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Urgent Help: Flame Detector Troubleshooting on MMO872 Controller

King_Cas
King_Cas Member Posts: 4
in Strictly Steam

Hello everyone,

I’m currently troubleshooting a 2-ton fire tube boiler equipped with an MMO872 controller. The issue is that the boiler fails to continue running, and we strongly suspect the flame detector is at fault. Unfortunately, replacing the flame detector isn’t an option right now as we don’t have a spare available.

We’ve already performed all standard troubleshooting methods on the flame detector (e.g., cleaning, wiring checks, continuity tests, and verifying the signal output), but nothing has resolved the issue.

To confirm whether the flame detector is indeed the culprit, is there a safe way to temporarily bypass it for diagnostic purposes without compromising safety? I’ve attached the schematic diagram of the MMO872 controller where the flame detector is connected for reference.

I fully understand the risks associated with bypassing safety devices and will ensure all precautions are taken to prevent unsafe conditions. However, I want to confirm the root cause to plan our next steps effectively.

Your guidance or alternative suggestions would be greatly appreciated!

Thank you!

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Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,170

    Sorry, I have no idea what that is. there is no information on the internet when searching "MMO 872" so I can't even get any background on that device.

    Who makes it and what does it do? Where does it mount on the burner. Is the burner using Gas or Oil. What is the firing rate? What is the brand name and model number of the burner? Brand name and model number of the boiler? And why is MZ 770 highlighted in yellow on the diagram?

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    King_Cas
  • King_Cas
    King_Cas Member Posts: 4
    https://forum.heatinghelp.com/discussion/comment/1838403#Comment_1838403

    Thank you for your response! I appreciate you taking the time to look into this.

    The MMO872 is a flame safeguard controller specifically used for fire tube boilers. It’s designed to monitor the flame detector and ensure safe burner operation. Unfortunately, it seems to be an older or less common model, which might explain why there’s limited information available online.

    Here’s some additional information:

    • Boiler Type: 2-ton fire tube boiler.
    • Burner Type: The burner is running on oil(LFO)
    • MZ 770 Highlight: The MZ 770 is highlighted on the schematic because it’s the terminal where the flame detector signal connects to the controller.

    To provide more context, I’ve attached the manual for the MMO872 controller, which might help clarify its functions and connections. Let me know if you need any other details, and thank you again for your assistance!

    Satronic MMO 872 Manual.pdf
  • 109A_5
    109A_5 Member Posts: 2,162
    edited January 28

    Decades ago I use to repair the flame detector amplifier boards for the large boiler maintenance folks at work. For the most part just the Electrolytic Capacitors would go bad. 15 minute job if we had the Capacitors. That made them very happy. I encircled the Electrolytic Capacitors in Red below (C1, C3, C6). In this case I probably be suspicious of C4 and C5 too. I'm guessing the schematic is in German or similar. Not sure if this helps you much, bringing back some memories for me.

    It may be just poor solder joints at the relays that need repairing. Or the relay contacts. The flame detector amplifier boards I repaired had no relays.

    There is probably no easy way to bypass the functionality of the board unless you can mimic the sequence and timing of the controlled events. Especially since you probably don't know which section of the board is the actual issue.

    image.png

    image.png
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • retiredguy
    retiredguy Member Posts: 1,002
    edited January 28

    I suspect that this boiler is not installed in the USA so we have no way to tell the size of this boiler. I have no idea the size of a boiler that is rated in tons. A firing rate in BTU's, GPH, fuel type etc would be helpful. Now, my one recommendation is to never repair a flame safeguard controller regardless of your knowledge of that control. I have seen too many accidents being caused by a "self repaired" control. It was always a much safer option to replace the controller with a factory rebuilt or brand new device. I say this from past experience. You may be the best at repairing this control but the price may be much too high.

    Now, as to what the problem is with the controls of your boiler and it's firing, a good service company is well worth the price. When I was still working, I had the replacement controls and the expertise to trouble shoot almost any firing system here in the states. So, before you make a wrong choice, call someone that has the expertise, PLEASE. Once it goes "BOOM" it is then too late too call someone. MY 2 cents.

    EdTheHeaterMan
  • 109A_5
    109A_5 Member Posts: 2,162

    OK I may have misread your original post, so I'll add this.

    If the MZ 770 is what you want to temporally substitute, I suspect it is just a Light Dependent Resistor (LDR) or cadmium sulfide (CdS) photocell, a two terminal device.

    Typically they have a very high resistance when dark and a much lower resistance when illuminated. Possibly verified with an Ohmmeter. A 1 Meg Ohm Potentiometer with a series resistor to set a minimum resistance may work or a resistor substitute decade box as a temporary substitute for the MZ 770.

    Be careful there may be mains voltage to ground in that circuit.

    image.png
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • GGross
    GGross Member Posts: 1,414

    Bypassing the flame sensor won't tell you if its bad, it will just tell you that the boiler wasn't detecting flame. Have you started with the basics of checking gas pressure? static and dynamic, incoming and outgoing from the gas valve if available and comparing that to the specified required values, and then confirming that your pressure drop is acceptable per the manufacturer?

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