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How to seal a boiler

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luna12
luna12 Member Posts: 10
edited May 7 in THE MAIN WALL

I have a used Weil-McLain CGa-6 cast iron boiler. I took off the sheet metal casing when I did the pressure test to inspect the sections. The factory used furnace cement to seal the collector hood and the area between the platform and boiler sections. The cement has cracked and fallen off all over. I removed it all and I need to decide how to re-seal everything. Weil-Mclain customer service recommended Do-All + furnace cement. I found a different brand at Menards. But I would rather not use furnace cement because it will likely eventually do the same thing as the origianal stuff. Is there any suitable, silicone-like, flexible sealant I could use? I found some Meeco's Red Devil clear hi-temp silicone sealant which says it works 450-500 degrees. Could I get by using something like that?

PXL_20260506_183048591.MP_small.png PXL_20260506_183118460_small.png PXL_20260506_192311088.MP_small.png

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 27,456

    Use furnace cement. While one can hope that the exhaust gas will never get much over 600 to 700 F, it will get that high from time to time — and under certain conditions can get much higher.

    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    luna12
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 20,306

    Hi temp red silicone is ok.

    Some boiler MFGs use "Kaowool". Its high temp gasket material. They make rope, tape and blanket style.

    HD sells it but they have to order it. for a 12" x 24" roll Ceramic fiber and it is cheap. Or Lynn Products

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 12,378
    edited 2:01AM

    If the factory uses furnace cement, it is for a reason. and it does dry out and peal away from the metal. The trick is to get it deep in the seam and use as little as possible. I have seen this stuff globbed on stove pipe that is wet into the base of the chimney and it looks like **** in just a few days.

    Screenshot 2026-05-07 at 9.58.36 PM.png

    If you are filling a gap that is more than 1/8" you need to use something else.

    Edward Young Retired

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

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