When your indoor air handler blower fan is running continuously, but your supply vents are only circulating unheated or cool air during the winter heating season, it points to a burner ignition failure or a heat pump malfunction. In a gas furnace, the blower fan turns on automatically after a brief warm-up delay once the burners ignite; if the burners fail to ignite due to a bad igniter, a dirty flame sensor, or a closed gas valve, the furnace safety controls shut down the burners while the blower keeps running to exhaust residual heat, blowing cold air. In a heat pump, it may indicate a stuck reversing valve or a defrost cycle. You can troubleshoot by checking your thermostat settings and furnace error codes. You can verify that the issue is resolved when the burners ignite successfully and warm air flows steadily from your vents.
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