Heat Pump in Cold Weather: Low Limits, Defrost Mode and Backup Heat

A standard air-source heat pump heats a home efficiently down to roughly 25 to 30°F (about -4 to -1°C), then loses capacity as the air gets colder, and a backup heat source usually takes over part of the load. Cold-climate heat pumps are built for harsher winters and can keep producing useful heat down to about -5 to -15°F (about -21 to -26°C), and some models go lower. In freezing, damp weather, every heat pump also runs short defrost cycles to clear ice from the outdoor coil, which is normal.

How cold can a heat pump work?

The low limit depends on the type of heat pump, and the drop in performance is gradual rather than a sudden cutoff. A heat pump moves heat from the outdoor air into the house, and there is still usable heat in air well below freezing. As the temperature falls, the unit has to work harder to collect that heat, so it delivers less warmth for each unit of electricity.

• Older or standard single-speed heat pumps handle mild winters well and start leaning on backup heat once outdoor temperatures drop into the 20s °F (below about -4°C).
• Variable-speed (inverter) heat pumps ramp the compressor up and down and hold their output better in the cold.
• Cold-climate heat pumps are designed and rated to keep a large share of their heating capacity in single digits and below zero °F.

Your model’s specification sheet or the installer’s paperwork lists the rated output at low outdoor temperatures. That figure tells you more than the brand name or the SEER rating does.

What is the balance point?

The balance point is the outdoor temperature at which the heat pump alone can no longer keep up with the heat your house loses. Above that temperature the heat pump covers everything. Below it, the system needs help from backup heat to hold the thermostat setting. A well-insulated, tightly sealed house has a lower balance point than a drafty one with the same heat pump, so sealing air leaks and adding insulation can cut how often backup heat runs.

What is defrost mode and is it normal?

Defrost mode is a normal, automatic cycle that melts frost off the outdoor coil. When the outdoor air is near or below freezing and humid, moisture freezes on the coil and blocks airflow. The control board senses this and briefly reverses the system so warm refrigerant flows through the outdoor coil.

During a defrost cycle you may notice:

• The outdoor fan stops while the compressor keeps running.
• A whoosh or click as the reversing valve switches.
• Clouds of steam rising from the outdoor unit, which is melting frost and not smoke.
• Cooler air from the vents for a few minutes, or backup heat switching on to cover the gap.

A typical cycle lasts about 5 to 15 minutes, and in frosty conditions it may repeat every 30 to 90 minutes. Defrost runs less often in very cold, dry air because there is less moisture to freeze.

When does backup heat kick in?

Backup heat, often called auxiliary or “aux” heat, turns on when the heat pump cannot raise the indoor temperature fast enough on its own. In most homes it is a set of electric resistance heat strips in the indoor air handler. In a dual-fuel system it is a gas or oil furnace. Common triggers are:

• The outdoor temperature drops below the balance point or a lockout temperature set on the thermostat.
• The indoor temperature falls a few degrees below the setting.
• You raise the thermostat by several degrees at once, for example after a night setback.
• The system is in a defrost cycle.

Emergency heat is different. “Em heat” is a thermostat setting that shuts off the heat pump and runs only the backup source. Use it only when the outdoor unit has failed or is buried in ice, and call a technician, because running on resistance heat alone costs much more.

Why does air from a heat pump feel cooler than a furnace?

Air from a heat pump register usually comes out around 90 to 100°F (32 to 38°C), while a gas furnace often blows air at 120 to 140°F (49 to 60°C). The heat pump runs longer at a lower temperature to deliver the same amount of heat, so the air can feel lukewarm on your hand even when the house is warming properly. Long run times in cold weather are expected and do not mean the unit is failing.

How do you keep a heat pump working well in winter?

A few habits keep a heat pump efficient through the cold months:

1. Keep the outdoor unit clear. Brush off snow and keep at least a couple of feet (about 60 cm) clear around it. The unit should sit on a stand or pad above typical snow depth.
2. Watch for roof runoff. Water dripping from a roof edge onto the unit can freeze into a solid block. A gutter or small cover above it, installed to the maker’s clearances, helps.
3. Use small setbacks. Lowering the thermostat only a few degrees at night, or not at all, avoids the big recovery that brings on backup heat. A thermostat made for heat pumps can stage the recovery slowly.
4. Change the indoor filter on schedule. A clogged filter cuts airflow and raises the chance of backup heat running.
5. Do not chip ice off the coil. The fins bend easily and the refrigerant lines can be damaged. Let the defrost cycle work, and if the unit stays covered in ice, switch to emergency heat and call a technician.

Quick answers

Is it normal for my heat pump to run all the time in very cold weather?

Yes. Near or below the balance point, a heat pump may run almost continuously. Call a technician only if the house cannot hold its setting or the backup heat never shuts off.

Why is my outdoor unit covered in ice?

A light coat of frost between defrost cycles is normal. A thick block of ice that stays for hours points to a defrost problem, low refrigerant, a blocked drain path or water dripping onto the unit, and a technician should check it.

Should I switch to emergency heat when it gets very cold?

No. The thermostat brings in backup heat automatically when it is needed. Emergency heat is meant for a failed or iced-up outdoor unit.

Does a heat pump work below 0°F?

A cold-climate heat pump can keep heating below 0°F (-18°C), though at reduced output. A standard model will rely mostly on backup heat at that temperature.