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Buderus G124

Ross_24 Member Posts: 82
A client of mine has visible corrosion of the smokepipe/venting due to sustained flue gas condensation. It's a Buderus G124, 3-zone system (zone 3 being a DHW priority w/ indirect)

After doing the math I've found that when all the baseboards are cooking and the inside temperature is at 70F the coldest the return water will get is 157F. Seems alright there.. BUT

The issue has got to be the fact that gases are vented into an exposed brick, outside chimney. It's currently piped in a branch header style. The only solution I'm seeing is to re-pipe as a primary/secondary to keep the return water as hot as possible.

Does anyone have any suggestions on how to combat this? Or, perhaps its another issue I'm not addressing?


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,541
    Chimney liner

    You need a liner with an exposed masonry chimney.

    Have a Logamatic,or a conventional aquastat? Either way,the boiler return temp will vary wildly,a bypass would be easier than repiping P/S or use something like this

    To learn more about this professional, click here to visit their ad in Find A Contractor.
  • heatpro02920
    heatpro02920 Member Posts: 991
    Combustion test?

    I would start there, piping primary sec is a good idea as is the liner, but make sure you are running rite... What is your stack temp? What does the venting config look like?
  • Ross_24
    Ross_24 Member Posts: 82

    The venting is about an 8 foot run to the outside chimney then approx 22 feet up. For some reason the installer came off the boiler with the usual 5" smoke pipe but then increased to 6" then ran to the chimney. This could be a factor as well. I'll try and take pictures in the next day or so.
  • heatpro02920
    heatpro02920 Member Posts: 991
    Ill bet the installers reason was

    the 6" was there from the old boiler, lol...

    Its a good place to start looking, check the drafts {all of the combustion for that matter, keeping a close eye on stack temps}, normally I find condensation issues with larger boilers, boilers using mixing valves, or emitters large enoug hto create huge deltas, since the g124's are smaller units, I am kind of surprised its condensing enough to create problems...

    If the problem is not the combustion or venting and is indeed an issue that adjustments, outside combustion air, reworking venting, ect can not fix, and you need to close the delta you have a few options, one of course is pri secondary or a bypass pipe, but another is a delta circ on each zone, like a bumble bee, set the delta and forget it...
  • Ross_24
    Ross_24 Member Posts: 82
    good point

    Now that I sit here and picture the system, the 6" section was much older than the 5" lol..

    I was initially concerned because the first floor zone is one massive series of baseboards and cast iron radiators.. it's a total jumbled mess. I ran the first floor series through siggy's HDS2 and it's showing it's within a safe temperature.

    You're right. Combustion test is a must. I also need to take a peak within the chimney. It's a very old home and I'm guessing it has a clay liner... if any at all.