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How to seal exhaust flue pipe?

cluelessandlost
cluelessandlost Member Posts: 13
in Chimneys & Flues

The cement around my flue pipe connection is cracking, and my city inspection won't pass the permit on my new boiler installation until I get it sealed airtight.

flue.png

I've been researching how to seal the flue pipe but getting mixed answers. I originally bought some furnace cement and some trowels, and planned to scrape off the old stuff and apply the cement to it. But then someone was saying furnace cement for flue pipes tend to crack because it contracts/expands a lot during regular operations. And looking further, all the sites that suggest using furnace cement have been for wood stoves, not steam boilers. Then I saw someone on YouTube use fire block sealant with a caulking gun for a flue pipe, though it was for a water heater.

Which is the best option? Any important things I need to take note of during the process is appreciated as well.

mattmia2

Comments

  • Grallert
    Grallert Member Posts: 935

    Unless things have changes furnace cement is all you need. Chip off the old and neatly apply the fresh stuff. It's not there to keep flue gasses from escaping it's there to assist with keeping a measurable, consistent and controlled draft at the flue.

    Miss Hall's School service mechanic, greenhouse manager, teacher, dog walker and designated driver

    mattmia2
  • cluelessandlost
    cluelessandlost Member Posts: 13

    Thanks! I did exactly as you said (minus the "neat" part - there was only so much my tiny t-rex arms could do behind all the pipes), and today it passed inspection!

    seal2.jpg
  • pecmsg
    pecmsg Member Posts: 5,836

    ain’t neat but it works. 👍

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 10,319
    edited February 14

    Screenshot 2025-02-14 at 10.33.58 AM.png

    I have always found that Structo-Lite is the best for sealing single wall vent pipe to a masonry chimney. It is fireproof. There is nothing that can burn in this product. It is strong enough to hold the vent in place. It does not shrink when it cures. It can be troweled and finished like mortar and will stay in place on a vertical surface. And it looks good when you are finished.

    Learned about this stuff from an old timer chimney sweep in Philadelphia.

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    Robert_H
  • cluelessandlost
    cluelessandlost Member Posts: 13

    Good info to know! If this current application ends up flaking or needing to be redone, I'll have that bookmarked.

    EdTheHeaterMan
  • Cyclist77
    Cyclist77 Member Posts: 137

    Similar question!

    What is used if a smaller diameter flue is going into a larger thimble? Just a reducer?

  • Long Beach Ed
    Long Beach Ed Member Posts: 1,713
    edited March 11
    https://forum.heatinghelp.com/discussion/comment/1842583#Comment_1842583

    I have a reducer made if my supplier does not have one.

    https://forum.heatinghelp.com/discussion/comment/1842567#Comment_1842567

    Structo-lite is the miracle material! It's like joint compound but stiffer and 50% lighter,so it doesn't plop out. It's like plaster mixed with vermiculite. Sets up quickly. Fantastic for filling large holes on lath in plaster, or for fireproofing. You can trowel it onto wire lath overhead like stucco. Great for fireproofing ceilings. The Heaterman speaks sooth!

    EdTheHeaterMan
  • mattmia2
    mattmia2 Member Posts: 12,137

    it is basically lime mortar. in the light weight version it is made with vermiculite instead of sand. It is plaster of paris and fine aggregate and probably some quicklime. The lime and the plaster make it sticky.

  • HVACNUT
    HVACNUT Member Posts: 6,664

    New boiler but the same boiler as the old?

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