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  1. #1
    Join Date
    Sep 2014
    Location
    NW Arkansas
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    PVC for exposed fresh air duct?

    We are looking at installing a fresh air intake and dehumidifier for an earth sheltered house. Inside we will use wrapped metal but we are having a hard time deciding on what to use outside. We will exit the building under the carport about 5' from the ground' then elbow up and through the roof. It was suggested that we transition to PVC before exiting the building. I have used PVC for inslab supply but that is all. Anyone have any suggestions or comments? I haven't run the numbers yet but guess it will be an 8".

  2. #2
    Join Date
    Sep 2014
    Location
    NW Arkansas
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    Thread Starter
    Looking into our MC I found PVC ductwork is only allowed below slab or in exhaust applications (though I was denied using pvc for bath exhaust a few years ago...). I may or may not be allowed an exemption for the portion outside of the envelope. Any suggestions on running exterior duct-work for fresh air?

  3. #3
    Join Date
    Aug 2002
    Location
    Southold, NY
    Posts
    27,246
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    If exposed Stainless Steel.

    8" is pretty large?

  4. #4
    Join Date
    Oct 2002
    Location
    Plano, TX
    Posts
    2,417
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    I can't understand the officials that write these rules. PVC is not allowed in common return because of flameability toxicity issues, a big headache for commercial remodel, yet it is allowed in underfloor duct and has the same properties!

    I can agree with PVC OK for exhaust issues.

  5. #5
    Join Date
    Nov 2014
    Location
    Chicago Illinois
    Posts
    216
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    Pretty interesting dilemma please keep us posted as this goes.

  6. #6
    Join Date
    Sep 2014
    Location
    NW Arkansas
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    Thread Starter
    Looks like, according to RHVAC9, I need 70cfm of fresh air. Could do with a 6" pipe but 7" might be better with all the turns. I'll put the questions to the inspector this week. Maybe they will let me with a fire damper ring?

    On a side note, we are putting in a dehumidifier. Already asked Teddy Bear some questions. Glad I did b/c now I know who sells their brand (though under a different label). The house is a monolithic concrete building except for southern wall might be cinder-block. Would you pipe the fresh air only into the dehum or would you split the pipe between the furnace and dehum and use controls/backdraft dampers? I was thinking of just running the FA to the dehum and the dehum supply to return of furnace. That way when just furnace is running it would still pull fresh air through the dehum. Is that assumption correct?

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