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chimney sizing for staged boilers sharing same

grumbug Member Posts: 7
I need some hard copy on sizing flue & thinble sizing
for multiple boilers sharing common the same



  • What Category are these

    boilers? What Make and Model number are the boilers? We need to start there and then we can go on. I also need some information on the BTU input of each, chimney or vent height, lateral run (how far from chimney or vent), what is the flue size on the boilers?

    What are you using the thimble for? Are you passing through combustible material?
  • jeff_51
    jeff_51 Member Posts: 545
    dig out your UMC

    Around here in Minnesnowta chimney size is based on total BTUH and of course horizontal run can only be a certain percentage of the overall vertical height. All of that should be in you UMC. We of course have a couple of addendums because {we} in Minnesnowta are smarter than everybody else and have to do it differant
  • grumbug
    grumbug Member Posts: 7
    chimney sizing for staged boilers sharing same

    Thanks, For the reply

    Here,s the facts

    2 buderus G115-28 Riello F40-F3 burners 112,000.

    Chimney Height 26' 8x12 flue 8" thimble, The project
    Manager thinks the 8" thimble is inadaquate wants 10"
    So thats why I need printed data to resolve this

    Also am planning to use a true 8" crotch to tie them


  • is that chimney....

    inside the house/build. or on the ouside/exposed side of the building. reason being the flue gasses on a buderus can be so "cold" that it will have a difficult time going up a large exterior flue. kpc
  • John Mills_5
    John Mills_5 Member Posts: 950

    Get a copy of the National Fuel Gas Code, it will tell you everything you need to know about flue sizing. Many mfrs reprint it in the installation instructions too.
  • Michal
    Michal Member Posts: 213
    careful how you twin them in

    to start Like mentioned before use the code books, second be aware of your elbows, turns, stack temperatures (because buderuses fue gasses can get low, they can condense in the flue, corrion will occor) and finally your height. You can go onto the selkirk-metalbestos website and get ideas and calculations. Also I would not twin the boilers together with pants ( I am assuming crotch as you have called it) You should run a horizontal header and twin them into the header individually. 8 inch might be fine if you have the correct height
  • Bob Harper
    Bob Harper Member Posts: 1,030
    NFPA 31 limitations

    No sizing charts in '31 for dual appliances common vented the way '54 does in detail. One solution would be to install a priority switch on one appliance then common vent them into one stainless liner sized for the largest single appliance. At under 1 gph and 26 ft. you're in the 5-6" liner range. I always get the boiler techies to rig the priority switches so I don't have all the poop on parts and techniques but it does work well when you have multiple appliances sharing an inadequate chimney.
    Having said all that, getting some good vent rise off the top, nice smooth runs with minimal elbows helps. Manifolding is an art with some science as I'm sure Jim Davis would tell you. I'd like to hear Jim's take on this.

    Always eager to learn, too.
  • Steve M_2
    Steve M_2 Member Posts: 121

    Here's a link that will tell you everything you need to know. Check with your local juridiction as well.

    Steve M.

This discussion has been closed.