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Igniter as flame rod

metrosilo Member Posts: 30
I was looking and couldn’t find very good info on this. 
What happens electrically when the igniter is used as a flame rectification?

im looking for good schematics or technical data. I realize it switches over but I’m looking for details. 

ie. Honeywell universal ignition module or Weil McLean ultra boilers. Etc. 


  • mattmia2
    mattmia2 Member Posts: 8,893
    I have no idea how the actually do it, but with some ingenuity you could probably run the polarity of the flame rectification sense circuit and the spark igniter circuit in opposite polarities and use a diode to protect the flame rectification sense circuit from the ignition voltage.
  • Tim McElwain
    Tim McElwain Member Posts: 4,597
    Are you talking about a Hot Surface Igniter (silicon carbide or silicon nitride)?
  • mattmia2
    mattmia2 Member Posts: 8,893
    I think they are asking about the flame rod when it is both the electrode for spark ignition and the sensor to prove flame in a flame rectification circuit, what the electronics inside the control look like to make it do both.
    EBEBRATT-Ed Member Posts: 14,560
    They were pretty common
    JUGHNE Member Posts: 10,914
    To me, IMO:

    The HSI heats up glowing red.

    Then the gas valve is opened. As that happens the HSI is no longer energized and becomes merely a conductive flame rod.

    The residual red hot glow is what lights the gas.

    All of this is controlled by the ignition module.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,763
    It's MAGIC.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • 109A_5
    109A_5 Member Posts: 1,029
    edited July 14
    Hello @metrosilo,
    The manufactures probably feel that information is proprietary or a trade secret. If you really want to know buy a failed one (cheap) and reverse engineer it, maybe repair it. Check on-line Patent information too. The resistance of the secondary of the High Voltage transformer is minimal as compared to the equivalent resistance of the flame. Think of the flame as a diode with a large forward voltage drop, like an old style tube rectifier. I suspect it is something like this inside (Block Diagram).

    Additionally a good flame detection circuit should be able to discern between these states;
    1) A valid flame (obviously), sufficient DC current (micro-amps).
    2) No or poor flame (obviously), no or insufficient current.
    3) A undesired or foreign resistance that would be in parallel with the flame, contamination, etc.
    4) Miss-wired in any way.

    Testing a board with an independent flame sense rod a 1 meg Ohm resistor in series with a common 1N4001 diode connected in the correct direction made the board happy. This may not work with one common flame ignition / sense rod, since the high voltage may destroy the diode.

    So maybe a better stated example.

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System