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Two boilers sharing one flu.

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Shahrdad
Shahrdad Member Posts: 120
in THE MAIN WALL
Over 20 years ago, I had the boiler chimney lined with a flexible metal liner. The boiler vent is 7" and the liner inserted was also 7". The boiler is in the basement, and the distance from the boiler vent to the top of the chimney is just over 45 feet.
If this boiler is replaced with two smaller boilers that have 6" vents with automatic dampers, can they share the 7" lined chimney, or will this liner need to be removed and replaced with a larger one?

Thank you!

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  • JohnNY
    JohnNY Member Posts: 3,230
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    You need to know the manufacturer of the liner and find out how many BTUs can be safely carried through it at the installed height. There are also charts to give you general tolerances but I prefer to hear it from the people who tested it and published their findings.
    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
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  • Shahrdad
    Shahrdad Member Posts: 120
    edited February 2021
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    Youngplumber said:

    Is your liner corrugated? That makes a difference in the load capabilities. @JohnNY is giving good advise. 


    I think about 30 feet of it are corrugated and the rest is smooth pipe. I found one photo that shows both. sections. They fed the corrugated portion down the chimney and the smooth part went on top. I'm not sure who the manufacturer was, as this was done over 20 years ago.

    Do you think this could handle two boilers with 6" vents with maximum 133 MBH input for each boiler? What would the max BTU be? There wouldn't be any more than four or five feet of horizontal vent. The top of the chimney is at least 40 feet above the boilers.





  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,519
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    @Shahrdad

    @JohnNY gave good advise. Go back to the manufacturer of the chimney liner used and ask them. We have no majic wand that's what we would have to do.

    If I had to guess I would say no but I am probably wrong 45' is a lot of height
  • Shahrdad
    Shahrdad Member Posts: 120
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    Thank you! That is very helpful. I'm assuming the BTU number refers to the BTU input of the appliance, not the output, is that correct? Would the efficiency also affect this? It would seem that the less efficient a boiler is, the more heat it has to send up the chimney.
    The two boilers I'm looking at are rated at 133,000 BTU/hr input, 112,000 D.O.E Capacity BTU/Hr. I saw in one chart that for corrugated liner one should deduct 20%. Even with 20% deducted from 379,000, I should still be well below the max number.
  • Shahrdad
    Shahrdad Member Posts: 120
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    Youngplumber said:

    I still think you should find the manufacturer of it, or one exactly like it, and get guidance. This isn't something to take lightly. 

    Yes, I'm looking to see what manufacturer's liner looks just like this one. Glad I have some photos of it.
  • Shahrdad
    Shahrdad Member Posts: 120
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    EBEBRATT-Ed said:

    @Shahrdad

    @JohnNY gave good advise. Go back to the manufacturer of the chimney liner used and ask them. We have no majic wand that's what we would have to do.

    If I had to guess I would say no but I am probably wrong 45' is a lot of height

    I measured the chimneys from the top to the ground back 20+ years ago when the scaffolding was up, and it was right over forty to the ground level, and the boiler hookup is a good five or six feet below that. I didn't measure all the way down to the chimney opening in the basement, but it should be right around 45 if not slightly over.

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