You can fix this
Likely cause: a clogged filter or closed vents
Restricted airflow can make a furnace overheat and shut off its heat while the blower keeps running. A new filter and open vents usually fix it.
Why this happens
Carrier, Lennox and Trane all list a dirty filter among the causes of a furnace blowing cold air, and Carrier adds closed vents. When air can't move freely, heat builds up inside the furnace and a limit switch shuts the heat off for safety. Carrier recommends replacing 1-inch filters every 1 to 3 months.
How to fix it
- Set the thermostat to Off before you pull the filter.
- Replace the filter with the same size, arrow pointing toward the furnace.
- Open closed supply vents and move furniture, rugs and curtains off vents and return grilles.
- Set the thermostat back to Heat, fan on Auto, and check for warm air after a few minutes.
- Write the date on the new filter's edge so you know when it's due again.
When to stop. If it still blows cold with a clean filter and open vents, call an HVAC technician. The limit switch itself may need attention.
If this does not fix it, tell the HVAC technician
- How dirty the filter was and when it was last changed.
- Whether vents were closed or blocked.
- Whether the heat came back after the fix, and for how long.
Build the full list: what to tell the HVAC technician.
Start this troubleshooter over
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