Most common causes
- Thermostat fan set to On. With the fan on On, the blower runs all the time, including between heating cycles, so you feel cool air. Set it to Auto.
- Furnace still warming up. Most furnaces wait 30 to 90 seconds before the blower starts, and the first air may feel cool for a moment. That is normal.
- Dirty flame sensor. The flame sensor proves the burner is lit. When it is coated, the furnace shuts the gas off after a few seconds, and the blower keeps pushing unheated air.
- Ignition failure. A cracked hot surface igniter or failed pilot means the burners never light.
- Clogged air filter. Low airflow lets the heat exchanger overheat, and the high limit switch shuts the burners off while the blower keeps running.
- Gas supply problem. If the gas valve to the furnace is off or the gas service is interrupted, the furnace cannot make heat.
- Condensate drain blocked (high efficiency furnaces). High efficiency furnaces make water. If the drain or trap is clogged, a pressure switch keeps the burners from firing.
What you can safely check
- Set the thermostat to Heat, raise the setpoint a few degrees above room temperature, and set the fan to Auto.
- Replace the furnace filter if it is dirty.
- Make sure the furnace door is fully closed so the door safety switch is pressed.
- Look through the small window on the furnace door for a blinking status light. Count the blinks and match them to the chart on the door, then share that with the technician.
- Check that the furnace breaker or the switch near the furnace is On.
- If you smell gas, do not touch any switches. Leave the house and call SoCalGas or 911 from outside.
Call a technician if
- The burners light and then go out within a few seconds
- You hear the igniter click or see a glow but no flame
- The status light shows an error code
- Water is pooling around a high efficiency furnace
- You smell gas (leave first, then call SoCalGas or 911)
What affects the repair cost
A dirty flame sensor is one of the simplest furnace repairs. Igniters, pressure switches, and limit switches are parts that vary by furnace model. Gas valve or control board replacement costs more. Older furnaces with repeated problems may be worth comparing against replacement.
