Why Is My Furnace Blowing Cold Air?
The three most common causes are a dirty flame sensor, a failed igniter, and a thermostat set to the wrong fan mode.
The last one takes ten seconds to check and costs nothing, so start there.
Check the thermostat fan setting first
If your thermostat fan is set to ON rather than AUTO, the blower runs continuously — including when the burner isn't firing. So you get air, but it's room temperature.
Switch it to AUTO. If the air comes out warm after a few minutes, that was it.
This catches a surprising number of "furnace blowing cold" calls, and it's usually someone in the house having changed the setting without mentioning it.
Then check the filter
Pull out the filter and look at it. If it's gray and matted, replace it.
A clogged filter restricts airflow, which makes the furnace overheat and trip a safety limit switch. The blower keeps running to cool the system down while the burner stays off — so you get cold air.
This is the second most common cause and also free to fix.
The flame sensor
If the furnace starts, runs for thirty seconds to a couple of minutes, then shuts the burner off and blows cold air — that's usually the flame sensor.
A flame sensor confirms the burner has actually lit. Over a season it builds up a film of residue, stops detecting the flame, and shuts the gas off as a safety measure.
The furnace isn't broken. It just can't see itself working.
It's one of the most common winter calls we take and one of the least expensive repairs.
The igniter
If you hear the furnace try to start but it never fires at all, the igniter is the likely cause. They weaken gradually over several seasons before failing completely.
Other causes
A pressure switch fault, often caused by a blocked flue or exhaust.
A thermostat problem — dead batteries, or wiring.
Low gas supply, or a closed gas valve.
A tripped limit switch from overheating, usually traced back to airflow.
In a heat pump, this behaves differently — a heat pump in defrost mode blows cool air briefly and that's normal.
What to do right now
Check the thermostat is on AUTO, not ON. Check it's set to heat and the temperature is above the current room temperature. Check the batteries.
Check the filter and replace it if it's dirty.
Check the furnace switch, which looks like a light switch and is usually near the unit. It gets knocked off more often than you'd think.
Check the breaker.
If all of those are fine, it's a technician's job.
One thing not to ignore
If you smell gas, leave the house and call your gas utility before calling anyone else. Don't operate light switches on the way out.
If you need it looked at
We repair furnaces across Salt Lake, Davis and Utah Counties, from Bountiful to Lehi. No heat in winter goes to the front of the line.
Every visit starts with a $99 diagnostic, credited toward your repair.