Why Does My Heat Pump Run Nonstop When It’s Below Freezing?

Quick answer: A heat pump runs nonstop in freezing weather because the colder it gets, the less heat it can pull from outdoor air, while your house loses heat faster. Below freezing, many run continuously with backup heat strips helping. It’s a problem only if the house can’t hold temperature or the outdoor unit stays caked in ice.

Why does my heat pump run constantly when it’s cold?

Because there’s less heat to move on a cold night. A heat pump doesn’t make heat; it carries it from outside to inside, and its output falls with the temperature. Meanwhile a 25°F night pulls heat out through your walls, windows and attic far faster than a 45°F one.

At some outdoor temperature those two lines cross. Below it, the heat pump can’t keep up alone, so it runs without stopping and the thermostat calls in backup heat. That’s normal on the coldest nights. Can a heat pump keep up with a Texas freeze? explains the whole setup.

Is it bad for the heat pump to run all the time?

No. Long, steady runs are easier on the compressor (the pump in the outdoor unit that does the heavy lifting) than constant starts and stops. Variable-speed heat pumps, which speed up and slow down instead of just switching on and off, are built to run for hours, and in the coldest weather that’s exactly their job. A heat pump that runs all night in a freeze is working, not failing.

Why does my thermostat say aux heat?

Seeing “aux” on the thermostat below freezing is normal. The heat strips in the indoor unit (electric coils, like a toaster’s) switch on to cover the gap, then drop out when the heat pump catches up. Aux heat vs. emergency heat explains each light, and why you should stay in normal Heat mode and not switch to emergency heat.

When is a heat pump running nonstop a problem?

When long run times come with other symptoms. Any of these calls for a closer look:

• The house keeps dropping below the temperature you set, even with aux heat running.
• The outdoor unit stays wrapped in ice that defrost cycles don’t clear. (A defrost cycle is a normal few-minute pause, often with steam, that melts frost off the unit.)
• The heat pump runs constantly on a 50°F day.
• The outdoor fan sits still while the unit runs, outside of short defrost cycles.
• Air from the vents feels cool for long stretches, well past the first few minutes. See why a heat pump blows cold air.
• A breaker for the air handler or outdoor unit has tripped, leaving part of the system off.

You can safely reset a tripped breaker once. If it trips again, leave it off and call for service.

How can I help my heat pump on a freezing night?

Hold a steady temperature, start with a clean filter and keep the outdoor unit clear.

• Hold a steady temperature. Turning it way down overnight and way up in the morning is the surest way to fire up the costly heat strips.
• Put in a fresh filter before the cold arrives.
• Keep leaves, mulch and plants clear of the outdoor unit so defrost water can drain, and never cover it while it might run.
• Close blinds at night, and open south-facing ones on sunny afternoons.

If your home runs out of steam every cold night, the usual culprits are leaky attic ducts, thin insulation, a heat pump that’s low on refrigerant, or one that’s undersized or tired. All of them are worth checking before next February. Gold Eagle’s technicians check heat pumps, ductwork and attic insulation in homes across the Austin area; see our heat pump service.

What should I do next?

• Check the outdoor unit on the next freeze and note whether ice clears after each defrost cycle.
• Keep the thermostat in normal Heat mode, not emergency heat.
• Write down any night the house drops below your setting even with aux heat running.
• Book a visit to check the ducts, insulation and heat pump if the house runs out of steam every cold night.

Related guides: Can a Heat Pump Keep Up With a Texas Freeze?, Aux Heat vs. Emergency Heat: What Your Heat Pump Thermostat Is Telling You and Heat Pumps vs. Electric Resistance Heat for Central Texas Homes.

About Gold Eagle Services: This guide comes from the team at Gold Eagle Services, a family-run Austin HVAC, indoor air quality and mold remediation company founded in 2017. We’re licensed (TACLA119235 and RCO1783), insured and bonded, and we serve homes across the greater Austin area.

Need help with this in your home? Learn about our heat pump repair and installation, or call 512-489-4653 to schedule a visit.

Related Gold Eagle service: Heating service