Carrier Air Conditioner Not Heating — Fix

What Does This Error Mean?

If your Carrier AC is set to Heat mode but not warming the room, check that the remote is in Heat mode (sun icon) and that the set temperature is higher than the current room temperature. Also ensure the outdoor unit is running.

Affected Models

Carrier Split System AC
Carrier Inverter AC
Carrier Mini-Split AC
Carrier Ductless AC

Common Causes

Mode set to Cool or Fan instead of Heat
Outdoor temperature too low for the heat pump to operate efficiently
Outdoor unit in defrost cycle — temporary reduction in heat output
Dirty air filter reducing airflow
Low refrigerant reducing heat pump efficiency
Reversing valve fault preventing switch from cooling to heating mode

How to Fix It

Verify Heat mode is selected.

Press the Mode button on the remote until the sun or heat icon is displayed. Set the temperature at least 3°C above the current room temperature. The compressor needs a temperature differential to activate heating.
Check the outdoor temperature.

Carrier heat pump ACs lose efficiency when outdoor temperatures drop below 5°C, and most stop heating effectively below -15°C. At very low outdoor temperatures, the unit may produce only lukewarm air — this is normal heat pump behaviour, not a fault.
Wait for the defrost cycle to complete.

In cold, humid weather the outdoor unit frosts over and the AC automatically enters a defrost cycle, pausing heat delivery for 5–15 minutes. The indoor unit may blow cool air briefly during defrost. This is normal — heating resumes when defrost completes.
Clean the air filter.

A dirty filter reduces both cooling and heating capacity. Clean the filters and allow the unit to run for 30 minutes — heating should improve if restricted airflow was the cause.

When to Call a Professional

If the mode is correct, the outdoor temperature is above -15°C, and the unit blows cool air in Heat mode, the reversing valve is stuck or has failed — this requires a technician.