An air-source heat pump's low-temperature limit is specific to the equipment and the home. Older systems may need supplemental heat at about 25°F to 30°F. Many modern cold-climate heat pumps continue heating below 5°F, and some models are rated to operate at approximately -13°F.
Heat Pump Temperatures at a Glance
| Heat pump type | Typical cold-weather behavior |
|---|---|
| Older or standard air-source heat pump | May need backup heat around 25°F to 30°F |
| Modern ENERGY STAR cold-climate heat pump | Tested at 5°F and designed to continue operating below 5°F |
| Advanced cold-climate model | Some provide heat to around -13°F |
| Ground-source or geothermal heat pump | Much less affected by outdoor air temperature because it draws heat from the ground |
Why Do Heat Pumps Struggle in Very Cold Weather?
An air-source heat pump pulls heat from outdoor air. As the air gets colder, less heat is available, so the system has to work harder to heat the home. The heat pump's output usually falls as the temperature drops, while the home's heating demand rises.
The temperature at which the heat pump can no longer meet the home's full heating load is called the balance point. Below that temperature, the heat pump may keep running while electric resistance heat, a gas furnace or another backup system supplies the rest.
The balance point depends on the heat pump model, insulation, ductwork, outdoor design temperature and system size.
A heat pump can therefore be operating normally even when it cannot heat the home by itself.
At What Temperature Does a Standard Heat Pump Need Backup Heat?
Many older or conventional air-source heat pumps begin using more supplemental heat at about 25°F to 30°F. One U.S. Department of Energy example describes a standard single-stage heat pump using more supplemental heat from around 30°F and producing little heat near 0°F. That example is not a universal cutoff because equipment and homes perform differently.
Backup heat may include:
- Electric resistance heat strips
- A gas or oil furnace in a dual-fuel system
- A boiler or another heating system
The thermostat may display AUX Heat when backup heat runs alongside the heat pump. It may display Emergency Heat when the heat pump is manually locked out and only the backup heater runs.
How Cold Can a Modern Cold-Climate Heat Pump Operate?
Many modern cold-climate heat pumps can continue heating below 5°F. ENERGY STAR cold-climate heat pumps must demonstrate performance at 5°F. Under the current criteria, a qualifying system must achieve a coefficient of performance of at least 1.75 at 5°F and retain at least 70% of its heating capacity compared with its rated capacity at 47°F.
ENERGY STAR says certified cold-climate heat pumps can operate below 5°F. At very low temperatures, backup heat may be more efficient than continuing to rely on the heat pump alone.
Some specialized models are rated for even lower temperatures. Mitsubishi Electric, for example, advertises certain Hyper-Heating INVERTER systems for heating operation down to -13°F. Check the installation manual for the specific outdoor unit before treating that temperature as its operating limit.
Does a Heat Pump Stop Working Below Freezing?
No. Freezing temperatures alone do not normally stop a heat pump from operating. Modern air-source heat pumps are designed to provide heat below 32°F.
The system may appear to stop during a defrost cycle. Frost can collect on the outdoor coil in cold, damp weather. The heat pump temporarily reverses operation to melt the frost, and backup heat may run during that time.
Short defrost cycles are normal. Continuous ice buildup, repeated shutdowns or an inability to maintain the thermostat setting may point to a problem with the equipment, airflow, insulation, refrigerant charge or controls.
What About Geothermal Heat Pumps?
Ground-source, or geothermal, heat pumps are much less affected by outdoor air temperature than air-source systems. Their underground loops exchange heat with the earth, whose temperature is more stable than the air above it.
The U.S. Department of Energy says ground temperatures about 30 feet below the surface generally remain between approximately 50°F and 59°F in much of the United States.
A geothermal heat pump can continue heating during subzero weather without using extremely cold outdoor air as its heat source. Its performance still depends on loop sizing, system design, maintenance and the indoor distribution system.
How Do You Find Your Heat Pump's Actual Low-Temperature Limit?
Check these sources:
- Outdoor unit model number: Find the model number on the equipment label.
- Manufacturer performance data: Look for heating capacity and operating temperature information.
- Thermostat settings: Check the compressor lockout, auxiliary heat and emergency heat settings.
- Installer's balance-point calculation: Ask whether the system was sized using the home's heating load and the manufacturer's low-temperature capacity data.
- Operating symptoms: If the system runs continuously but cannot maintain the set temperature, the cause may be sizing, airflow, insulation, refrigerant charge or a control setting rather than outdoor temperature alone.
Bottom Line
Use the manufacturer's rated minimum temperature and the home's balance point when judging a heat pump's cold-weather performance. Those two figures are more useful than a general claim that a heat pump stops working at a particular temperature.