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Low Water Cut-off Sensor RB-122-E Probe Replacement

ZBoilerZBoiler Member Posts: 11
Recently, due to voltage surge caused by snapped neutral power wire, I lost all of my oil boiler controls. I had to discover it painfully one by one as each of them in the control chain was preceded by the other one.

The first one that was discovered as failed was McDonnell RB-122-E. As I was replacing its electrical part, using picture and the terminals diagram, I didn't want, wisely I think, to mess with the probe assembly. I didn't have a proper tool and material to seal its pipe thread connection. The old probe that is still in use is definitely older than 10 years. A technician who installed the last failed control, 3250 Plus, didn't want to deal with the probe either as, according to him, the probe cavity area can't be isolated, piping wise, and water from the system has to be drained in order to replace the probe. There is only one valve above the probe position but not below it where the pipe goes directly into the boiler. The technician somehow discounted importance of the probe replacement as he said it is only useful in case of a major/catastrophic leak in the system, which, according to him again, is extremely rare.

Should I be concerned about replacing the probe as it is older than 10 year old? Perhaps, 15+. The sensor manufacturer is recommending its replacement every 10 year.

Thank you.


  • EdTheHeaterManEdTheHeaterMan Member Posts: 566
    edited December 2019
    That is the problem with lowest bid installation. By not paying for a few extra valves, every service call that requires replacing a part that has water pressure on the other side of it becomes a major project.
    I have replaced relief valves, boiler drain valves, aquastat wells and even a LWCO probe without completely draining the system. If you can have the replacement probe ready, and are able to break the old probe loose (without breaking the probe itself) you can try this: 1. Let the water temperature drop to below 100°(so you don't scald yourself) 2. Let the boiler pressure drop to 0 PSI. (This way if you open the system air will need to go gurgle into the closed system before water can gurgle out) 3. Loosen the probe with a wrench until you can turn it by hand 4. Have the replacement probe ready with pipe sealant applied. 5. Remove the old probe and quickly block the opening with the new probe as fast as you can reasonable do it. 6. Once the threads are engaged and hand tightened you can add water pressure slowly while you use a wrench to secure the probe to a point where the seal won't leak.
    Most LWCO manufacturers recommend using a pipe sealant other than teflon tape. This is because the electrical connection from the probe to the pipe is part of the circuit that senses that the water is present. The Teflon will act as an insulator.
  • ZBoilerZBoiler Member Posts: 11
    Thank you for the advise. Based on your description, I would rather wait until the season is over and, then, perhaps, ask a technician to add valve below the probe. Please see attached for my piping arrangement.

    1) Would closing all valves on the return reduce water flow from hole where the probe is installed?

    2) Is pressure in the boiler piping generated only by heat or also by a house water pressure? Another words, if the boiler is cold is there any pressure in the system except one caused by weight of water column above the boiler?
  • Robert O'BrienRobert O'Brien Member Posts: 3,190
    Why do you need a separate LWCO if you have a 3250 with integral LWCO
    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • ZBoilerZBoiler Member Posts: 11
    My 3250 doesn't have LWCO integrated. The probe is part of McDonnell & Miller RB-122-E and is wired through that.
  • ZBoilerZBoiler Member Posts: 11
    Would you mind to cover my two earlierquestions?
    Thank you.
  • HVACNUTHVACNUT Member Posts: 3,349
    edited December 2019
    Turn off the power.
    Close the ball valve above the LWCO. Close all return isolation valves. Close all air eliminators.
    Close the boiler feed valve.
    Get two 5 gal buckets and a 6 ft. washer hose. You'll need to see the flow.
    Connect to the drain on the bottom right of the boiler.
    Open and drain into the bucket.
    There will be pressure at first so be careful.
    Drain the boiler until it slows to a stop. Maybe have a second bucket handy. Once you see flow stop, quickly close the drain valve while the bucket end of the hose is below the buckets waterline so air doesn't siphon back into the boiler.
    Replace the sensor. No tephlon tape.
    You want to prep the new sensor threads with compound, then like Indiana Jones swapping the sack o sand for the jewels, do it quick as to not break the vacuum.
    Tighten and connect the LWCO to the sensor.
    Open the boiler feed.
    Open all air eliminators.
    Check for a leak around the sensor.
    Open supply.
    Open return(s).
    Check boiler pressure.
    Start, cycle, check all zones.
    Recheck system pressure at limit.
    If your fast, you shouldn't need to purge any zones.
  • ZBoilerZBoiler Member Posts: 11
    Thank you. Will keep for my records, and see if my technician follows all the steps in the spring. :-) I don't want to learn the trade using my own boiler.
  • HVACNUTHVACNUT Member Posts: 3,349
    > @ZBoiler said:
    > Thank you. Will keep for my records, and see if my technician follows all the steps in the spring. :-) I don't want to learn the trade using my own boiler.

    That's how I do it. It's not a code that must be adhered to.
    Some might want to purge.
    As long as the heat works, pressure maintains, and the burner circuit opens on limit, then it's all good.
  • ZBoilerZBoiler Member Posts: 11
    So, is there any pressure in a system when domestic boiler is cold?
    Most likely there should be remaining pressure down from circulators if there is a check valve before them. But what about in the piping in immediate area of the boiler, when it is cold? Does an expansion tank retain pressure in the loop when boiler is cold?
  • HVACNUTHVACNUT Member Posts: 3,349
    The PRV should be factory set to 12 psi. From a cold start at 12 psi and limit at say 180°, the pressure shouldn't be higher than approx 20 psi at limit. Give or take a couple.
  • EdTheHeaterManEdTheHeaterMan Member Posts: 566
    You want to prep the new sensor threads with compound, then like Indiana Jones swapping the sack o sand for the jewels, do it quick as to not break the vacuum.
    I like that description! @HVACNUT
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