Home › Heat pumps › Heat Pump Not Heating: Reading a System That Runs Both Ways

Heat pump not heating the house

By · Updated

Because a heat pump uses the same equipment to heat and cool, a heat pump that will not heat can fail in ways a furnace never does. And some of what looks broken is the system deliberately working around outdoor temperature.

Check the mode before anything else

Confirm the thermostat is actually set to “heat” and not “cool” or “auto,” and that the setpoint is above room temperature. It sounds basic, but heat pump thermostats often have more modes than a furnace-only thermostat, including an emergency heat setting. And it is easy to have the wrong one selected without noticing.

Air that feels cool but is not cold

A heat pump moves heat from outdoor air into the house. The air coming out of the vents in heating mode is often only body-temperature-ish rather than furnace-hot: somewhere in the low-to-mid 90s Fahrenheit is typical and normal, not a sign of failure. If the air is genuinely room temperature or cooler, that is the actual problem, not a normal characteristic of how heat pumps work.

The outdoor unit is where heat pumps do their work

In heating mode, the outdoor unit is absorbing heat from the outside air. That is why you may see it running and the fan turning even in cold weather. If the outdoor fan is not spinning while the system calls for heat, that points at an electrical fault, commonly a weakened capacitor. Do not try to diagnose it by opening the unit yourself.

Frost on the outdoor unit, and the defrost cycle

Some frost on the outdoor coil in cold, humid weather is normal. And the system periodically runs a defrost cycle to clear it, briefly reversing to cooling mode and running the outdoor fan while a supplemental heat source keeps the house comfortable. Heavy ice buildup that does not clear over a full day, or a defrost cycle that seems to run constantly, is different and worth reporting to a technician.

Auxiliary or emergency heat

Many heat pump systems have a backup heat source (electric resistance coils or a furnace). It kicks in automatically when outdoor temperatures drop below what the heat pump alone can handle efficiently. You can also switch it on manually as “emergency heat” if the heat pump itself has failed.

If your system only warms the house on the “emergency heat” setting, that is a strong sign the heat pump side has a real fault and needs a technician, even though the house is technically getting heat.

What not to do

  • Do not assume lukewarm air from the vents means the system is broken: confirm the room is actually failing to reach the setpoint first.
  • Do not open the outdoor unit's electrical compartment to check the fan motor or capacitor yourself.
  • Do not leave the system on “emergency heat” indefinitely without having the heat pump itself diagnosed: it is a backup, not a fix.

Frequently asked questions

Why does my heat pump run almost constantly in cold weather?

Long run times in cold weather are often normal. A heat pump works harder to extract heat from colder outdoor air, so it runs longer than a furnace would to deliver the same warmth. Constant running paired with a house that still is not warming is the actual red flag.

Is it normal to hear the system reverse direction periodically?

Yes, that is usually the defrost cycle clearing frost from the outdoor coil, and it is followed by a return to normal heating within several minutes.

Sources

What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.

Keep reading