The short answer
A Minnesota heat pump that runs almost constantly in winter is usually normal: inverter units run long and slow, and any heat pump runs nearly nonstop in a cold snap. Call if the house won't hold the setpoint, aux heat runs all day above 40°F or the coil stays iced.
Key points
- Variable-speed (inverter) heat pumps are meant to run almost continuously at low output.
- Any heat pump runs longer as it gets colder, because its capacity drops as the house needs more heat.
- Warning signs: the house can't hold the setpoint, aux heat runs all day above about 40°F, the coil stays iced, or the bill jumps.
- On dual fuel, the switchover and lockout temperatures decide how long the heat pump runs before the furnace takes over.
- Check the filter, the outdoor unit and the thermostat settings before you call.
Older single-speed heat pumps work like a light switch: full output until the thermostat is satisfied, then off. An inverter model varies its compressor speed instead, so on a 30°F day you may see the outdoor fan turning slowly almost all day, and on a 0°F night you'll find it at full speed without a break. Both are the system doing what it was designed to do. The tell that matters is whether the house holds its temperature, and the sections below separate long run times from the symptoms that really need a look.
Why are inverter heat pumps built to run for long stretches?
An inverter heat pump varies its compressor speed. The Department of Energy's Energy Saver guide describes inverter-driven systems as adjusting between low and high speeds, with energy savings and better humidity control. Daikin puts it plainly in its own guide to inverter systems: instead of turning off once the desired temperature is reached, the unit keeps running at a low power level.
Why does a heat pump run more as it gets colder?
A heat pump pulls heat out of the outdoor air, and it gets less of it as the air gets colder. At the same time your house loses heat faster. The DOE's guide calls the crossing point the balance point: the outdoor temperature where the heat pump's capacity equals the house's heating load. Below it, the heat pump runs continuously and backup heat or the furnace makes up the difference.
Where your balance point falls depends on the heat pump and the house. A cold-climate model holds more of its output in deep cold; our guide to HSPF2 and COP ratings shows how to read capacity at 5°F on a spec sheet. A drafty 1920s story-and-a-half in Minneapolis reaches its balance point at a warmer temperature than a tight 2010s house with the same unit. Our post on heat pumps at -20°F covers what happens hour by hour on the coldest nights.
When is a constantly running heat pump a problem?
Long run times alone aren't a symptom. These are:
- The house can't reach the setpoint in weather the system should handle. The thermostat keeps sliding a degree or two below where you set it.
- Aux heat runs all day in mild weather. Aux on a -10°F night is expected. Aux for hours at 40°F points to a heat pump that isn't carrying its share or a setting that's wrong. Our post on aux heat vs emergency heat covers the difference.
- Ice that doesn't clear. Carrier says a light frost layer under 1/4 inch is normal and should clear in defrost; thicker ice, or ice that stays, is not.
- A jump in the electric bill without a cold snap to explain it. On an all-electric system, a heat pump that quietly stopped working can hide behind the heat strips for weeks.
Carrier lists the usual causes of a heat pump that runs constantly without keeping up: a frozen coil, a malfunctioning reversing valve, low refrigerant and compressor trouble. We'd add airflow: a clogged filter or closed registers make any heat pump work harder for less heat.
How do switchover and lockout settings affect run time on dual fuel?
On a dual-fuel system (a heat pump paired with a gas or propane furnace), the thermostat or control decides when the heat pump stops and the furnace takes over. That switchover temperature, and any compressor lockout temperature, have a big effect on how long the heat pump runs.
- Switchover set too cold on an older standard heat pump: it runs nonstop on cold days while barely keeping up, and the house drifts.
- Switchover set too warm on a cold-climate heat pump: the furnace takes over on days the heat pump could have handled, and you give up heat pump hours you paid for.
The DOE's guide notes that many heat pump controls are wired to bring on backup heat whenever the thermostat is raised by more than 3 degrees, regardless of the outdoor temperature, and that energy codes require controls to prevent unnecessary strip heat. Our dual-fuel guide explains how to pick the switchover point from your own gas and electric rates.
What can you check before calling?
Troubleshooting a heat pump that won't keep up follows the same order as any no-heat call:
- Thermostat. Confirm it's set to Heat with the fan on Auto. The DOE recommends the auto fan setting unless the system has a variable-speed fan motor. Look at whether aux heat shows on, and note the outdoor temperature when it does.
- Breakers and the furnace or air handler switch. Make sure nothing is tripped, including any separate breakers for heat strips. If a breaker trips a second time, leave it off and have it looked at.
- Filter. A clogged filter cuts airflow. The DOE's heat pump maintenance guide says to clean or change filters every 3 months or as the manufacturer recommends. Many Minnesota houses with pets need it more often in winter.
- Outdoor unit clear. Brush snow off the top, clear drifts from the sides and make sure a gutter isn't dripping onto it. Leave ice on the coil alone.
- Condensate. If a dual-fuel system's furnace is a condensing model, a frozen or plugged condensate line can shut the furnace down, leaving the heat pump to carry the house alone. Our post on a frozen furnace condensate line covers the fix.
Don't open the electrical panels on the outdoor unit, touch the capacitor, add refrigerant or keep resetting the system.
What does a technician check on a heat pump that won't keep up?
A technician compares what the heat pump delivers with what it should: temperature rise across the indoor coil, refrigerant charge, the outdoor fan and defrost control, the reversing valve and the airflow through the filter and ducts. The thermostat's installer settings, including the switchover and aux lockout temperatures, should be set from the equipment's specs and your house rather than the factory default.
When should you call about a heat pump that runs nonstop?
Call if the house won't hold temperature, aux heat runs for hours above about 40°F, the coil stays iced after a defrost, or the bill jumped with no cold snap. The $79 trip charge on a service call is waived when you approve the repair, and the price is written and flat-rate before work starts. See our heat pump service page or heat pump service in Minneapolis. No-heat calls get priority; call 612-429-3465.
Common questions about heat pumps
Does a heat pump running all the time use more electricity?
Not necessarily. An inverter heat pump running at low speed for hours can use less power than one that starts and stops at full speed, because starting up is the least efficient part of the cycle. What raises the bill is backup electric heat running when it doesn't need to.
Why does my heat pump run constantly when it's below zero?
Because it has to. A heat pump's output drops as the outdoor air gets colder, while your house loses heat faster. Below the balance point, the heat pump runs flat out and backup heat or the furnace fills the gap. Carrier says it can be completely normal for a unit to run all the time in extreme temperatures.
Should I turn my heat pump off at night to give it a break?
No. It doesn't need rest, and a deep setback on a heat pump with electric backup can cost more, because the recovery in the morning tends to bring on the strips. The Department of Energy advises against setting back a heat pump's thermostat if it causes the backup heat to come on. Small setbacks with a heat-pump-aware thermostat are fine.
Does a heat pump running all the time mean it is undersized?
Only if the house will not reach the setpoint on the coldest days. Long run times alone are normal. A properly sized system comes from a Manual J load calculation, and an undersized one shows up as falling indoor temperatures while backup heat runs. Ask for the load calculation before assuming the equipment is too small.
Sources
- U.S. Department of Energy energy.gov
- Carrier manufacturer documentation carrier.com
- Daikin manufacturer documentation daikincomfort.com