Loss of flame after establishment

What is going on

Burners light, then drop out. Flame is established and then it is gone. Often a few seconds later it tries again. Look at what changes right after light off: gas, draft, or flame sensing. Start with the easy stuff.

Likely causes

• Interrupted gas supply
• Lazy burner flames from improper gas pressure or restriction in flue and/or combustion air piping
• Front cover pressure switch opening
• Improper induced draft blower performance
• Coated or oxidized flame sensor
• Weak or poorly aligned igniter

Check in this order

1. Locate and correct the gas interruption. Gas work is for a licensed gas tech
2. Check front cover pressure switch operation (hose, wiring, contacts). Correct if needed
3. Replace or realign the igniter
4. Check the flame sense signal. Sand the sensor if coated or oxidized
5. Check flue piping for blockage, proper length, elbows and termination
6. Verify proper induced draft blower performance
7. Watch the flame on a full trial. Note whether it goes out before or after the inducer changes speed

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)