Furnace Blowing Cold Air: What to Check
Cold air from your vents can mean a thermostat fan setting, overheating, or an ignition problem.
A furnace blowing cold air is frustrating but usually traceable. The fix depends on whether it's blowing cold constantly or only sometimes.
Check the thermostat fan setting
If the fan is set to ON instead of AUTO, it runs constantly and blows room-temperature air between heating cycles. Switch it to AUTO so the blower only runs when the burners are producing heat.
Overheating and ignition faults
If the furnace starts warm then turns cold, it may be overheating (dirty filter, blocked vents) and shutting the burners off while the fan keeps running. Repeated ignition lockouts can also leave the blower pushing cold air.
Key takeaways
- Set the thermostat fan to AUTO, not ON
- Replace a clogged filter to prevent overheating
- Open and unblock supply and return vents
- If it locks out repeatedly, book a diagnostic
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