A heat pump that successfully cools your home during summer but fails to provide heat during winter is typically experiencing a specific failure in the components responsible for reversing the refrigeration cycle. The most common cause is a stuck or failing reversing valve, which is the internal mechanical valve that shifts refrigerant flow between cooling and heating modes; if the solenoid coil fails or the internal slider sticks, the system remains locked in cooling mode. Another frequent cause is a faulty defrost control board or outdoor sensor that leaves the unit trapped in a continuous defrost cycle. Additionally, a miswired thermostat control cable or a failed auxiliary heating strip can prevent proper warmth. You can verify the fault by checking if the outdoor compressor runs and testing voltage signals at the reversing valve terminal during a heating call.
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