Why Is My Heater Blowing Cold Air?
A heater blowing cold air usually means the burners aren't igniting while the blower runs. Common causes include thermostat set to FAN instead of AUTO, a failed igniter, dirty flame sensor, or overheating from a clogged filter. Check your thermostat settings and filter first. If the furnace clicks but won't light, the igniter or flame sensor likely needs service.
Common Causes
Thermostat Set to Fan Only
If the thermostat is set to FAN ON instead of AUTO, the blower runs continuously, even when the furnace isn't heating. This circulates unheated air.
Pilot Light Out
For gas furnaces with standing pilot lights, if the pilot goes out, the furnace can't ignite. The blower may still run, circulating cold air.
Igniter Failure
Modern furnaces use electronic ignition. If the hot surface igniter cracks or fails, the burners won't light even though everything else works.
Flame Sensor Dirty
The flame sensor verifies that burners are lit. If it's dirty with carbon buildup, it can't detect the flame and shuts off gas flow as a safety measure.
Overheating/Limit Switch
If the furnace overheats (often due to a dirty filter or blocked vents), the limit switch shuts off the burners while the blower continues running to cool the unit.
Gas Supply Issue
The gas valve may be off, the gas line may have an issue, or there could be a problem with gas pressure. No gas means no heat.
What Should You Do?
Try This First
- Check thermostat, ensure it's set to HEAT and AUTO (not FAN ON)
- Check and replace the air filter if dirty
- Make sure all vents are open and furniture isn't blocking returns
- Verify the gas valve near the furnace is in the ON position
- Check if other gas appliances are working (to rule out gas supply issues)
Call a Pro If...
- You smell gas (leave immediately and call 911)
- The furnace clicks repeatedly but won't stay lit
- You see a flashing error code on the furnace
- The problem persists after checking obvious issues
- The furnace is cycling on and off frequently