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Buderus GB142 "drain plug" leaks. (what is it used for?)

Paul Wolf
Paul Wolf Member Posts: 36
Please see attached pic. My GB142 is leaking (only when cool) from what the parts listing calls the "drain plug" Part #80 on the diagram. I have been through the service manual many times, and can find no procedure at all that mentions this "drain plug". In the hopes of just cleaning it, I released the water pressure on the boiler, pulled the retainer clip (part #82), and tried (not very hard for fear of damaging something) to pull the drain plug out. It wouldn't come out. The diagram indicates it's square, so I didn't try and twist it. It appears to be loose in its socket. IE I can wiggle it ever so slightly so I don't think it's "crusted in solid", though there is mineral buildup around it as you can see, which I believe is the source of the leak

I'd like to take it out and clean it, and hopefully clean the orifice it seats into to resolve my leak. This is the first time I've seen this part leak in the 12 years I've had the boiler.

I've had to replace the pressure sensor O-Ring about every 3 years for leaking.

Does anyone have any ideas on getting this drain plug out safely ?

Thanks in advance.



    HVACNUT Member Posts: 5,025
    edited April 2019
    I would order a new plug and clip.
    Pry the existing out with a flat head screwdriver.
    My manual shows it as a cap not a plug.
  • Paul Wolf
    Paul Wolf Member Posts: 36
    Hello HVNut, Thanks for the reply. I am curious, I wonder if I am somehow missing a cap. Does part #80 in your manual look like mine above ? Does it list it as a "cap" 7099364? I can't imagine mine is missing, but it really does look like a plug.

    Do you know if it supposed to just pry out? I tried yesterday (not very hard admittedly). I've never had to take this out before. I'm not even sure why they put it in. I've drained the boiler a couple of other ways without removing this.

    Here is another pic of mine if it helps.

    Thanks again.

    HVACNUT Member Posts: 5,025
    Same part number.
    It's there to drain the water side if the boiler only when you close the valves on the LLH.
    I've never used it.
    Just order a new one.
  • Paul Wolf
    Paul Wolf Member Posts: 36
    UPDATE: I pulled it out today to have a look-see. The documentation is not accurate. There is an O-ring on the plug not shown in the diagrams. The o-ring was deformed and etched. The plug had mineral buildup on it. Mostly on the exposed/outside of it. I was able to clean it up with a soft wire brush. I also ran the wire brush lightly into and around the "socket" and flushed it out. Regretfully, I didn't take a picture of it to post. I forgot.

    Like the o-ring on the pressure sensor, it's difficult to determine the size because it's already deformed. Several years ago when the initial O-ring on the pressure sensor went bad, I tried to find a part number. Unsuccessful. I called Buderus and they told me to "just go to a hardware store or supply house and match it up" They couldn't give me the dimensions. They didn't have them. I was able to get the pressure sensor O-ring successfully replaced with a #8 O-ring. As mentioned above, this item leaks about every 3 years and the O-ring needs to be replaced.

    This drainplug also had an o-ring on it. Not shown in the diagram. A different size than the pressure sensor. I replaced it with a #7 O-ring. I should know in a day or two if cleaning the plug and replacing the O-ring was successful or not. If folks are interested, I can try and find the actual dimensions of these, but again, I am not sure these are the BEST O-rings for the purpose. I will suggest that the original O-rings are not only metric, but also heat resistant.

    I don't know if the drainplug ($51) or the pressure sensor ($105) comes with the O-rings or not, but it seems like a lot of money just to get the proper O-rings, when the parts themselves are functioning just fine.

    I am going to try and call Buderus tomorrow and see if they have any updated documentation, or if they NOW have part numbers/dimensions for the O rings.

    Thanks for the help and I hope this helps someone.

  • Paul Wolf
    Paul Wolf Member Posts: 36

    Official response from Buderus/Bosch:

    "We do not have a size for these O-rings not do we have a separate part number for them. You will have to match them up or buy the replacement part that comes with the O-rings."

    The O-rings I used were round cross section:
    Drainplug: 1/2 OD x 3/8 ID x 1/16 CS
    Pressure sensor: 9/16 OD x 3/8 ID x 3/32 CS.

    No guarantees. These are just the ones I have used and seem to work At least for a few years.

    In my search recently, I found some "square" cross section O rings. The original ones I have pulled out of the boiler "appeared" to be square, but again, it's hard to tell whether they are truly square, or have adopted that shape from being installed for so long and under extreme temps.

    According to Tech Support above, the new parts come with the O-rings. I'd be very curious is anyone has ready access to these two parts, if they could confirm if the O rings are round or square. Better still, if they could measure them and post the actual dimensions of OEM O rings, that would be great.


    HVACNUT Member Posts: 5,025
    I dont see the issue. Order a new drain plug and the 5 pack of retainer clips.