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Sealing 4" gas WH vent to masonry?

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Jells
Jells Member Posts: 576

I used to use furnace cement around where a 4" duct penetrated the flue till I saw some post saying it was innapropriate. I've noticed it shrink and crack. I managed to cut the hole quite a close fit, can I use silicone sealant? What about thinset? I have a recently opened bag. Seems a good choice considering the tight fit will dry too quickly for regular cement.

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  • Jamie Hall
    Jamie Hall Member Posts: 23,640
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    Not silicone. It won't take the heat — and it's flammable. The furnace cement is the best bet — that's what it's for and minor cracking shouldn't be a problem. Assuming your chimney has adequate draught, that is.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • JohnNY
    JohnNY Member Posts: 3,244
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    Use furnace cement like my company did last year on the 60 or so chimney vented boilers we installed.

    Contact John "JohnNY" Cataneo, NYC Master Plumber, Lic 1784
    Consulting & Troubleshooting
    Heating in NYC or NJ.
    Classes
  • Jells
    Jells Member Posts: 576
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    Interesting. I just had a boiler chimney relined, and they cemented the pipe into the flue with a regular masonry cement. See why us civilians get confused!

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
    edited March 2023
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    I prefer a product called Structo-Lite base coat plaster https://www.usg.com/content/usgcom/en/products/walls/drywall/plasters/structo-lite-basecoat-plaster.html because it does not shrink and crack like the gray Furnace Cement shown here

    When this High Heat Furnace Cement cures and dries out, it often peels away from the block wall or brick chimney opening. Next year when you open up the chimney to inspect it, you must be very careful of the sharp edges of that cement that you use for caulking that seam.

    An old chimney man in Philadelphia showed me this plaster base coat that you mix with water. It will fill gaping holes between the vent pipe and the masonry wall around a chimney opening. It is fireproof and not as heavy as regular cement, so it sticks to the wall and vent pipe without peeling away when it dries up and cures. It is easily workable for up to 15 minutes. Plenty of time to trowel the surface smooth and professional looking. Much better looking than the cracked cement pipe connections shown here.

    I have used it for years with great success. 

    Mr. Ed

    Edward Young Retired

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

  • Jells
    Jells Member Posts: 576
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    Thanks, I think I had seen an old post of yours on that before. I've seen it used on walls, but I thought a gypsum based product didn't have heat resistance enough for this job. Doesn't gypsum degrade from heat, unlike Portland cement? If I were going to use that, I might as well use it's cousin Plaster of Paris! Sets fast, doesn't shrink.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 8,166
    edited March 2023
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    I believe the chimney man from Phila selected it because of the additional sand that is included in the mixture. It has a rough Stucco like finish when you trowel it smooth. Like an very heavy grit sand paper. I believe it is that way to accept the finish coat plaster (which I believe is plaster of paris based product). I think it is a lighter sand product making it stick to the walls and vertical surfaces more easily. I have used it for over 40 years and found that it does not crack, discolor or separate from the wall or the metal pipe. Later on, when you need to reomve the pipe for chimney inspection, it breaks away easier than portland cement, or mortar mix. In some cases, I have found that the sheet metal pipe will stay in the wall and the first connection elbow gets removed. Then there is no need to re-cement next tune up/inspection. So that cement job can be there for 10-20 years without discoloring.

    Mr. Ed

    Edward Young Retired

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

  • STEAM DOCTOR
    STEAM DOCTOR Member Posts: 2,040
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  • STEAM DOCTOR
    STEAM DOCTOR Member Posts: 2,040
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    Been using for years, on advise of my chimney guy. Gone back on jobs years later and zero changes. The furnace cement has a tendency to chip

  • HVACNUT
    HVACNUT Member Posts: 5,888
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    If it's not too hard, I would enlarge the hole another one inch diameter. As it is, most of the cement will only be on the surface. Then you'll get the peeling and cracking.

    I'll sometimes fill the circumference gap with steel wool, about an inch deep so the cement has something to set to. Only seal what needs to be sealed