My furnace is running but blowing cold air instead of hot air. The thermostat is set to heat but only cold air comes out.
A furnace that runs but blows cold air has a few possible causes. First, check the thermostat fan setting. If the fan is set to ON instead of AUTO, the blower runs continuously even when the furnace is not actively heating, pushing unheated air through the vents between heating cycles. Switch it to AUTO. Next, check the air filter — a severely clogged filter restricts airflow over the heat exchanger, causing the furnace to overheat and the high-limit safety switch to shut off the burner while the blower continues running. Replace the filter. For gas furnaces, check if the pilot light is out or the igniter is not firing. If you hear the furnace start up and then shut down after a few seconds without the burner lighting, the flame sensor may be dirty. The flame sensor is a small metal rod in the burner area — cleaning it with fine sandpaper or steel wool often fixes the issue. A clean flame sensor allows the gas valve to remain open after ignition. If none of these fixes work, the gas valve, control board, or heat exchanger may have failed and you need a professional HVAC technician.
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