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Community and Q&A

Deflecting Furnace Heat Away from Basement Wall

cgran | Posted in General Questions on

Hello!  We have a furnace approximately 1 1/2 feet from our stone basement wall. The heat from the furnace is drying out the mortar around the stone which is concerning. How can we protect the wall? How would we deflect the heat away from the wall to stop further drying? Our home is over 100 years old. Thank you!

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  1. Patrick_OSullivan | | #1

    Other than when it's applied, mortar shouldn't be wet, so I'm curious what you mean as to the heat drying it out.

    What are you actually noticing that's concerning? I suspect you're noticing increased efflorescence in this area and perhaps degradation of the mortar.

    If so, the problem to control is water getting into the wall in the first place, not the furnace drying it out.

  2. cgran | | #2

    Hello Patrick,
    Apologies for not being more clear. We noticed that the mortar around the stones right behind the furnace and mortar around the pipe out of the furnace into the wall appears to be discolored (turning white) and at touch is very dry but not crumbling (yet). The rest of the mortar around the stones in the remaining walls is natural color and cool to touch. We are concerned the heat from the furnace will completely dry out the mortar and begin to crumble. We are purchasing an aluminum heat shield to reflect the heat away from the wall. Wondering if that is enough.

    1. Expert Member
      BILL WICHERS | | #3

      Masonry doesn’t really “dry out”, but heat can sometimes be an issue. If the problem is just right around the flue pipe, I’d put in a thimble which should solve the problem. A thimble is a metal assembly that allows hot flue pipes to penetrate walls and other things and provides an air space between the pipe and the surrounding wall.


      1. cgran | | #4

        Great advice! Thank you!

  3. Jon_R | | #5

    Rockwool will block the radiant heat without fire concerns. But consider properly insulating the entire wall, which as a side effect, is likely to reduce efflorescence.

  4. Expert Member
    Michael Maines | | #6

    You are probably seeing efflorescence--minerals that leach out of mortar. Your furnace heat is not likely hurting the masonry at all.

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