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Outside Air Kit in Concrete Slab

Post in 'The Hearth Room - Wood Stoves and Fireplaces' started by jhambley, Nov 22, 2012.

  1. jhambley

    jhambley Member

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    East Central Kansas
    I've decided on a BlazeKing Princess for the new home I'm building. I want to use the Outside Air Kit but have a few questions:

    1. I'm building on a concrete slab so I have to add the vent under the slab before pouring the concrete. Can I just use 3" drain pipe?

    2. How long can the vent pipe be?

    3. Can the vent pipe run up the wall into the attic to draw air?

    Any other suggestions would be appreciated.

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  2. WhitePine

    WhitePine Feeling the Heat

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    Talk to the building inspectors, assuming you have them in your location. They may have something to say about what you want to do. The may also have some insights into the best way to go about it.
  3. begreen

    begreen Mooderator Staff Member

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    That should work, but why does the OAK have to penetrate the slab? I would prefer it to go out the wall in back of the stove.
  4. Malatu

    Malatu New Member

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    As far as code is concerned, I can't help you.

    I don't know if the 3" is large enought, though if your question is asking if it will deliver an appropriate volumn of air, I would think so. If not you could always use 4" pipe.

    For the most part, I would think there would be no limit to the length of a vent pipe for fresh outside air.
  5. WidowMaker

    WidowMaker Member

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    I have seen kits advertised the stated 16 ft max run....
  6. nate379

    nate379 Guest

    The stove isn't near an outside wall?
  7. Malatu

    Malatu New Member

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    That's interesting. I hope someone chimes in with an explanation.
  8. burnt03

    burnt03 Burning Hunk

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    Regency spec'd a max length of 20' for my F2400. Might want to check with the manufacturer.
  9. jhambley

    jhambley Member

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    No building inspectors out here in the country. Stove is located in the center of the house, 16 feet from an outside wall.
  10. burnt03

    burnt03 Burning Hunk

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    Forgot to mention, if it's anything like a dryer vent, 90's and 45's will add "length" to the pipe. I'm pretty sure that a 90 adds 5' and a 45 adds 2.5'.
  11. WhitePine

    WhitePine Feeling the Heat

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    Can you put in a raised hearth and run the vent pipe inside an interior wall to get to the outside? You' d probably need to go to a 2x6 interior wall, in order to get the largest pipe possible (4" ???) in the wall. That would give you the longer run you need. Burnt03 is correct about the elbows adding effective length.
  12. northernontario

    northernontario Member

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    Been a long time since I calculated pipe flow resistance... but long story short is, anything moving through a pipe (air, water, solids) experiences friction. Long pipe runs and small diameter pipe can create enough resistance to flow that you won't get sufficient airflow to fire your stove. Imagine the difference between sucking water out of a straw vs. out of a 1/2" copper pipe. Small increase in diameter allows significant increase in flow at reduced restrictions.
  13. northernontario

    northernontario Member

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    And some searching online shows 5" OAK's available for some stoves (drolet).

    If you're doing something in a slab that you can't change later, you may want to oversize now. Too much air can't hurt, because you can throttle it back. I wouldn't run vent pipe up the wall to draw attic air... your creating another chimney with the fresh air pipe... you may have problems with the smoke backdrafting into the fresh air tube. Best to run it outside, down low.

    You might want to consider running two pipes under the slab (if that's the route you go)... run two 3-4" pipes (or larger if your budget allows and the distance is long)... where you come out of the slab, use a "T' to combine to a single pipe and connect that to the fresh air to the stove. Remember that the larger you can go, the better...

    These numbers are for water (can't find air flow right now)... http://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html
    3" Sch. 40 PVC - 20 gal/min - friction loss of 0.05psi/100ft
    4" Sch. 40 PVC - 20 gal/min - friction loss of 0.01psi/100ft
  14. WhitePine

    WhitePine Feeling the Heat

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    And any open pipe will naturally be occupied by the first "right sized" critter that finds it. Preventing that requires a screen or grill, which can be blocked by leaves or other detritus sucked or pushed up against the opening. An intake high up above the ground or structure such as a deck is much preferred, as a blocked outside air pipe will cause all kinds of grief when the stove is going.

    A ground level air intake can also be blocked by storm water under the right circumstances.

    All part of the reason that when decision time came, we went with no OAK. We saved money, too. We don't miss it a bit.

    Also, there is a discussion of the pros and cons of OAKs on woodheat.org that comes down against them.
  15. jhambley

    jhambley Member

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    BlazeKing recommended a 3" intake. I've stubbed it in and ran it over to the front exterior wall about 8' away. I figured I can place the vent up high to prevent blockage.
  16. Highbeam

    Highbeam Minister of Fire

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    For a 16' run I would use 4" for the under slab portion and then hook up to the stove using the 3" stuff. You only get one chance to get the under slab portion right. Any pipe will work so long as it is strong enough to withstand the weight of the concrete and installers during the pour. I've done this with a 4" dryer vent in a slab using schedule 40 pvc. Some stoves, maybe a future stove, requires 4" so you want to be ready.

    OAK setups are great and there is no good reason to not install one. Generally it is a lazy installer that wants to skip the OAK.

    Don't go up any higher than necessary on the intake end, the OAK becomes a chimney to some extent as a previous poster wrote.
  17. jhambley

    jhambley Member

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    Too late. The 3" is already buried in gravel with concrete coming tomorrow. The distance to the exterior wall is less than 8'.

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