When your residential heat pump blows cold air while your wall thermostat is set to heating mode, it is usually experiencing one of several normal operational behaviors or minor mechanical faults. The most common and benign cause is that the heat pump has automatically entered its defrost cycle to melt frost or ice accumulated on the outdoor condensing unit during cold winter weather. During this defrost cycle, the system temporarily reverses its refrigeration cycle to act like an air conditioner, which causes the indoor unit to blow cooler air for a brief period of ten to fifteen minutes before resuming normal heating. Another frequent cause is a stuck reversing valve that fails to shift the refrigerant flow from cooling to heating mode properly. You can verify whether the system is simply in a defrost cycle by checking the outdoor unit for steam and waiting to see if warm airflow resumes automatically. If cold air continues indefinitely, professional diagnostic service is needed.
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