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new thimble question-- for the inspectors out there

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by stovepipe?, Dec 6, 2005.

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  1. stovepipe?

    stovepipe? New Member

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    My mason is putting in a new thimble into my masonry chimney for use with a new wood stove (jotul F3CB). There is wood framing around and in front of the chimney. He wants to build a header to create six inches clearance from the thimble hole in the existing chimney to any studs. Then he will brick out from the chimney using the header to frame the brick. This will create a thimble that passes through 9 inches of brick and is surrounded by 6 inches on all sides. (Does this make sense? Brick is 6 inches on both sides, above and below the thimble and 9 inches deep between thimble opening to connect to stove and SS liner in chimney).

    So my question: is 6 inches of brick between the galvanized thimble and wooden framing sufficient? He seems to think that it is and meets code requirements. My intuition tells me that that brick is going to get pretty seriously hot.

    If I wanted to play it very safe, what should I do? Could I get him to use an insulated thimble i.e. those used to pass through combustible walls? Is this an appropriate application for something like that?

    thanks for any input

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  2. bruce

    bruce Member

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    the bricks get hot but not burning hot, when i build chimneys i take a 8" thimble and then put a 6" in the center of it, that makes for expansion and a second fireproofing, the main clay thimble will get hot and crack, i have changed many of these, with the bricks i build a 28" square and center the 8" thimble in it, as far as the pass threws i always wonder about rust,,
    ask about refactory cement on the flues
  3. elkimmeg

    elkimmeg Guest

    Why guess? Ask the inspector that will be doing the inspection what he requires or code requires not what the mason thinks will work.

    In real world situations 9" of masonry should not burn through

    Bruce very few do the type of work you described with the thought process. I wish more did. My job would be so much easier
  4. stovepipe?

    stovepipe? New Member

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    according to my town inspector woodstove installs are unregulated here. My main concern is not the 9 inch depth of the thimble, but the 6 inches of brick between its opening and the wood framing. Are there national codes which dictate the amount of masonry that should surround a galvanized thimble? I am more concerned that it be safe than that it meet code requirements (or, rather, I would like it to meet code requirements AND be safe).

    also, if I have single wall pipe connecting with the thimble, will this set-up be acceptable? I understand the 18 inch clearance to walls, etc, but don't really understand how this works when the pipe is running horizontally into the thimble.

    thanks
  5. bruce

    bruce Member

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    is the new thimble metal or clay?
  6. stovepipe?

    stovepipe? New Member

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  7. bruce

    bruce Member

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    i wood recomend a double clay thimble, the metal stove pipe will rust and corrode from the mortar contact when building the surround of bricks, even faster if winter admixes are used, remember the temps in this area could get 500+
  8. stovepipe?

    stovepipe? New Member

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    okay, thanks. that's good to know.

    But with respect to the 6 inches to the wood framing, seems ok to you?
  9. Metal

    Metal Minister of Fire

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    Be sure that you meet or exceed the clearance to combustibles stated in the pipe manufacturers installation instructions. Clearances are clearances, you can put 6" of space shuttle heat shields between the pipe and the studs and you will still have only 6" CTC.
  10. elkimmeg

    elkimmeg Guest

    I'm in the process of lookingg your question in NFPA 211 I will post the response
  11. elkimmeg

    elkimmeg Guest

    Minium 3.5" brick flamed by a a combustiable wall the minium distance shall be 12" from the combustiables to the fire clay liner connector This means you hace a clay liner plus 12 " each side to combustiables 6" as your mason stated is about half as needed

    For reference NFPA 211 Chapter 6 Chimney Connectors and Vent Connectors
    Figure 6-7.5 Chimney connectors systems and clearances from combustiable walls for residential heating appliances
    (Sub title diagram description ) SYSTEM A
  12. stovepipe?

    stovepipe? New Member

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    thanks, that answers my question. really appreciate you taking the time for that.

    Do you have any suggestions for an alternative? Should I have him use a piece of insulated chimney or an insulated thimble?

    thanks very much
  13. elkimmeg

    elkimmeg Guest

    What you are actually asking, without doing so on purpose, is for me to offer suggestions where I am writing my own code ITs not so much the thimble, but the single wall pipe that creates the heat situation you want a safety factor. Suggestions: Double wall black connector pipe into an 8" round clay masonry thimble. I wish I had a sample here to test the fit find out the interior demention of the 8" masonry connector thimble (Bruce are you able to shed some light on this?) and find out the exterior demention of the double wall balck pconnector pipe (you may only need one section for the pass through) see if it will work. At that point you can use the double waLL pipe manufactures specs for clearance to combustiables, anywhere between 6" to 9'' depending on the manufacture s and their specs. I also supose a galvanized metal thimble can be used. I am thinking you only need one section of double wall pipe about 16" to 18". ( 9" pasing threw the brick and 6 or 9" extend beyond the brick, the same distance it is spected from combustiables)
  14. stovepipe?

    stovepipe? New Member

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    so let me get this straight. you are saying that the reason there is a problem with my mason's proposed thimble is that with single wall pipe running into it it will get too hot. I can solve this by running a double wall pipe in so that I can take advantage of the reduced clearances. THe double wall pipe I have seen often comes with an adaptor for the thimble which is in effect a reduction down to single wall which slides inside it. You are saying this is insufficient, and my thimble will be no good uinless I have some sort of insulation of the thimble from the pipe itself, and double wall running through the thimble to my liner would solve this problem. Am I correct in thinking that this would be a problem with single wall pipe both inside and outside the thimble? in other words, the section of insulated/double wall pipe ought to run not just inside the thimble, but from the inside end of the thimble out to the 90 degree elbow that take me from my vertical run from the stove to the horizontal run into the thimble. Is this correct? I as because I think I could simple use a zero clearance pass-through thimble, one of those insulated deals, and it would accomplish the same thing as double wall for the 9 inches of thimble. But it would sit flush with the wall surface and a single wall pipe would connect to it. You are saying that such an arrangement is still problematic?


    That makes sense to me. so then the question is whether I can transition from single to double wall pipe for this run into the thimble. I am using single wall with heat shields up to my elbows-- reason being that because I have a small, odd off-set, I need to be about to use the adjustable 90's. The fixed 45s/90s in double wall won't work for me. So is this possible and will it solve my problems? vertical stretch and elbows in single wall with heat sheilds, transition to double wall and run all thr way through the thimble? or can I get away with: single wall all the way to the thimble opening, and then thimble itself is an insulated wall pass through thimble that doesn't require the 12 inches of masonry around it.

    thanks very much. even if you can tell me what is code/acceptable, just knowing what you think is reasonable and safe would be very helpful since it's not easy to know whose judgment to trust with something like this (e.g. my mason and fire department), especially when it is my house, not theirs.

    also, elk, awesome house. you must feel extraordinary pride at that accompishment and pleasure each day living in it (and rightly so!)
  15. elkimmeg

    elkimmeg Guest

    I have to go back and read threw your post a couple time to best advise this has become more complicated
    Yes proud of the house but a lot less than seeing my children graduate from Villanova Cornell and Wheaton
  16. elkimmeg

    elkimmeg Guest

    I still am having a hard time visulizing what you are asking the reduced clearance metal thimbles are for through the wall ceilings roof installations. I have never seen them used with Masonary flue connectors. Basically to gain the clearances with the metal throught the wall flanges one must used the speced double wall pipe suggested or manufactured by the same company as the pipe
    if venting into a masonry flue the the clearances requirements are what the manufacture supplies all single wall pipes are the same the differences is with double and triple wall pipes
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