Why are my vents blowing cold air when the heat is on?

When your indoor supply vents are blowing 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, which causes the indoor unit to blow cooler air for a brief period 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. Additionally, low refrigerant levels or a malfunctioning thermostat configuration can prevent proper heating. 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.