When an HVAC system blows cold air while the thermostat is actively set to a heating setting, it points to a control communication failure, a stuck reversing valve, or a normal defrost cycle activation. In heat pump systems, the reversing valve controls the direction of refrigerant flow; if it fails to energize or gets stuck mechanically, the system continues to operate in cooling mode even when heating is requested. Alternatively, if outdoor temperatures are low and frost accumulates on the outdoor coil, the heat pump enters a temporary defrost cycle, which mimics cooling mode and blows cool air indoors for ten to fifteen minutes. Other potential causes include miswired thermostat cables or a failed control board relay. You can verify the cause by checking for outdoor coil icing or testing voltage at the reversing valve terminals.
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