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How do I hookup McDonnell & Miller Guard Dog RB-122 to Weil McLain CGI-5 Boiler

HP_JonathanHP_Jonathan Member Posts: 4

I have a Weil McLain CGI5 boiler. Previously I had a McDonnell & Miller Guard Dog RB-24 Low water cutoff (LWCO) hooked up to Weil McLain CGI-5 boiler. Since they are not sold anymore I bought a McDonnell & Miller Guard Dog RB-122. The wiring is a bit different. When I connect the 2 yellow controls from the RB-122 to the Yellow wiring (t-stat controls) on the WM control the T-stat is always on (calling for heat). I tried a different connection to the WM but ended up blowing the control module (fortunately it only cost me $70 from an Ebay seller to replace it instead of $150 & up from the local dealer). So to get my boiler to be controlled by my thermostats and not the RB-122 I have disconnected one of the RB-122 yellow cables. How should it be hooked up properly?

Thank you,



  • heatpro02920heatpro02920 Member Posts: 991
    edited March 2013
    not a 24v control

    That is a 120v lwco, so you will have to wire it different, don't hook your t-stat to it at all, instead wire it to your service switch, high voltage..

    Take the black wire from your incoming power after the boiler service switch and disconnect it, hook it to the black wire and one yellow wire, then hook the white to the white wire, and lastly hook the remaining yellow wire to the black wire that you disconnected from the service switch power... So the yellow wires just break that incoming power.... You can also just hook the yellows to the same test at wire so it breaks it and not feed the yellows 120 I believe..

    I dont like using high voltage lwcos on gas boilers... Should have just bought a cheap 24v lwco...
  • HP_JonathanHP_Jonathan Member Posts: 4
    That instruction is helpful, but a still a bit confusing

    The boiler power goes into the WM control 120 volt power in. I split that and have hooked up the 120 volt wires from the 120 volt service switch to the Guard Dog and then to the WM. Here is the diagram:

    White (Service Switch) + Red (Guard Dog) -> WM 120 volt in

    Black (Service Switch) + Black(Guard Dog) -> WM 120 volt in

    Green -> WM ground

    Yellow 1 (Guard Dog) & Yellow 2 (Guard Dog)?

    The LWCO has power, but how do I interrupt power to the WM when the LWCO is in Test or detects a fault?
  • HP_JonathanHP_Jonathan Member Posts: 4
    Directions from Weil McLain

    When installing a probe-style LWCO on Weil-McLain residential gas boilers, wiring can be

    simplified if the120-volt version is used (McDonnell & Miller Model RB-122). Wiring the RB-122 into the service switch box after the service switch is quick and easy. The white wire of the RB-122 connects to the neutral wire (L2), and the black wire of the RB-122 connects to the hot lead (L1) of the service switch. The two yellow wires from the RB-122 are then connected in series with the limit switch to break the 24-volt limit circuit.

    What is meant by "The two yellow wires from the RB-122 are then connected in series with the limit switch to break the 24-volt limit circuit."? How are the 2 yellow wires hooked up?
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