HomeAnswersWhy does my furnace blow cold air?
Why does my furnace blow cold air?
A furnace that blows cold air is almost always one of three things: the blower is running without a heat call, the burner tried to light and shut back down, or the furnace got too hot and the limit switch stopped the burners. Which of the three you have shows up in what you hear before the air ever feels cold.
Check the two free things first
Set the thermostat fan switch from ON to AUTO. In ON, the blower runs constantly and blows room-temperature air between heat cycles. It is not broken; it is just moving air you did not ask to heat. Next, replace the filter if you cannot see light through it. A clogged filter chokes airflow until the high-limit switch shuts the burners off while the blower keeps pushing cold air. During a Green Bay cold snap, a dirty filter is the single most common cause of cold air with no other fault.
The burner is not staying lit
If the air starts warm and goes cold after a few minutes, the flame is dropping out. The usual reasons are a flame sensor coated with white silica film, a hot-surface igniter that is cracked, or a pressure switch that will not prove draft. The furnace tries, fails, and locks out; the blower finishes its cycle on cold air. These parts are covered in detail on the furnace repair page, and the parts themselves are typical furnace parts repairs in the low hundreds.
When to shut it down
Stop the furnace at its switch and request an urgent visit if the cold air comes with a burning plastic smell, a clicking that never stops, or if a carbon monoxide alarm is sounding. Leave the house on a CO alarm and call emergency services first. For everything else, note whether the igniter glows, how many seconds the burner burns, and whether the small draft fan runs. Those three facts shorten the first visit considerably.
Most cold-air calls resolve in a single visit with a part in the $150 to $600 range typical for the metro. The company that takes the visit confirms the fault and the price before work begins. If the house is already cold, say so on the request form and we line up the first available furnace company.