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guard dog low water cut off question

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mhooker32
mhooker32 Member Posts: 1
in Thermostats and Controls

guard dog lwco rb-122-e. no heat, guard dog relay chatters rapidly. no lower water condition. had to by pass to get heat. if i disconnect the wire from the board wing nutted to the probe itself, buzzing stops. with the wire wing nutted back on the probe, the chatter returns. if i jump from the wingnut to the pipe with a clip lead, still chatters. leads me to believe its a board issue. possibly bad solder joint on board. is there a spec to test the actual probe? is it on/off ? or a variable resistance ? it bugs me to replace units that can be repaired . i know mant will just call me cheap. ive been repairing board my whole life. obviously, sometimes the juice isnt worth the squeeze, but i think here it might and will also same me having to drain and bleed the system. thanks

Comments

  • Big Ed_4
    Big Ed_4 Member Posts: 3,471

    It could be bad , did you pull the unit and clean the probe ?

    There was an error rendering this rich post.

  • 109A_5
    109A_5 Member Posts: 3,798

    Since it is buzzing and with your tests, I would suspect an Electrolytic Capacitor has failed. For the size of the Electrolytic Capacitor(s) in that unit they are very inexpensive. They can be replaced if you or a friend is competent to do so. Probably not worth the cost to have a professional repair facility do the work.

    Since it is a safety device the HVAC industry and most likely the manufacture of the boiler and the LWCO probably recommends replacement of the whole unit if it is defective.

    image.png

    Instruction Manual

    https://pexuniverse.com/uploads/literature/mcm-rb122e-inst.pdf

    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • 109A_5
    109A_5 Member Posts: 3,798

    Here is a (bit crusty) example. The Electrolytic Capacitor(s) (that I can see and your board may be different) have the Yellow circles around them, the values are printed on the sides and they are polarized, they only go in one way correctly, however they can be easily installed backwards (incorrectly) if you do not heed the polarity. Soldering tools and skills needed.

    image.png
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • 109A_5
    109A_5 Member Posts: 3,798
    edited 6:22AM

    They state 20,000 Ohms probe sensitivity. I suspect if the resistance of the water at the probe to the piping is higher than 20,000 Ohms the LWCO unit thinks there is no water. With some LWCO units if the probe is shorted that will cause an error also. I would think 100 Ohms to 20,000 Ohms would be a reasonable detection range.

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

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