When a heat pump is powered on and running in heating mode but fails to blow hot air, it is often experiencing a reversing valve issue, a defrost cycle delay, or low refrigerant charges. The reversing valve is responsible for switching refrigerant flow between heating and cooling; if it sticks or fails electrically, the system cannot provide heat. Additionally, if outdoor temperatures are low and frost accumulates on the outdoor coil, the unit enters a temporary defrost cycle, which mimics cooling mode and blows lukewarm or cool air indoors for ten to fifteen minutes. Other potential causes include restricted airflow from dirty filters or malfunctioning auxiliary heating elements. You can verify if it is in defrost mode by checking for steam outdoors and waiting to see if warm airflow resumes automatically.
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