What is Flame Rectification?

By: Peter Kirtschej, Service Coordinator, ABR Wholesalers, Inc.

January 6, 2014

Flame rectification occurs when an AC (Alternating current) gets converted to a DC (Direct current) by using the conductivity of the flame as the path.

Rheem furnaces apply voltage - usually between 80 and 95 volts - to the flame sensor whenever there is power to the control board.  When flame is applied across the flame sensor, electrons flow from the sensor to ground through the flame.  The burner assembly is a ground as it is connected to the furnace chasis.  Because of the size difference of the flame sensor compared to the burner face, electrons are more likely to flow from the small flame sensing rod to the large grounded burner, than flow from the ground to the rod.  Therefore, more electrons flow from rod to ground than ground to rod.  The control board reads this very weak signal as a DC micro amp current thus proving flame and keeping the gas valve on.  The amber colored light on the control board lights up solid when there is 1.5 or greater micro amp DC voltage.  If the flame signal is weak, between 0.5 and 1.5 micro amps DC voltage, the amber light has a slow blink.  If the flame signal is under 0.5 micro amp DC voltages, the amber light is off.  When this happens, the board does not see flame presence, and the control board closes the gas valve. During the ignition sequence, the control board only has 8 seconds to determine if there is flame presence; if the flame signal is weak or non-existent, the furnace shuts down.  The furnace will make 4 ignition attempts before locking the furnace out for an hour.

When checking for flame sense:

  1. Set your multi meter to (uA).
  2. Pull the spade terminal from the flame sensor and connet one of the leads from your multi-meter to that spade and the other to the flame sensor.
  3. Find the furnace and when the flame is present you then will know exactly what the signals are.

It is rare to see a flame sensor fail; they are usually just dirty and need to be cleaned.  The impurities in the air and/or gas tend to create a film on the rod causing low or no flame sense, so the flame sensors need to be cleaned annually.  Home owners storing chemicals close to the furnace such as paint, fabric softener, laundry detergent, and air fresheners can cause premature sensor failures.  When cleaning a flame sensor, I recommend using steel wool and wiping the sensor with a clean rag or towel after cleaning it. If a technician cleans a sensor and the board still does not see flame, the next step would be to check all the ground and neutral connections in the furnace and the panel box.  Remember, without a good ground connection, the flame sensor is more likely to fail.  For more information on how to check ground and neutral connections, stay tuned for next month's post.

For any questions, or to purchase the UEI multi-meter referenced in this article (DL379), please contact your local ABR branch - or visit us on our website:  www.abrwholesalers.com