Furnace Blowing Cold Air

Air moving without heat behind it means the fan side is healthy and the fire side quit, which is worth sorting out before the house falls further behind.

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When this becomes an emergency

Registers are moving plenty of air, but none of it feels warm.
The thermostat has been calling for an hour and the room has not gained a degree.
The burner sound never arrives, though the fan clearly does.
The unit runs a few minutes, then blows cool air until it stops.

What a pro does

The split between a working blower and a dead burner narrows things down fast. A technician watches a full call for heat from the start, looking for whether ignition is even attempted, whether the flame lights and then drops out seconds later, and whether the control board has latched into a lockout after too many failed tries. A grimy flame sensor is the classic version of this on gas equipment, since the board reads a real flame as no flame and cuts fuel while the fan keeps running for comfort. Oil burners land in the same place by a different road: a worn nozzle, a weak pump, or a cad cell that can't see the fire. Repeatedly resetting past a lockout is the one thing worth avoiding.

Frequently asked questions

Why would the fan keep running if the burner has stopped?

Most controls run the blower through a cooldown or leave it running on a heat call, so a house full of moving air says nothing about whether anything is actually burning.

Is a lockout a sign of a broken part?

Not by itself. A lockout means the control counted several failed ignition attempts and stopped trying, which is protection working rather than a component that's failed outright.

Could this be the thermostat instead?

It can be, especially if the fan setting has been left on rather than auto, which makes a system that's idle between cycles feel like one that's blowing cold.

Does the same symptom mean the same thing on oil?

The symptom looks identical from the living room, but the causes sit elsewhere, usually in the nozzle, the fuel pump, or the flame sensing eye rather than in a spark ignitor.

Furnace Blowing Cold Air?

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