every gap in your walls, around your windows, and along your doors is a place where heated air escapes. Your heat pump has to replace that air, so it runs longer and switches to backup heat sooner. Sealing these leaks is the single most effective winter efficiency step you can take, and most of it costs almost nothing.

Start with the places you can feel air movement: the frame around exterior doors, the edges of windows, and any visible gaps where pipes or wires enter the house. Use weatherstripping (foam tape or rubber strips) on door frames and window sashes. Caulk gaps smaller than a quarter-inch with paintable silicone caulk. For larger gaps—around rim joists in the basement or where the foundation meets the sill plate—use expanding foam sealant, then trim it flush and caulk over it.

Check your attic for gaps around the attic hatch, light fixtures, and where ducts pass through the ceiling. Attic leaks are often the biggest culprit because hot air rises and escapes through the roof. Seal the hatch with weatherstripping and a latch, and use duct sealant (not tape) on any visible duct seams.

Keep the outdoor unit clear and unobstructed

The outdoor unit of your heat pump needs to pull in air to extract heat from it. Snow, ice, leaves, and debris block that airflow and force the compressor to run longer to move the same amount of heat. In winter, this is a weekly task in cold climates.

After snow or ice storms, clear away snow from around the unit and gently brush ice off the coils if it has built up. Do not use a pressure washer or sharp tools—you can damage the aluminum fins. Make sure there is at least two feet of clear space on all sides of the unit and nothing blocking the top. If you have a heat pump with a defrost cycle (most do), it will periodically reverse to melt ice off the outdoor coils automatically, but it works better when the unit is not already buried.

In fall, before the cold weather arrives, have a technician inspect the outdoor unit and clean the coils if needed. Dirty coils reduce heat transfer and force the system to work harder all winter.

Adjust your thermostat settings for winter operation

The way you use your thermostat has a direct effect on how much backup heat your system uses. Every time you raise the temperature suddenly—jumping from 68°F to 72°F, for example—your heat pump cannot keep up, so the electric heating element kicks in to close the gap quickly. That element uses three to four times more electricity than the heat pump itself.

Instead, set your thermostat to one steady temperature and leave it there. If you want it warmer, raise it by one or two degrees per hour. Better yet, lower the temperature by a few degrees at night or when you are away, then raise it slowly when you return. A programmable or smart thermostat can do this automatically. You will save more energy by running the heat pump steadily at 66°F than by running it at 68°F with frequent jumps to 72°F.

Some thermostats have a setting called auxiliary heat lockout or emergency heat mode. Auxiliary heat lockout tells the system to use only the heat pump, never the backup element, even if it takes longer to reach your set temperature. In very cold climates, this can reduce winter heating costs by 10 to 15 percent, though your home will warm more slowly. Check your thermostat manual to see if this option is available.

Close off unused rooms and lower temperatures in those spaces

Your heat pump heats every room in your home equally, even rooms you do not use. Closing doors to unused bedrooms, guest rooms, or storage areas and lowering the temperature in those spaces reduces the total square footage your system has to heat.

Close the door to the room and lower the thermostat for that zone if your system has zone control. If you have a single-zone system, close the door and partially close the supply vents in that room so less heated air flows in. This forces the heat pump to work less overall. In a typical home, closing off 20 percent of the space can reduce heating load by 10 to 15 percent.

Do not close off so many rooms that you create pressure imbalances—if you seal off half your home, return air cannot flow properly and the system will strain. Closing one or two rooms is usually safe; if you are closing more than that, talk to your HVAC technician about whether your ductwork can handle it.

Have your system serviced before winter arrives

Two common problems make heat pumps inefficient in winter, and both are invisible until a technician checks them: low refrigerant charge and a defrost cycle that is not working properly.

If your heat pump is low on refrigerant, it cannot move heat as efficiently, so the compressor runs longer and the backup heat comes on sooner. A slow refrigerant leak can develop over months or years. A technician can measure the charge against your system's specifications and top it up if needed.

The defrost cycle is the mechanism that melts ice off the outdoor coils when they freeze. If it is not working correctly, ice builds up, airflow drops, and the system overworks. A technician will test the cycle and clean the coils at the same time. Schedule this service in fall, before the coldest weather, so you are not waiting for an appointment in January.

Use supplemental heat wisely on the coldest days

On days when the outdoor temperature drops below 20°F or 10°F—depending on your heat pump model—your system will use backup heat no matter what you do. At that point, the heat pump is straightforward not powerful enough to meet the demand. This is normal and expected.

What you want to avoid is triggering backup heat on days when the heat pump could handle the load on its own. If you follow the steps above—sealing leaks, keeping the outdoor unit clear, adjusting your thermostat slowly, and closing off unused rooms—you will delay the point at which backup heat comes on. On a 25°F day, you might avoid it entirely. On a 5°F day, you cannot, and that is okay.

If you have a gas furnace as backup instead of electric heat, the math changes slightly—gas is often cheaper than electric, so backup heat is less costly. But the principle is the same: the less often your system has to switch to backup, the lower your bill.

Frequently Asked Questions

Should I use the emergency heat setting on my thermostat in winter?

No. Emergency heat mode turns off the heat pump entirely and runs only the backup electric heating element, which is much more expensive. Use it only if your heat pump breaks down and you need heat while waiting for a repair. In normal winter operation, leave it in heat pump mode.

Does covering the outdoor unit in winter help?

No, and it can hurt. A cover blocks airflow and traps moisture, which can damage the coils. Clear snow and ice off the unit, but do not cover it. The unit is designed to operate in cold weather.

What temperature should I set my heat pump to in winter?

Whatever temperature keeps you comfortable is the right one. The efficiency gain comes from keeping it steady, not from the number itself. A steady 66°F uses less energy than a fluctuating 68°F to 72°F, but if 66°F is too cold, a steady 70°F is better than constantly adjusting it.

Can I improve efficiency by running my heat pump on a timer?

Only if you lower the temperature when you are away or asleep. Turning the system off and back on repeatedly forces it to use backup heat to catch up, which wastes energy. A programmable thermostat that lowers the temperature gradually and raises it slowly is more efficient than one that cycles the system on and off.

How much will these steps save on my heating bill?

That depends on your climate, your home's insulation, and how much backup heat your system uses. Sealing air leaks and adjusting your thermostat behavior typically save 5 to 15 percent. Keeping the outdoor unit clear and having it serviced can add another 5 to 10 percent. Combined, these steps often reduce winter heating costs by 10 to 25 percent, though results vary widely.