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Convect air behind wood stove's back wall?

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by Leroy_B, Oct 8, 2012.

  1. Leroy_B

    Leroy_B Member

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    Building the back wall for a wood stove install and why is it all the pictures I see of hearths built I don't see any space on the bottom or top where cement board or veneer for the back wall. I was advised on another post I'm still working thru to get it done right. New to the forum and asking for help!! Not looking to do it wrong. I'll install metal soffit vents and blend it in rather than a gap on top and bottom. Just a gap doesn't look right. The metal studs doubled up are secured to a plywood wall with liquid nails and decks every 12" next is 1/2" cement board.

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  2. begreen

    begreen Mooderator Staff Member

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    It all depends on the installation and whether one needs a clearance reduction or not. Many folks just do it for cosmetics or extra peace of mind. A full brick wall will achieve a 33% reduction without ventilation.
  3. Leroy_B

    Leroy_B Member

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    A clearance reduction? What's getting cleared and what reduction? please explain. Cosmetic or peace of mind.....I'm still confused. And full brick as in a complete brick mortar wall? I'm not doing brick..... I work best with details explained.
  4. Heatsource

    Heatsource Minister of Fire

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    really? is that per nfpa?
  5. Wood Duck

    Wood Duck Minister of Fire

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    If you have enough clearance between the stove and the wall, then you do not need a vented cement board wall. Any wall is OK as long as it is far enough from the stove to provide the required clearance specified by the stove manufacturer. You mention that you are going to use the design where you have a non-combustible material like certain types of cement board held away from the wall by spacers, and with the space behind the cement board vented at top and bottom to permit air flow - this design is used in installations where the space between the stove and the wall is not large enough to meet the required clearance limits set by the stove manufacturer.

    I don't have a special wall behind my wood stove because the wall is far enough from the stove. I guess a lot of the photos you have seen also do not have a special wall for the same reason.
  6. Highbeam

    Highbeam Minister of Fire

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    You can't reduce the clearances below the manufacturer's spec unless your particular stove specifically allows for this in the manual. Otherwise, the NFPA reductions only apply to stoves installed using the standard 36" clearance spec and not to your BK (random brand) with a 6" factory spec. As such, there is usually no reason for a clearance reducing wall, they are usually for looks only.
  7. Leroy_B

    Leroy_B Member

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    Now that's the kind of responses I can work with!! Thanks guys for the reply, gives me plenty to work with. I have the PDF file on the stove specs and I'm building to what the manufacturer requires the distance away from the wall even with 1/2 cement board, doubled steel studs, and field stone veneer.
  8. theonlyzarathu

    theonlyzarathu Member

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    Stay with the specs but if you are worried put your hand on the wall behind your stove when its running. If your can leave your hand there for any legth of time then its not going to burn. Remember it takes 451 degrees to start paper on fire. boiling water is 212 degrees and you can't touch that for even just a second without being burned. It has to get pretty hot to start a fire even on dry wall.
  9. begreen

    begreen Mooderator Staff Member

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  10. Heatsource

    Heatsource Minister of Fire

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    ^ ok, so a 33% reduction, down to 24"
    so this alpplies to non-listed appliances as most newer units will already have closer than 24" clearance
  11. begreen

    begreen Mooderator Staff Member

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    Correct, the whole table is based on non-listed appliances. But with permission in the docs of the stove mfg. these reductions are permissible for modern stoves. Good to know for highly radiant stoves like the big VCs or Jotuls.
    Heatsource likes this.
  12. Highbeam

    Highbeam Minister of Fire

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    Not the same thing. You can easily touch 200 degree metal. 200 degree water is way different not only in heat transfer but also it surrounds your hand vs. a single surface.
  13. begreen

    begreen Mooderator Staff Member

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    I don't disagree that if you can hold your hand on the surface for a few seconds without pain that the surface temp is most likely safe. But one should know that long term exposure to heat will lower the combustion point of wood (and paper) over time. Sometimes by a couple hundred degrees. Pyrolysis can occur as low as 170::Fgiven enough heat and time. That's why you want to be conservative here. Sounds like Leroy has taken the right precautions.

    http://www.doctorfire.com/low_temp_wood1.pdf
  14. Heatsource

    Heatsource Minister of Fire

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    ^ good info there begreen!
  15. Leroy_B

    Leroy_B Member

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    Damn I hope so, been lurking since Dec 2010. How else was I going to learn and understand any thing and everything involved with wood stoves and everything related. My family thinks I'm nuts.
  16. begreen

    begreen Mooderator Staff Member

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    They're probably right. You came to the wrong place for a firebug head check. LOL.
  17. Leroy_B

    Leroy_B Member

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    Good just where I need to be. I'm in many of the wrong places. This just happens to one of many.
  18. theonlyzarathu

    theonlyzarathu Member

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    Perhaps so, but remember that it takes a much hotter temperature than 212 degrees F to ignite paper, and even more to ignite drywall. And I don't think you would want to put your hand flat on boiling water for very long, or put hour hand flat on 200 degree metal for very long.
  19. begreen

    begreen Mooderator Staff Member

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    I think Highbeam meant just briefly. As noted, pyrolysis can dramatically lower the combustion point of paper or wood.

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