Key takeaways
- A thermostat set to ON instead of AUTO makes the fan blow cool air between cycles.
- A dirty filter or flame sensor can cause the burners to shut off.
- An overheating furnace may blow cold as a safety response.
- Some causes are quick DIY checks; others need a technician.
You set the thermostat to heat, the furnace kicks on, but the air coming out is cool. Before you shiver through a chilly Sacramento night, run through these common causes, several of which you can check yourself.
Quick answer
The most common reason is a thermostat fan set to ON instead of AUTO, which blows room-temperature air between heating cycles. Other causes include a dirty filter, a dirty flame sensor, or an overheating furnace shutting off its burners. Start with the thermostat and filter.
1. Thermostat fan setting
If your fan is set to ON, it runs constantly, even when the burners are off, so you feel cool air between cycles. Switch the fan to AUTO so it only blows when the furnace is actively heating.
2. A dirty filter
A clogged filter restricts airflow and can cause the furnace to overheat and shut off its burners as a safety measure, leaving the fan blowing cool air. Replace it and see our filter guide.
3. A dirty flame sensor
The flame sensor confirms the burners lit. When it is dirty, the furnace shuts the burners off for safety but the fan may keep running. This is a common, inexpensive professional fix.
When to call a professional
If the thermostat and filter check out, or you smell gas, call a technician. Pilot, ignition, and gas-supply issues should always be handled professionally. See our furnace troubleshooting checklist for more.
Furnace not heating?
We will get the warm air flowing again, with heating service across Sacramento.
