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  • Flame Sensors

    What causes flame sensors to fail? I know you can keep sanding them and they will continue to work for a little while, but you will end up replacing them. I have replaced a flame sensor twice on a Tappan furnace in the past 3 years, why do they keep going bad? What can I do to make them last longer?

  • #2
    Re: Flame Sensors

    Dirty air, dirty natural gas, chemicals from laundry detergents. I clean at least a dozen a year with open mesh sanding cloth. Ive started wiping them down with a towel after sanding and that has seemed to help a little.

    Andy

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    • #3
      Re: Flame Sensors

      honestly i always use sandcloth also but they say your supposed to use steel wool because the scratches absorb dirt and cause an insulation film on the sensor

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      • #4
        Re: Flame Sensors

        Might help to sand where the FS mounts. I was told that is where the FS grounds and it must have a good ground to get a good reading. Anyone know if you can test a flame sensor? Maybe like testing a thermocouple?

        Comment


        • #5
          Re: Flame Sensors

          I to have mixed results cleaning flame rods....

          I to have found steel wool cleans them nicely.

          Some oil systems if I remember right use the flame rod as a ground loop ( the flame contacting the rod completes a circuit to ground)

          (from Fieldpiece website)

          Here’s the way it a typical flame rod works works....

          The controller applies alternating voltage between the flame sensing rod and the base of the flame (ground). The ions in the flame provide a high resistance current path between the two. Because the surface of the base flame is larger than the sensing flame rod, more electrons flow in one direction than the other. This results in a very small DC offset current. If there is a flame present, the DC offset is detected by the controller, which tells the gas valve to remain open. If there is no current flow, the controller will close the gas valve and the system will purge itself of any remnant gas before trying to re-ignite or lock-out. The DC offset is small, only in micro amps.

          It’s very important that the flame sensing rod works properly. Dirt, corrosion, or bad connections in the flame sensing circuit can cause the controller to think the flame didn’t ignite. The gas valve will be shut down prematurely. Controller manufacturers publish specifications for the flame diode DC offset current.

          You can measure the current in the flame sensing rod by putting an instrument in series with the flame sensing rod. The instrument must be capable of measuring 1 to 10 micro amps DC and have a resolution of .1 micro amps.


          Okie


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          • #6
            Re: Flame Sensors

            i have been in hvac for 18 years and never replaced a flame sensor. unless the wire is bad. keep them clean and they will work. better check control.

            Comment


            • #7
              Re: Flame Sensors

              you can ohm out flame rods. use meter, one lead on end of rod the other on end of wire. should read 0 resistance run lead down side of rod if you get resistance on different spots clean it. flame rods create milla amps.

              Comment


              • #8
                Re: Flame Sensors

                One of the most common no heat or intermittent lock out problems that I find in Minnesota is loss of flame sensing. When natural gas burns with the oxygen obtained from the air trace amounts of contaminates are also burned. These contaminates are from many sources including the gas supply, laundry detergent trace vapors, cat litter vapors, bleach vapors and others. The trace quantities of the contaminates can be very small but the important thing is that the products of combustion include silicon oxide. Now you have to understand the process of flame rectification. The ignition control circuit board applies an AC voltage to the flame sensor which is just a stainless steel rod. A interesting thing occurs inside a burning flame called ionization. That is free electrons are produced which can conduct electricity through the flame itself. The electrons will normally flow from the flame sensor, through the flame when present, and back to ground through the grounded burners. The ignition system must prove that a flame is present to continue the gas flow or if no flame shut off the gas flow through the gas valve to prevent a possible explosion. It also must not be fooled into thinking there is a flame present by a flame sensor that is touching the ground from being broken or bent. The way it does this is by a diode effect where the sensor surface area is less than 10% of the ground surface area. This produces a half wave of electrical current out of each full wave. The ignition control circuit detects the half wave to determine if the sensor is merely touching ground. Now back to the original question, the silicon oxide that is produced by the contaminates burned in the gas will coat the flame sensor. The electrons cannot penetrate this coating which acts like an insulator. The ignition control then thinks that there is no flame present and locks out. You should clean the flame sensor of this almost invisible coating using steel wool or a Scotchbrite green pad. Never use sandpaper as it scores the metal which makes the oxide build up faster and also contains silicon in the sand particles.

                Comment


                • #9
                  Re: Flame Sensors

                  Originally posted by HeatingExpert View Post
                  One of the most common no heat or intermittent lock out problems that I find in Minnesota is loss of flame sensing. When natural gas burns with the oxygen obtained from the air trace amounts of contaminates are also burned. These contaminates are from many sources including the gas supply, laundry detergent trace vapors, cat litter vapors, bleach vapors and others. The trace quantities of the contaminates can be very small but the important thing is that the products of combustion include silicon oxide. Now you have to understand the process of flame rectification. The ignition control circuit board applies an AC voltage to the flame sensor which is just a stainless steel rod. A interesting thing occurs inside a burning flame called ionization. That is free electrons are produced which can conduct electricity through the flame itself. The electrons will normally flow from the flame sensor, through the flame when present, and back to ground through the grounded burners. The ignition system must prove that a flame is present to continue the gas flow or if no flame shut off the gas flow through the gas valve to prevent a possible explosion. It also must not be fooled into thinking there is a flame present by a flame sensor that is touching the ground from being broken or bent. The way it does this is by a diode effect where the sensor surface area is less than 10% of the ground surface area. This produces a half wave of electrical current out of each full wave. The ignition control circuit detects the half wave to determine if the sensor is merely touching ground. Now back to the original question, the silicon oxide that is produced by the contaminates burned in the gas will coat the flame sensor. The electrons cannot penetrate this coating which acts like an insulator. The ignition control then thinks that there is no flame present and locks out. You should clean the flame sensor of this almost invisible coating using steel wool or a Scotchbrite green pad. Never use sandpaper as it scores the metal which makes the oxide build up faster and also contains silicon in the sand particles.
                  Excellent post and very informative Thanks!

                  Comment


                  • #10
                    Re: Flame Sensors

                    Originally posted by servicefitter View Post
                    i have been in hvac for 18 years and never replaced a flame sensor. unless the wire is bad. keep them clean and they will work. better check control.
                    i have seen on more than one occasion that the flame rod would no longer carry the required signal to the burner control even after cleaning... after replacing the proper signal was there. and i have also found them with cracks in the porclyn causing them to ground out

                    Comment


                    • #11
                      Re: Flame Sensors

                      I take my statement back, I have found cracked porclyn. but i can not remember the last time i found the flame rod to be the root of a problem.

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                      • #12
                        Re: Flame Sensors

                        You blow a lot of smoke. I know you have to selling goodman. flame rod has no power to it. When heated it creates milla amps 4-15 dc. Which keeps a micro current relay closed and allows power to the gas valve. I bet you are a scab company but charge union scale. local 553 pipefitters .

                        Comment


                        • #13
                          Re: Flame Sensors

                          No need to take the low road servicefitter. You have your OPINION and others have theirs.

                          I also use the green scotch brite pad and found it works pretty well at cleaning them. I can see where sandpaper would be a bad move.
                          Anyone can tear a man down, few can build one up.

                          Comment


                          • #14
                            Re: Flame Sensors

                            Originally posted by Dr. House View Post
                            i have seen on more than one occasion that the flame rod would no longer carry the required signal to the burner control even after cleaning... after replacing the proper signal was there. and i have also found them with cracks in the porclyn causing them to ground out


                            ??? You sure you are not thinking of Ignition electrodes on an oil fired furnace???

                            Comment


                            • #15
                              Re: Flame Sensors

                              Originally posted by HeatingExpert View Post
                              One of the most common no heat or intermittent lock out problems that I find in Minnesota is loss of flame sensing. When natural gas burns with the oxygen obtained from the air trace amounts of contaminates are also burned. These contaminates are from many sources including the gas supply, laundry detergent trace vapors, cat litter vapors, bleach vapors and others. The trace quantities of the contaminates can be very small but the important thing is that the products of combustion include silicon oxide. Now you have to understand the process of flame rectification. The ignition control circuit board applies an AC voltage to the flame sensor which is just a stainless steel rod. A interesting thing occurs inside a burning flame called ionization. That is free electrons are produced which can conduct electricity through the flame itself. The electrons will normally flow from the flame sensor, through the flame when present, and back to ground through the grounded burners. The ignition system must prove that a flame is present to continue the gas flow or if no flame shut off the gas flow through the gas valve to prevent a possible explosion. It also must not be fooled into thinking there is a flame present by a flame sensor that is touching the ground from being broken or bent. The way it does this is by a diode effect where the sensor surface area is less than 10% of the ground surface area. This produces a half wave of electrical current out of each full wave. The ignition control circuit detects the half wave to determine if the sensor is merely touching ground. Now back to the original question, the silicon oxide that is produced by the contaminates burned in the gas will coat the flame sensor. The electrons cannot penetrate this coating which acts like an insulator. The ignition control then thinks that there is no flame present and locks out. You should clean the flame sensor of this almost invisible coating using steel wool or a Scotchbrite green pad. Never use sandpaper as it scores the metal which makes the oxide build up faster and also contains silicon in the sand particles.
                              Super post thanks. Man have I cleaned a lot with sand paper. I'll put some steel wool on the van. Thanks again.

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