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Advice needed about correcting an install problem with block off

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by paredown, Nov 19, 2010.

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  1. paredown

    paredown Member

    Joined:
    Jan 11, 2009
    Messages:
    62
    Loc:
    Lower Hudson New York
    Heard a big thump the other day, and investigated up on the roof while blowing the leaves. Our chimney has a large slate top over the flue, and the noise from a piece of the slate breaking of and landing on the roof. From the soot in the crack I think it had been waiting to go for some time.

    While up there, though I took a good look at the block off that the installers had put on last year, and was depressed by what I saw. They were back cleaning last week, & the guy mentioned that the liner had shifted, but he had "put some more silicone on it to seal it." What I found did not impress.

    I have to say, I did not take a look last year after they finished the install--a mistake on my part. We have this enormous round flue on a large chimney, that has this large slate sitting up above on four rock "posts". (Slate can be lifted down for cleaning.)

    To cap their install at the top, they used a large round piece of galvanized sheet metal (16 ga?--pretty light) with a simple collar, ran the liner through it and held it in place with some sheet metal screws. All they did to hold the assembly in place on top of the flue tile was to shoot a bunch of silicone on it--no mechanical attachment. Then, because there is not enough height to put a proper cap on the liner, they jammed some crumpled up screening into the end of the liner.

    So after one season of burning the liner has shifted, and the piece of sheet metal has pulled off-center and off-plane. There are gaps between the sheet metal and the top of the flue, despite the cleaner's assurances that he had put more silicone on it (and there is a lot!)
    Edited to add a pic of the chiimney

    So what would cause the liner to shift?
    How heavy should a reasonable block-off plate on the top be? Should it be mechanically fastened to the flue tile?
    Does someone make a collar-style clamp that you could use to clamp around the round flue tile (I remember making large connectors for commercial round ducts with dad back in the day..) that could be mechanically fasted to the block off?

    As usual, more questions than answers, and one more confirmation that if you want it done right, you might well have to do it yourself. (although in this case our insurers insisted on a "professional" install.)

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  2. Retreadsme

    Retreadsme Member

    Joined:
    Feb 14, 2009
    Messages:
    90
    Loc:
    NW TN
    Not knowing what type liner (flex vs solid) might skew some answers as well as not knowing what the dimension of an "enormous round flue" is since they come anywhere from 4" to 36" diameter, and larger if special ordered.

    So what would cause the liner to shift? - many things from a sloppy install to tornadic winds to ground shift. If it is a flex liner and it was installed loosly (waving on the way down) it could settle and move.
    How heavy should a reasonable block-off plate on the top be? - varies by mfg; 14-18 ga probably in my opinion.
    Should it be mechanically fastened to the flue tile? - Many people do not, but I do.
    Does someone make a collar-style clamp that you could use to clamp around the round flue tile that could be mechanically fasted to the block off? - The block-off plate could have been made with flanges extending down the side of the round clay tile and a band-clamp used to secure the assembly. It would not be difficult to add flanges to your existing flat plate if you are handy with shears and have a pop-rivet gun.

    I never rely on silicone to do more than seal out moisture, some folks do though. A proper spark arrestor should be installed instead of stuffing mesh inside the liner. Hopefully there is 7-8" height from the liner top to the slate cap. If not you may want to consider removing the slate cap.
  3. paredown

    paredown Member

    Joined:
    Jan 11, 2009
    Messages:
    62
    Loc:
    Lower Hudson New York
    Thanks for the response.

    Yes, it is an insulated flexible liner, so movement while installing seems a likely explanation. Is this easily correctable?

    I'll get back on the roof to measure the flue size--I knew I have the measurements written down somewhere & have been too sick to get back up to look.

    So to add the extra fastening, I'm assuming the easiest would a series of tabs attached to the plate, and then use the band clamp around the whole flue?

    And thanks for the 8" estimate--since I did think that the slate may have cracked since the hot gases were more less directed at that spot after it shifted. If I take the slate off, will I need to add a cap along with the spark arrestor?
  4. Retreadsme

    Retreadsme Member

    Joined:
    Feb 14, 2009
    Messages:
    90
    Loc:
    NW TN
    Since it is flex, I would be inclined to check it for extra length left inside clay-tile allowing the liner to slump and/or shift, thus moving the top plate. If there is extra liner, I would be inclined to cut it and re-attach to the top plate. If excess liner is not the issue then the movement could occur from many possibilities, and all the examples I gave earlier are typical of what occurs in our part of the country making more than just top plates shift. :(
    Yes sir - the easiest and cheapest route would be to add 4 tabs 1 - 1 1/2" wide (approx) to the top plate, bend them down the clay-tile and band clamp it. Depending on age of the clay-tile one may be able to use masonry screws, but I try not drilling/screwing into tile that is old. Again - others may, that's just me.
    Yes - you will need a cap along with the spark arrestor; and most are mfg'd that way. There are literally 100's of styles, materials, price range to chose from.
    My house has a brick chimney on an external wall that measures 56"x42" with a 12" square clay-tile flue. I had a SS chase cover made with a 52"x38" roof raised 9", with 1/2" screen on all 4 sides for spark arrestor. The roof is removable to allow access for liner cleaning. That was NOT the most economical way to approach my setup, but it accomplished the visual affect I wanted and it gave the protection required. I don't find many people that want to invest the $$ for full chase covers and caps the way I did mine though.
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