Symptoms
- Air from vents feels cool or only slightly warm in Heat mode
- System runs but doesn't seem to be catching up on cold days
- Outdoor unit may be covered in ice
Likely causes
- CommonNormal reduced capacity in very cold outdoor temperaturesStandard heat pumps lose heating capacity as outdoor temperatures drop, and many rely on backup heat below a certain point.
- CommonOutdoor unit heavily iced over beyond a normal defrost cycleExcess ice blocks airflow and heat exchange, reducing performance significantly.
- PossibleStuck reversing valve keeping the system in cooling modeThis valve switches the refrigerant flow between heating and cooling and can stick with age.
- PossibleLow refrigerantReduced refrigerant hurts heating capacity just as it hurts cooling.
- PossibleBackup/auxiliary heat not engaging when neededMany systems rely on electric backup heat in extreme cold, and a failure there leaves a capacity gap.
What to do
- Check the outdoor unit for excessive ice buildup beyond a light, temporary frost that clears during normal defrost cycles.
- Confirm the thermostat is set to Heat, not Emergency Heat unless intentionally needed, and check for any heat pump specific settings.
- Feel the air from vents; heat pump air in heat mode is often warm but not as hot as a gas furnace, which is normal.
- Check if backup/auxiliary heat is engaging during very cold weather, often shown by a specific indicator on the thermostat.
- Replace the air filter to rule out an airflow-related performance drop.
- Call an HVAC technician if the outdoor unit is heavily iced, air stays cool rather than just moderately warm, or backup heat isn't engaging.
Tools and parts
AC/heat pump air filterSearch: “heat pump air filter size replacement”
When to call a pro
Call an HVAC technician if the outdoor unit is heavily iced, the reversing valve is suspected, or backup heat isn't engaging in cold weather.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.