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How Furnace Flame Sensors Work - Flame Sensor _____________________________________________________________________________________

By Saini Vonih -

In order for a furnace to operate the flame must be proved. This means the furnace needs to know that there is a flame present and burning and if not then it has to shut off the gas supply so a build up of explosive gas does not occur.

What Is Flame Sensor

Most newer furnaces use a technique called flame rectification to determine this. This is accomplished with a simple device called a flame sensor. The sensor is a small stainless steel rod that is insulated with a ceramic insulator. A wire is attached to the sensor on the bottom. The sensor is positioned in front of the burner which is the furthest burner from the burner that has the ignitor. When the furnace turns on the ignitor glows red hot. Then the gas valve opens and allows gas to flow through the burners and ignites when the gas passes over the hot ignitor.

Next the furnace must quickly determine if the flame is lit. A voltage is applied to the wire that is connected to the flame sensor. If a flame is present, some electrons will flow through the sensor and through the flame to the metal burners which are grounded. The flame will conduct the electricity because free electrons are produced in a burning flame. If no electrons are flowing then the furnace knows the flame is not there and it will close the gas valve to protect against a dangerous gas build up.

A very common problem that can make your furnace stop working is a build up of silicon oxide on the flame sensor. Silicone oxide is produced when small traces of common chemicals that are present in the air burn with the gas in the burners. Silicone oxide is almost invisible but it prevents the electrons from leaving the flame sensor and the furnace will think that there is no flame present even when there is a flame. The furnace will shut down leaving you with no heat. To correct this problem first shut off the electric supply to the furnace. Remove the door to the burner area and locate the flame sensor. Next remove the screw holding the sensor in place. Now you can take the sensor out and clean it with a Scotch Brite pad. Replace the sensor and door and torn on the electric supply.

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Flame sensor