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  1. #1
    Join Date
    May 2008
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    N. Syracuse, NY
    Posts
    36

    Default chimney as supply duct

    Thoughts on conversion of brick chimney with no liner to supply duct for new attic bedroom. Both funace and hot water changed to direct vent; chimney carried both exhausts for many years. Brick from attic floor thru roof removed, floor grate and skylight added. Flue is only 4x12, so flex duct would be small, CFM's limited. Can the open flue be used as duct? Fire away.

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  2. #2
    Join Date
    Mar 2007
    Location
    Spring City/Surrounding Philadelphia area
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    3,473

    Default Re: chimney as supply duct

    As long as the old chimney interior is completely sealed off from the exterior, is water tight, flue interior is not deteriorating, and has no other flue within the same chimney structure like from a fireplace or from an attached house (many row homes and twins share chimney structures), it sounds like it should be OK.

    "It takes a big man to cry. It takes an even bigger man to laugh at that man". - Jack Handey

  3. #3
    Join Date
    Jul 2008
    Location
    Snowbird (this means I'm retired and migrate between locations), FL/MI
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    4,086

    Default Re: chimney as supply duct

    I see your location is listed as Syracuse, NY. Start your planning process with your local building office and review your State Code adoptions and local ammendments. Next discuss your plan ideas (since you will require submitted plans) with those officials, OR hire a planner. This is not a DIY site, even for "licensed electricians".

    Really have no details, and a lot of assumptions here. Even if "chimney" is central to the structure (not outside wall), I definately would NOT recommend it.

    Especially as you say it was unlined when in use I don't know if you actually mean it was parged originally, or if it had or has tile flue which measures 4x12. I would still nix the idea. If masonry chimney serving oil fired appliances, "sour gas", or at one time coal, coal gas, or wood in the past, would completely demolish.

    Have no idea the vintage of same, but if last century or current, lime mortar is unlikely, stability of same questionable, etc.

    Either way soot, acids, and other deteriorating effects can continue to erode. Contact of masonry to metal ductwork sweating, errosion, corrosion cycle.

    Not to mention the moisture conduit (masonry) between floors of same will continue, (like a straw "wicking") and never being exposed to the higher carrying and drying temperatures during its previous use, may even sweat. Bricks continue to take on moisture and swell. You'll also likely have negative effects to your conditioned air temperature with no air space or thermal break between ducts and masonry "chase".

    No matter what, with forced air, you're going to need returns somewhere. Where is the air handler, basement?

    What better time than to do ahead and dismantle completely, properly frame and block the cavity securing duct supports and floor/ceiling transitions, and possibly re-work some of the newly reclaimed chimney space outside of the chase for storage, etc or use for properly sized, smooth, guaged ductwork.

    See no real practical way to block transitions between floors, maintain spacing and isolation between ducts and masonry, properly support and secure ducts, seal duct transitions, eliminate vibrations, and use correct gauge materials for multi-story delivery and maintain strucutral integrity below. Suspect you plan major breaches below to get trunks interconnected - so you'd already have questionable issues with supporting and reworking the old masonry structure of the chimney anyway.

    Take the time, do it right, remove it and dispose of it responsibly, and remediate the framing, floor/ceiling transitions and be mindful of blocking and stopping (spread of fire and smoke) and enjoy better environmental control comfort as well. You'll likely need some adjustments to properly "zone" the converted attic to be 3rd floor habital space, for comfort as well.

    Do your attic conversion with plans, permits and inspections. Hire professionals to assist you or perform the planning process to the building codes, mechanical codes, etc.

    P.S. the whole "floor grate added" ("skylight above" and "open flue") is just spookily, ambiguously, scary.

    Last edited by H.G. Watson, Sr.; 08-02-2010 at 11:04 AM.

  4. #4
    Join Date
    Mar 2007
    Location
    Spring City/Surrounding Philadelphia area
    Posts
    3,473

    Default Re: chimney as supply duct

    HG does raise some good points in regard to residual combustion products inside the chimney. Pulling a flex duct through the old flue could result in damage to the duct and allow any air within the chimney to enter the supply air through damaged areas on the duct.

    "It takes a big man to cry. It takes an even bigger man to laugh at that man". - Jack Handey

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