Heat Pump Running Constantly in Winter: When It's Normal and When It's Not

A heat pump that runs almost nonstop in cold weather is usually doing its job. Carrier says it “can be completely normal for a unit to run all the time in extreme temperatures” as it pulls enough heat from the outdoor air, and variable-speed models are built to run nearly continuously at low speed. The warning signs are a house that never reaches the thermostat setting, AUX heat that stays on in mild weather, ice that won't clear, or constant running when it's only moderately cold. Carrier says constant running in moderate weather may point to a dirty air filter, a wrong thermostat setting, or a dirty outdoor coil.

This page covers what is normal, the causes from the easiest to check, what you can fix, and where a technician takes over. For cool air from the registers, see heat pump blowing cold air; for ice on the outdoor unit, see heat pump frozen.

When Running Constantly Is Normal

It's cold outside. A heat pump moves heat in from outdoors, so the colder it gets, the harder it works. Carrier says that as temperatures fall, the heat pump may run at higher capacity and run longer to hold the setting, and Lennox says extremely low temperatures can tax a heat pump, so it takes longer to reach the temperature you set. Long, steady cycles on a frigid day, with the house holding the setting, are what you want to see.

You have a variable-speed or inverter heat pump. These don't switch fully on and off. Carrier says a variable-speed heat pump adjusts its output anywhere from 25 to 100 percent of capacity and its compressor “runs almost continuously at a lower, optimized speed,” which uses less electricity than frequent on-and-off cycling. Trane says the same kind of system runs longer in mild weather while using less power. Daikin says inverter systems “are designed to run continuously at low speeds,” and that one that seems never to turn off is “completely normal.” If you aren't sure which kind you have, the model number on the outdoor unit and the owner's manual will tell you.

It's below the balance point, so AUX joins in. Carrier defines the balance point as the outdoor temperature at which the heat pump's heating capacity meets the home's heating demand; below it, auxiliary heat may run to add capacity. Carrier and Bryant say there is no single temperature for every system; Bryant notes that one of its cold-climate models is designed to keep full heating capacity down to 0°F. ENERGY STAR tests cold-climate heat pumps down to 5°F and says they keep working below that but heat most efficiently paired with backup heat. AUX on the thermostat in deep cold, or for a few minutes during defrost, is normal (Trane).

Defrost adds run time. In cold, damp weather the heat pump stops to melt frost off its outdoor coil. Lennox says a cycle usually lasts 5 to 15 minutes and that one extended cycle on an unusually icy day isn't cause for panic. The heat pump frozen guide explains what normal defrost looks like.

Signs It's a Problem

• The house never reaches the setting. Carrier says a system that keeps running without reaching the set temperature needs a qualified HVAC technician to find whether the cause is airflow, sizing, refrigerant, or something else. Bryant lists non-stop running while the house doesn't feel warm as a sign the heat pump or auxiliary system isn't meeting the home's heating demand.
• It runs constantly in moderate weather (Carrier).
• AUX stays on when it isn't that cold. Trane says to investigate if it's above 40°F and the thermostat shows AUX heat for more than an hour or two. Bryant says AUX should run as needed during temperature drops or defrost, not continuously.
• Defrost comes too often or runs long. Lennox says repeated long defrost cycles should be checked by an HVAC professional, and Trane says AUX that comes on often because defrost is more frequent is a reason to call.
• Ice that doesn't clear, or electric bills that suddenly spike, which Bryant says can mean the system is leaning on its electric backup heat.

Most Likely Causes

Check them in this order. The first four are yours to check; the rest are a technician's.

1. A thermostat setting. Carrier's first check is whether the heat pump is set to cooling; switching to HEAT should fix it. ENERGY STAR recommends HEAT rather than AUTO, so the system doesn't switch modes on an unusually warm or cold day, and the fan on AUTO. A wrong heat pump setup on a new thermostat can also cause trouble; see thermostat not working.
2. A dirty filter or blocked vents. Carrier says a dirty or clogged filter “can cause the system to run longer and less effectively,” and that dirty filters can push the system onto backup heat even in mild weather. Lennox says airflow problems can keep AUX running constantly.
3. Thermostat setbacks. Turning the heat way down at night or while you're out and back up later can bring on the backup heat. Carrier says recovering from a big nighttime drop often forces AUX on, and that raising the setting more than 2 to 3 degrees at once can call for it too; Bryant says resistance heat can run during recovery from a setback. ENERGY STAR says heat pumps, unlike furnaces, don't save energy from being turned down; pick a comfortable temperature and leave it there.
4. Snow, leaves, or a dirty outdoor coil. Carrier says to check the outdoor unit for leaves or snow when it isn't heating well or runs constantly, and that an extremely dirty coil can leave the system unable to keep up. Trane says to make sure snow isn't covering or surrounding the unit when AUX won't quit.
5. Ice and defrost problems. Lennox says a failed control board can make the system act as if it's in defrost far more than needed, or all the time, so AUX runs far more than normal. Ice that won't clear is covered in heat pump frozen.
6. A refrigerant leak, sensor, or mechanical fault. Carrier says AUX running above 40°F may point to a dirty filter, a refrigerant leak, a malfunctioning sensor, or a thermostat raised too quickly. If it's only mildly cold, the system was serviced recently, and it still runs constantly, Carrier names a frozen coil, a malfunctioning reversing valve, low refrigerant, or a compressor issue.
7. A system that's too small. Carrier says an undersized heat pump may struggle to reach the set temperature, and ENERGY STAR says a system too large or too small for its space will struggle; a contractor should size it with a Manual J calculation.

What You Can Fix Yourself

1. Check the thermostat. Set HEAT, not COOL, AUTO, or EM heat, with the fan on AUTO (Carrier, ENERGY STAR).
2. Replace the filter. Hold it to the light and replace it if it's dirty; our filter guide walks through it. The Department of Energy says to clean or change filters once a month or as needed.
3. Open the vents. Open closed registers and move furniture and rugs off supply and return vents.
4. Clear the outdoor unit. With the power off at the disconnect (Lennox), brush snow off the top and clear snow and leaves from the sides. Never chip at ice; the frozen heat pump guide covers safe thawing.
5. Stop deep setbacks. Hold a steady temperature (ENERGY STAR), and when you do change it, raise it a degree or two at a time, as Trane suggests. The DOE says not to set back a heat pump's thermostat manually if it causes the backup electric-resistance heat to come on. Our winter thermostat guide has the settings.
6. Check the breakers. If AUX never seems to help in deep cold, the backup heat may be off. Reset a tripped HVAC breaker once (Lennox); if it trips again, leave it off and call. See breaker keeps tripping.

What Not to Do

• Don't run EM heat to “help it along.” Carrier says EM heat shuts the heat pump off and runs only the backup, and is only for a heat pump that has broken down; it calls electric backup heat 100 percent efficient against as much as 300 percent for a heat pump. Trane says to call for service right away if you switch to EM heat.
• Don't crank the thermostat up to warm a cold house faster; big jumps call for AUX (Carrier, Trane).
• Don't open the panels or touch refrigerant lines (Lennox).

When to Call a Pro

Refrigerant, the reversing valve, the compressor, defrost controls, sensors, and electrical parts are a licensed HVAC technician's job. EPA rules under Section 608 of the Clean Air Act require technicians who service equipment that could release refrigerant to be certified, and prohibit venting refrigerant. Call when:

• The house can't reach the setting even after you've checked the thermostat, filter, and outdoor unit (Carrier).
• AUX shows for more than an hour or two above 40°F (Trane), or runs constantly (Lennox).
• Defrost comes too often or runs long again and again (Lennox), or ice won't clear.
• It runs constantly in mild weather even after a recent service (Carrier).
• A breaker trips again after one reset, or you've switched to EM heat (Trane).
• The house falls behind in every cold snap; ask about sizing and the backup heat (Carrier, Bryant).

If you'll be away while you wait, keep the heat no lower than 55°F, the American Red Cross minimum for protecting pipes.

Prevent It

• Pick a temperature and leave it. ENERGY STAR says that's the best way to run a heat pump. If you want setbacks, the DOE says to use a smart or programmable thermostat with functions suitable for a heat pump. Google's Nest thermostats, for example, have a Heat Pump Balance setting that trades comfort against how much AUX heat the system uses. See our smart thermostat guide.
• Check the filter monthly and change it as needed (DOE).
• Keep the outdoor unit clear of snow and leaves after every storm (Carrier, Trane).
• Book regular service. Carrier says regular service by a qualified technician gives more efficient, reliable operation, and Lennox recommends a yearly tune-up. The winter guide schedules it.

Frequently Asked Questions

Is it normal for a heat pump to run all the time in winter?

Often, yes. Carrier says running all the time can be completely normal in extreme cold, and Daikin says inverter heat pumps are designed to run continuously at low speed. It's a problem when the house can't reach the setting or it runs nonstop in mild weather.

How long should a heat pump run in cold weather?

There's no single number. Run time depends on the outdoor temperature, the home, and the system; Carrier says a heat pump runs longer as it gets colder, and variable-speed models may barely stop. The test is whether the house holds the setting.

Why is my heat pump running on AUX all the time?

In deep cold, below the balance point, that can be normal. Above 40°F, Trane says AUX for more than an hour or two is worth investigating, and Carrier lists a dirty filter, a refrigerant leak, a bad sensor, or a thermostat raised too quickly. Check the filter and outdoor unit, then call.

Should I turn my heat pump down at night?

Not much. ENERGY STAR says heat pumps don't save energy from being turned down, and Carrier says the morning recovery often brings on AUX. The DOE says to set back only with a thermostat made for heat pumps.

The Bottom Line

Long, steady running in cold weather, especially from a variable-speed heat pump, is normal as long as the house holds the setting. Set HEAT with the fan on AUTO, replace the filter, clear the outdoor unit, and stop deep setbacks. If the house still can't keep up, AUX stays on above 40°F, or defrost won't quit, call a licensed HVAC technician.

Sources

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