Why Is My Furnace Blowing Cold Air?

Dowe & Wagner

Cold air blowing out of your home’s vents in the summer:  nice. Cold air blowing out now, with the furnace on?  Something’s wrong, which may be a simple problem, or indicate a major repair.   In the fall, when people switch their thermostat to heat, they hope to hear the reassuring hum of a functioning furnace, and feel the gentle stream of warm air.  After 10 minutes of waiting and no activity, or room temperature air blowing from the vent, what should homeowners do?

Tom Eppers, co-owner, Dowe & Wagner, a heating, ventilation, and air-conditioning (HVAC) company serving residential and commercial customers in Illinois and Wisconsin, advises to check the furnace’s air filter.  A filthy filter can impede airflow, and cause the furnace to overwork, overheat, and sometimes shut off as a safety feature.   Filters should be replaced regularly, according to the furnace’s manufacturer’s instructions.  If there are allergies or pets in the house, the filter may need to be changed more often than the typical monthly recommendation.

According to HVAV experts, a simple cause of a furnace blowing cold air is not switching your fan setting to “auto.”  The “on” position instructs the fan to work continuously, even when it’s not heating the air.  In the “auto” mode, the fan will only blow warm air, operating when the furnace is running.

For older furnaces, cold air streaming from the vents could indicate a blown out pilot light.  The owner’s manual details how to re-light it safely.  If it doesn’t remain lit, an HVAC professional may need to be called.

If the air starts warm but soon turns cold, the flame sensor may need professional cleaning.   An annual maintenance visit to clean and service your furnace twice a year helps prevent equipment breakdowns.

For more information, contact Dowe & Wagner, a full-service heating, ventilation, and air-conditioning company at (815) 678-3000, or visit


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