Zero-Clearance Wood Stove Draft Issues

MrShocker Posted By MrShocker, Jan 13, 2018 at 10:23 AM

  1. MrShocker

    MrShocker
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    Jan 13, 2018
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    Loc:
    Knoxville, TN
    I am in the process of replacing my old fireplace with a new zero-clearance wood stove. I would like to use combustion air from inside the home, but I have had pretty bad downdrafting. It seems to work ok when I use outside combustion air, but I have read several articles on why this is bad.

    Here is what I have going on:
    1. Flame Monaco Zero-Clearance Wood Stove
    2. 22ft of Duravent 6" ID Double Wall Class A chimney
    3. Exterior un-insulated chase.
    4. 17 ft vaulted ceilings in room where the fireplace is located.
    5. Highest elevation of conditioned space is 23 ft higher from where the fireplace is. This room is pretty far away on the opposite side of the home.

    I am debating on insulating, drywalling, installing a firestop, and bringing that 17 ft into the conditioned space of the home. This would still leave almost 8 ft of chimney in unconditioned space. I am really not sure if it is going to be worth the effort or if I should just get the outside air kit and be done with it.

    Any insight into this issue would be appreciated.

    See the picture below for a diagram of this setup
    CHIMNEY.jpg
     
  2. begreen

    begreen
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    Nov 18, 2005
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    Outside air is not bad, particularly when is solves a negative pressure situation causing downdraft. There's no firestop showing in the exterior chase. There should be one at the first floor level and definitely one at the attic level. These stops may reduce or eliminate the downdraft issue.
     
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  3. MrShocker

    MrShocker
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    Jan 13, 2018
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    Loc:
    Knoxville, TN
    Thanks for the response. I was reading articles that talk about potential fire issues with outside air kits where a strong wind could blow in the opposite direction of the air intake and actually turn the outside air kit into a vacuum pulling the fire through it. Also, the outside cold air is what gets heated instead of room temp. air which lowers the efficiency of the fireplace.

    After reading the "cold-hearth syndrome", I convinced myself that insulating the chase is the only option, but it will end up being a lot of additional work. So the question is, will the benefits of insulating the chase be worth it?

    As for the firestops, the attic level one makes plenty of sense to me, but I am not sure I understand the the first floor level one. The room that the fireplace is in has vaulted ceilings all the way up the attic level. What purpose would the additional firestop provide?
     
  4. begreen

    begreen
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    FWIW, although I have read about that concern with an OAK I have never heard of this actually happening in the past 11yrs here. Not saying it can't happen, but it seems unlikely if the intake is properly placed.

    Code says there needs to be a firestop at the 10' level as well.
    R302.11 Fireblocking. In combustible construction, fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space.

    Fireblocking shall be provided in wood-frame construction in the following locations:
    1. In concealed spaces of stud walls and partitions, including furred spaces and parallel rows of studs or staggered studs, as follows:
    1.1. Vertically at the ceiling and floor levels.
    1.2. Horizontally at intervals not exceeding 10 feet (3048 mm).
    2. At all interconnections between concealed vertical and horizontal spaces such as occur at soffits, drop ceilings and cove ceilings.
    3. In concealed spaces between stair stringers at the top and bottom of the run. Enclosed spaces under stairs shall comply with Section R302.7.
    4. At openings around vents, pipes, ducts, cables and wires at ceiling and floor level, with an approved material to resist the free passage of flame and products of combustion. The material filling this annular space shall not be required to meet the ASTM E 136 requirements.
    5. For the fireblocking of chimneys and fireplaces, see Section R1003.19.
    6. Fireblocking of cornices of a two-family dwelling is required at the line of dwelling unit separation.

    R1003.19 Chimney fireblocking. All spaces between chimneys and floors and ceilings through which chimneys pass shall be fireblocked with noncombustible material securely fastened in place. The fireblocking of spaces between chimneys and wood joists, beams or headers shall be self-supporting or be placed on strips of metal or metal lath laid across the spaces between combustible material and the chimney.
     
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  5. MrShocker

    MrShocker
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    Jan 13, 2018
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    I have looked at the code, but I am not sure I am interpreting it the same way. I don't have a second story floor on the same wall was where the chimney is. The code only mentions fireblocking at 10' horizontal intervals (not vertical). It says vertically at ceiling and floor levels. The only ceiling level I have would be my attic ceiling at the 17' level.
     
  6. begreen

    begreen
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    10' horizontal intervals meaning ten feet over the horiz. plane of the floor.
     
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