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Flue cement chronically lifting around patch edge

We have had the area around where the flue pipe exits the foundation patched twice, at last boiler installation, then re-patched a few months later, but the cement keeps lifting off around the outer edge of the patch where it blends up against the foundation wall. There's about 1/8" to 1/4" of space between the patch material and the foundation wall, all around the edge of the patch. Any idea why this lift-off keeps happening?


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,268
    Got a picture?
  • pecmsg
    pecmsg Member Posts: 4,206
    Expansion contraction?
    Bob Harper
  • Jamie Hall
    Jamie Hall Member Posts: 21,568
    Is this a feather edge or nearly so? It is remarkably hard to get a decent bond to concrete, and a feather edge is almost guaranteed to lift. Any movement of the flue relative to the foundation from expansion WILL lift it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Bob Harper
    Bob Harper Member Posts: 986
    The seal between the chimney connector "stovepipe" should be where it ends flush to the face of the flue tile. It should not project into the flue nor stop short of it. The junction between the pipe and the wall should be essentially cosmetic. If you're worried about it you can pack a little fiberglass stove gasket into the gap. If there is not a clay or steel thimble to act as a conduit or sleeve then it is not properly installed. Where a stainless steel liner is installed, the liner can be pulled or swept into the room and connected within a few inches of the wall. Any pipe cemented into the wall should be stainless steel.

    I agree it is popping loose from thermal expansion and contraction cycles. The furnace cement is much harder than the wall probably and will expand more breaking loose.
  • 307TurboFire
    307TurboFire Member Posts: 18
    I'd call it a feather edge. Stainless stovepipe goes into the cement, or rather, cement was placed around where the stovepipe leaves the basement through the plane of the inner foundation wall. I am not sure what the rest of the connection to the chimney flue looks like, behind that, I have not excavated the cement, and the gap all around now is too small to see much. I feel confident the rest of the connection to the flue was probably configured correctly, because everything else appears to have been done correctly. I am not really surprised this is happening. I have done some mortar pointing in my time. I am just wondering if there is some way to minimize the lift off, as with better cement, some kind of bonding agent, spritzing the foundation with a little water before applying the cement. This does not look like ordinary Portland cement to me. It looks like it might involve some sort of polymer.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,160
    edited September 2022
    I hate that stuff for large patches at a chimney base sealer. By the nature of the product it shrinks while curing. As a youngster we used real asbestos.insulating cement. You can't find that anymore, thank God!

    Back in the day, we referred our customers to a professional chimney sweep that really understood chimneys and how they work. I was installing a new boiler (as the helper) while this man with a thick Italian accent plastered the chimney base with a product he called Structo-Lite.
    It turned out that this was used by builders back in the day for the sub base of a plaster wall. The first and second coat. You mix the powder with just the correct amount of water to get the consistency required to stay in place. He was an artist and when he installed the crock at the base of the chimney for the vent connector pipe. The finish was seamless. After it dried and cured (about 2 hours) it still looked perfect. No cracks, no curling of the feathered edge. It looks like the rest of the basement wall and blended in perfectly.

    When I started my own business, I always went to the building supply yard and purchased 2 50# bags of Structo-Llte and kept a small metal Wheeling trash can of that on my truck. The trash can lid was great as a mixing tray, Structo-Lite does a great job for those customers that need to complain about the shotty work around the chimney pipe. Another Wheeling trash can had Oil Dry in it, and a third had powder furnace cement. They fit nicely under the parts shelf by the back door of a standard service van shelf system.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    STEAM DOCTOR Member Posts: 1,791
    Home Depot as something called Rapid Set Mortar Mix. Seals gaps much better than furnace cement  Won't curl or curdle. Chimney expert gave me this info. Instantly better product for this application.
  • 307TurboFire
    307TurboFire Member Posts: 18
    That is helpful, I appreciate.
  • mattmia2
    mattmia2 Member Posts: 8,440
    edited September 2022
    You can get Structo Lite at home depot although it comes in 80# bags. Structo is USG's brand name for pre mixed base coat plaster. Structo lite is light weight base coat plaster.