Furnace Blowing Cold Air

When this becomes an emergency

• Registers are pushing room-temperature air even though the fan is clearly running.
• You hear the igniter click and the burner try, then everything drops back to just the fan.
• The unit warms up briefly at the start of a cycle and turns cold partway through.
• One end of the house never catches up while the fan seems to run forever.

What a pro does

Cool air at the registers tells you the fan side works and the fire side doesn't. Where it gets interesting is why. A tech watches a full call for heat with the panel off, timing each stage, because the sequence itself names the failure: inducer spins but the pressure switch never proves, or it proves and the igniter never glows, or the flame lights and dies three seconds later at the sensor. That last one is the classic. A flame rod with a film of oxide on it stops passing enough current for the board to believe there's fire, and the board does exactly what it should by dropping the gas and letting the fan purge. Blocked flue draft counts too, and in tight older basements a water heater sharing an undersized chimney can steal enough draft to knock the furnace out on its safety. Forcing repeated retries just cooks the igniter.

Frequently asked questions

Furnace Blowing Cold Air?

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