When a residential heat pump blows hot or warm air while your thermostat is actively set to cooling mode, it usually points to a significant mechanical or electrical control failure. The most frequent culprit is a stuck or failing reversing valve, which is the internal component responsible for shifting refrigerant flow between heating and cooling modes; if it sticks in the heating position, hot refrigerant is sent indoors. Another common cause is a failed compressor capacitor or contactor, which prevents the outdoor compressor from engaging while the indoor fan continues to blow unconditioned air. Additionally, a miswired thermostat control cable can cross the reversing valve voltage signals. You can verify the fault by checking if the outdoor compressor runs. Professional HVAC service is required to test voltages and repair reversing valves.
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