Outside Air Kit in Concrete Slab

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

  1. jhambley

    jhambley
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    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

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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

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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.
     
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  4. Malatu

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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

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

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

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

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

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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.
     
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  10. burnt03

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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'.
     
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  11. WhitePine

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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

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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.
     
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  13. northernontario

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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
     
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  14. WhitePine

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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

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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.
     
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  16. Highbeam

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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.
     
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  17. jhambley

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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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