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Buy a spare hot surface ignitor for an 11-year-old furnace?

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  • #31
    Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

    Originally posted by blue_can View Post
    The flame sensing in water heaters is different to that in a furnace (or for that matter tankless with a non-standing pilot). The principle of the thermocouple is that the junction of two dissimilar metals when heated will produce a voltage and that voltage is used to keep the gas valve open for the pilot.

    If the thermocouple is shorted to the burner it will not be able to develop a potential difference since the current will flow to the burner (and probably to ground) so yes it will stop working.
    Sorry to confuse you. In a power vent water heater as apposed to gravity vent water heater a flame sensor is used just like in a high efficient furnace. same principle nothing but a stainless steel rod used to conduct electricity. no standing pilot unless there is a call for heat then a hot surface ignitor or an arc sparks to lite the pilot.

    But I'm still curios as to why a shorted flame sensor won't show pilot?

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    • #32
      Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

      Originally posted by Robert Gift View Post
      Does a residential water heater have a flame sensor?
      Thought it had only a standing pilot light and thermocouple, otherwise it would need to have power to operate the flame-sensing circuitry. (I wish it did so that I could install an electric damper to stop all the heat loss up the vent.)

      A circuit couldetect AC and know it is a short and not DC caused by flame conduction.

      Yep they are refered to as power vent water heaters.

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      • #33
        Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

        Originally posted by saysflushable View Post
        Sorry to confuse you. In a power vent water heater as apposed to gravity vent water heater a flame sensor is used just like in a high efficient furnace. same principle nothing but a stainless steel rod used to conduct electricity. no standing pilot unless there is a call for heat then a hot surface ignitor or an arc sparks to lite the pilot.

        But I'm still curios as to why a shorted flame sensor won't show pilot?
        One of two things could happen.

        1. The flame would have been acting as a high impedance and so directly shorting to the burner could have caused the driver circuit to overload and current limit. In that situation if they are using AC the peaks could be clipping and distorting the waveform.

        2. Since there is no flame no rectification would be taking place and so even if there was no overload the AC would be a full sinewave and so since the detection circuit looks for a half wave signal it would be unable to detect a flame.

        My guess is that both 1 and 2 would have applied thereby not sensing a flame.

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        • #34
          Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

          Originally posted by blue_can View Post
          One of two things could happen.

          1. The flame would have been acting as a high impedance and so directly shorting to the burner could have caused the driver circuit to overload and current limit. In that situation if they are using AC the peaks could be clipping and distorting the waveform.

          2. Since there is no flame no rectification would be taking place and so even if there was no overload the AC would be a full sinewave and so since the detection circuit looks for a half wave signal it would be unable to detect a flame.

          My guess is that both 1 and 2 would have applied thereby not sensing a flame.
          Thanks

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          • #35
            Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

            Do not know if anyone is still watching this thread but most units check for flame before trying to start so if you short the flame sensor it might think there is already flame and something is wrong. Not sure need an electrionics expert. Just my .02

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            • #36
              Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

              Originally posted by trykonxlgold75 View Post
              Do not know if anyone is still watching this thread but most units check for flame before trying to start so if you short the flame sensor it might think there is already flame and something is wrong. Not sure need an electrionics expert. Just my .02
              That's possible but in order to determine that you need to watch how the unit behaves. If the unit attempts to light the flame and then determines there is no flame and shuts down then it has not been looking for a flame from the beginning. If it shuts down before even activating the igniter then what you say is possible.

              It all depends on the people who designed the furnace/water heater. The only way to know for sure would be to look at the software/embedded code for the controller on the electronics board. Personally I'm not sure if I were designing the system why I would want to look for a flame at cold boot - if the igniter has not been activated yet, how and why would a flame be present. Makes no sense but it all depends on the logic of the people who designed a particular model.

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              • #37
                Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

                On some equipment, especially larger commercial and industrial burners, it is possible for a flame to still be present if there is a pilot valve failure or main valve failure. It is necessary to insure that this is not the case for safetey. Some residential equipment also has this feature, and I am quite certain that all 90%+ does this.

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                • #38
                  Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

                  Originally posted by shock1964 View Post
                  On some equipment, especially larger commercial and industrial burners, it is possible for a flame to still be present if there is a pilot valve failure or main valve failure. It is necessary to insure that this is not the case for safetey. Some residential equipment also has this feature, and I am quite certain that all 90%+ does this.
                  Maybe I missed something here. If the main flame is present when the unit is turned off why would it be a safety issue if the unit is turned on. In fact you may want to run the vent fan continuously in such a situation to prevent buildup of exhaust gases.

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                  • #39
                    Re: Buy a spare hot surface ignitor for an 11-year-old furnace?

                    Also forgot to add to my previous post but don't all gas fired appliances such as furnaces have a hi limit switch that trips due to overheating which is what would happen if the flame got stuck on. That or a thermal fuse. But in any event I cannot see how a flame can get stuck on let alone why the controller should have to detect such a condition given it should never happen. Actually if it did whoever owned the unit is going to have serious problems.

                    Did I miss anything in my assessment?

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