This article will take you through the most common reasons why the burner shuts off when the blower comes on and what you can do about them. Many are DIY fixes, and you’ll find links to pages here on Pick HVAC that walk you through how to repair the problem.
Why The Furnace Flame Goes Out When Blower Comes On
Before doing any work on the furnace, make sure it is turned off and the gas valve is closed.
Air Flow Problems
Your furnace needs proper air flow into the burner, or the burner will turn off. First, check your furnace filter. A clogged filter can cause the furnace to shut off.
Keeping the furnace in a cramped area like a utility closet or surrounded by items in a storage room may restrict the necessary air flow required for the furnace to run properly.
Make sure nothing is blocking the airflow inside of the home. If your furnace draws air from outside the home, check to see that there is nothing blocking the air intake such as leaves, other debris, or ice and snow.
A Problem with the Flame Rollout Switch
A furnace flame rollout occurs when the flames burn outside of the combustion chamber of the furnace. This can happen due to a blocked flue or the build-up of gases in the chamber. Both of these issues can harm your furnace and allow carbon monoxide to enter your home.
The flame rollout switch is a safety component that monitors the amount of heat within the furnace and shuts off the supply of gas to the furnace if the temperature outside the combustion chamber exceeds a certain level.
The flame rollout switch turns off the furnace just as it is heating up so, in this case, the blower is not causing the flame to go out, it’s just a matter of timing. About the time when the blower turns on is the time that the flame rollout switch recognizes the problem and shuts down the furnace.
If the switch or switches are not functioning properly, they are easy to replace. If the switches are not functioning properly or the furnace continues to malfunction, you will need to hire an HVAC repair professional.
How to Test the Flame Rollout Switch
The flame rollout switch is located on the front of the burner. Your furnace may have as many as three or four switches, and they should all be tested. Sometimes the rollout switches are behind a cover that needs to be removed.
Disconnect the wires from each switch then unscrew and remove them. You will need a multimeter to determine if the switch is faulty. Set the multimeter to the lowest ohms of resistance and touch the probes on the multimeter to each terminal on the switch. If the switch is faulty it will show infinite ohms. Reset the switch by pressing the button in the middle of the switch and retest it. If resetting the switch doesn’t correct the problem, it will need to be replaced.
If the new switches continue to trip, you will need an HVAC professional.
A Problem with the Flame Sensor
If the problem isn’t the flame rollout switch, next check the flame sensor. The flame sensor monitors the gas burner. If the furnace starts but shuts off after the blower turns on, it might be that the sensor is not recognizing the flame. The flame sensor makes sure there is actually a flame when the gas is on.
Most of the time the problem can be fixed by cleaning the flame sensor. The flame sensor is located on the burner assembly and sits in front of the pilot flame inside the furnace. It is usually attached with a single screw and is a thin metal rod and generally is bent at a 90° angle.
How to Clean a Flame Sensor
Make sure the furnace is turned off and remove the screw that holds the sensor. Gently pull the sensor out. Clean the sensor by rubbing it with a mildly abrasive pad or fine Emery cloth to remove build-up but without scratching the sensor. In a furnace that is working efficiently, the flame sensor should only need cleaning about once a year.
A Problem with the Control Board
After testing the flame roll out switch and the flame sensor, the next thing to check is the control board. The control board regulates power to all of the components within the furnace so when it is faulty it won’t deliver the necessary voltage. A faulty control board may also cause the furnace to cycle on and off for very short periods.
Common signs that your furnace control board is not working include flashing lights on the control board, fluctuations in temperature, and interruptions in the normal sequencing of your furnace.
It is possible to check the control board yourself but there are a number of complicated steps that might be difficult for someone without HVAC experience. It is also difficult to replace and will likely require an HVAC professional.
A Problem with the Pressure Switch
The furnace pressure switch is an automatic safety feature that is located near the motor. It makes sure there is enough air flow through the furnace and will shut the furnace down if it senses negative pressure created by the draft inducer motor. It protects your home from back drafting which allows exhaust fumes back into the system. A defective pressure switch can cause your furnace to shut off about the time the blower would come on.
How to Test a Pressure Switch
You will need a multimeter to check a pressure switch. Multimeters can also function as ohmmeters.
First turn off the furnace and close the gas valve then locate the round-shaped pressure switch which is usually near the draft inducer motor and is easy to identify once you’ve removed the furnace door. Place a multimeter lead on both of the pressure switch terminals. The reading should be zero or very close to zero. A high reading means that the switch has failed.
Replace the switch by disconnecting the hose on the pressure switch and remove the wires. Unscrew and remove the old pressure switch. Align the new switch, tighten into place, reconnect the wires, and reattach the hoses.
Pressure switch ratings will be different depending on your location and altitude so make sure you purchase one with the same pressure rating as the original.
Bad High Limit Switch
Aka the limit switch, this part of your furnace senses the heat level inside the furnace cabinet. It could be that your furnace is malfunctioning and overheating, and that would cause the limit switch to properly shut down the burner about the time the blower comes on.
But more often, the limit switch is defective, and it “thinks” the furnace is overheating when it isn’t. The high limit switch is pretty easy to replace. See a couple Pick HVAC pages that discuss this part, why it would shut down a furnace and step-by-step instructions (just a few steps) for limit switch replacement.