Why is my furnace blowing cold air?

Cold air from a furnace is alarming, but the cause is usually one of four things, and two of them cost you nothing to fix.

Check the thermostat fan setting first. If the fan is set to “on” instead of “auto,” the blower runs continuously, including between heating cycles, and between cycles it is pushing unheated air. That reads as “furnace blowing cold air” when nothing is broken at all. Flip it to “auto” and see if the complaint disappears.

Check the filter second. A clogged filter chokes airflow, the furnace overheats, and a safety device called the high-limit switch shuts the burners off while the fan keeps running to cool things down. Result: cold air, then maybe warm air, then cold again. If you cannot see light through the filter, replace it. Bakersfield dust makes this the most common furnace call there is.

If those two check out, it is our territory. A dirty flame sensor makes the furnace light and then shut down within seconds, over and over. Cleaning or replacing one runs $189 to $260 flat. A failed hot surface igniter means the furnace never lights at all, and that runs $220 to $340. Both are same-visit repairs in most cases, and the $89 diagnostic is waived when you book the fix.

The one that is not a repair call: if you ever smell gas, do not troubleshoot. Leave the house first, then call the gas utility and 911 from outside. Furnace repair comes after everyone is safe, every time.

The quiet lesson in this list is that a fall heat check catches three of the four before the first cold morning does. That is half of what the Bloom Plan is for.