Furnace Blowing Cold Air

If FAN is set to ON, the blower runs continuously — including when the furnace isn't actively heating. Between heat cycles, you get cold air. This is by design, not a malfunction.

Look at your thermostat. There should be FAN options of AUTO and ON. AUTO = blower runs only during heat call. ON = blower runs always.

The flame sensor is a thin metal rod that detects the burner flame. When it's dirty (oxidized), it can't detect the flame and the gas valve shuts off as a safety measure. Burner lights for 5-10 seconds, then shuts off. Blower keeps running, blowing cold air.

Watch the furnace through the inspection window. If burners ignite then shut off after 5-10 seconds (sometimes after 2-3 attempts), it's classic flame sensor failure.

Modern furnaces use a hot surface ignitor (HSI) — a small ceramic component that glows red-hot to ignite the gas. They typically last 5-10 years, then fail. Without ignition, no flame, no heat.

Watch the furnace at startup. The ignitor should glow bright orange-red within 30 seconds. If you see no glow, or it cracks visibly, it's failed. Sometimes you hear gas valve open but no ignition.

If your furnace is 20+ years old with a standing pilot light, the pilot may have been blown out (by a draft) or the thermocouple has failed.

Look through the inspection window for a small blue flame near the burners. No flame = pilot is out.

The high-limit switch shut off the gas to prevent overheating. Caused by restricted airflow — dirty filter, closed vents, or failing blower.

Furnace runs for 5-10 minutes then shuts off the burners while the blower continues to cool the heat exchanger. Sometimes you'll see a red blink code.

Gas is shut off at the supply valve, or the gas valve on the furnace itself has failed.

Test other gas appliances (water heater, stove). If they work, problem is at the furnace. The manual shutoff valve at the furnace should be parallel to the gas line = OPEN.

90%+ efficient furnaces produce condensate water that must drain. If the drain clogs or freezes, the pressure switch trips and shuts off the furnace. Blower may run.

High-efficiency furnaces have PVC venting. Look at the drain line and trap — water should flow freely. Check for ice in cold weather if vents are exposed.