Flame Sensing and Rectification

A flame sensor is a single metal rod placed in the burner flame. The control board applies alternating current to the rod and the flame, being conductive and asymmetric, allows current mostly in one direction. The resulting small direct current tells the board flame is present.

Flame sensing is the safety that prevents raw gas from flowing without combustion. It is also one of the most common causes of intermittent no heat calls, because a thin oxide layer on the rod slowly reduces the signal until the board drops out after a few seconds of burner operation.

1. 1Turn off power and gas, then place the meter in series with the sensor wire using the microamp range.
2. 2Restore power and gas and start a heat call.
3. 3Read the signal once the burners are fully lit and stable.
4. 4Compare to the manufacturer minimum. A signal near the minimum will fail as the rod ages.
5. 5If low, clean the rod gently with a non abrasive pad, never sandpaper, and recheck.
6. 6If cleaning does not restore the signal, check the ground path, burner condition, gas pressure and flame quality.

• Meter with a microamp direct current range
• Alligator jumper leads
• Non abrasive cleaning pad
• Combustion analyzer to confirm flame quality