If your furnace burners ignite but then go out before your home reaches the temperature set on the thermostat, something is interrupting the heating cycle. The cause could be as simple as a dirty flame sensor, but repeated burner shutdowns can also be related to overheating, airflow problems, ignition issues, venting problems or a failing furnace component. The timing of the shutdown can provide an important clue.
A dirty flame sensor is a common cause
If the burners light normally and then shut off after only a few seconds, the flame sensor is one of the first things that should be checked. The flame sensor confirms that the burners are actually producing a flame. If it does not detect the flame properly, the furnace control board shuts off the gas supply as a safety measure. A dirty or oxidized flame sensor may still be physically intact but unable to send a reliable signal. The furnace may then try to ignite again, only to repeat the same cycle.
The furnace may be overheating
If the burners stay on for several minutes before shutting off, overheating may be the problem. A furnace that gets too hot will normally shut the burners down to protect the system.
Common causes of overheating include:
A dirty furnace filter
Closed or blocked registers
A dirty blower wheel
A weak or failing blower motor
Restricted ductwork
An overly restrictive filter
Other airflow problems
If the furnace cools down and then starts again, this can create a repeated cycle of heating, shutting down and restarting.
A dirty filter can restrict airflow
A clogged filter is one of the easiest problems for a homeowner to check. As the filter becomes loaded with dust and debris, less air can pass through the furnace. That can cause heat to build up inside the heat exchanger. If the furnace reaches an unsafe temperature, the high-limit switch may shut the burners off. Check the filter and replace it if it is visibly dirty. If the burner problem continues after the filter is replaced, the furnace should be inspected.
The high-limit switch may be shutting the burners down
The high-limit switch is a safety device that monitors furnace temperature. If the furnace gets too hot, the switch shuts the burners off. The blower may continue running to help cool the furnace. Once the temperature drops, the burners may ignite again. A furnace that repeatedly trips the high-limit switch usually has an underlying airflow or overheating problem that needs to be corrected.
The burners may be dirty
Dirt, rust or debris can interfere with proper burner operation. The flame may become uneven, unstable or fail to travel properly across the burner assembly. This can affect ignition and may also interfere with flame sensing. Burner cleaning and adjustment should be handled carefully because the burner assembly is part of the furnace combustion system.
The gas supply may be inconsistent
The burners need a steady supply of gas to remain lit. Problems with gas pressure, the furnace gas valve or another part of the fuel supply system can cause the flame to drop out. If the gas valve is not opening correctly or gas pressure is unstable, the furnace may shut the burners down. Gas supply problems require professional diagnosis.
A venting problem can interrupt furnace operation
Gas furnaces must safely move combustion gases out of the home. Many furnaces use a pressure switch to verify that proper venting conditions exist before and during operation. If the venting system becomes blocked or the pressure switch loses its signal, the furnace may shut the burners off.
Possible causes include:
A blocked intake or exhaust pipe
A weak inducer motor
Damaged pressure-switch tubing
A clogged condensate drain
A venting restriction
A faulty pressure switch
These safety controls should not be bypassed.
A clogged condensate drain can cause problems
High-efficiency furnaces produce water as part of the combustion process. That water must drain properly. If the condensate drain, trap or tubing becomes clogged, it can interfere with pressure-switch operation and cause the furnace to shut down. This is one reason burner shutdowns are not always caused by the burner assembly itself.
The control board may be shutting the furnace down
The control board monitors the furnace’s safety systems and operating sequence. If it detects an improper flame signal, pressure-switch problem, overheating condition or another fault, it may shut the burners off. A failing control board can also cause unusual operation, although it should be tested before being replaced. Many furnaces display a diagnostic code when a fault occurs. If you can see a flashing LED through the furnace viewing window, the flash pattern may help identify what caused the shutdown.
Pay attention to how long the burners stay lit
The amount of time the burners remain on can help narrow down the problem. If the burners go out after only a few seconds, a flame sensor problem is more likely. If they run for several minutes and then shut off, overheating or airflow problems become more likely. If the burners stop and restart unpredictably, the problem could involve gas flow, venting, electrical controls or another intermittent fault. This timing information can be very useful when you describe the problem to an HVAC technician.
Is it normal for furnace burners to turn off?
Yes, but only as part of a normal heating cycle.
Once the thermostat reaches the desired temperature, the furnace should shut the burners off. The blower may continue running briefly to move the remaining heat through the ductwork. The problem is when the burners repeatedly go out before the thermostat is satisfied. That usually means the furnace is being interrupted by a safety control or another operating problem.
When should you call for furnace repair?
You should have the furnace checked if:
The burners repeatedly light and go out
The burners shut off after only a few seconds
The furnace keeps restarting
The blower runs but the burners repeatedly stop
The furnace goes into lockout
The furnace is overheating
The diagnostic light shows a recurring fault code
You smell gas or burning
Replacing the filter does not solve the problem
Repeated burner shutdowns should not be treated as normal furnace operation.
Need help with furnace burners that keep going out?
If your furnace burners keep going out, the problem may involve the flame sensor, airflow, high-limit switch, gas supply, venting system, pressure switch or another furnace control.
Roberts Brothers Comfort can inspect the furnace, determine why the burners are shutting down and make the necessary repairs. We provide expert furnace repair service throughout Ohio’s Miami Valley region including Casstown, Covington, Fletcher, Piqua, Rosewood, Saint Paris, Sidney, Tipp City, Troy, Urbana, West Milton and all the surrounding areas.
