Furnace Blowing Cold Air

Air that stays cool while the fan keeps running says the burner side has stopped and the fan has not, and that is worth a call before the house loses ground.

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When this becomes an emergency

The registers are moving air that never turns warm.
The blower runs long past when it should have shut off.
The burner ignites and then drops out within seconds.
The furnace goes into lockout and will not restart on its own.

What a pro does

Cool supply air is a heat-source complaint, not a fan complaint. The technician checks whether ignition truly happens, looks at the ignitor and the flame sensor for the residue that stops a sensor from proving flame, and determines whether a safety lockout is keeping the burner off while the blower carries on. Forced-air gas systems in houses of a certain age here often have return ducts that were reworked during a basement finish, and starved return air overheats the cabinet and trips a limit that reads as this exact symptom. Repeatedly resetting a lockout doesn't address whatever caused it.

Frequently asked questions

How can one small sensor stop the heat?

The flame sensor is the board's only proof of fire. Once residue coats it, the board sees no flame and shuts the gas valve almost immediately after ignition.

What does a finished basement have to do with it?

Return ducts get rerouted or partly blocked during remodels, and a furnace short on return air overheats and trips a safety limit while the fan keeps running.

Is a clogged filter enough to cause this?

Indirectly, yes. The filter restricts airflow, the cabinet overheats, and the limit switch cuts the burner while leaving the blower going.

Should the system be left calling for heat?

Better to set it to off or fan only. Dozens of failed ignition attempts overnight wear the ignition components for nothing.

Furnace Blowing Cold Air?

Get connected with a local pro now.

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