Flashback at ignition or burner shutoff

What is going on

Flashback is when the flame burns back into the burner venturi because the burning speed is faster than the gas and air coming out of the port. You see it at ignition, after the burners warm up, or at shutoff as a pop when the flame goes out. In my experience gas pressure and a shifted burner are the usual culprits. Delayed ignition can look the same from the outside, so work out which one you actually have before you start changing parts.

Likely causes

• Improper gas pressure
• Burner misalignment
• Improper orifice size or obstructed orifice
• Dirty or rusted burner ports disturbing the gas-air mix

Check in this order

1. Check gas pressure and adjust to rating plate values (see gas pressure checks). Gas work is for a qualified gas technician
2. Check burner alignment and replace the burner if needed
3. Check the orifice for obstruction and correct size
4. Watch the burner light and shut off to see exactly when the pop happens
5. Inspect burner ports and clean out debris with power and gas off
6. Recheck flame pattern after repairs

More furnace guides

• Airflow lower than demanded, furnace runs on low stage when high stage is expected
• Alternate combustion air opening (upflow or horizontal)
• Alternate vent/flue location (upflow or horizontal left)
• Annual inspection checklist
• Auxiliary limit control (location and purpose)
• Auxiliary limit switch open (blower compartment)