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  1. #1
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    May 2015
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    Split OA between CA and FA for RA.

    After reading a lot of material on the benefits of OA (Outside Air) I am attempting to bring OA into the Return Air (RA) supply.

    All pictures can be found at http://salestime.com/HVAC/.

    Currently I have 2 13' long horizontial OA ducts (labeled Lower CA and Upper CA)



    ending in the FAU closet off the hallway (one high and one low).


    (Inside screens 5x13 1/4" screen x2 = 65si each)

    I know that one should not route CA (Combustion Air) from the closet to the RA (just below the FAU).

    But, is it possible to share the OA ducts and tap into the
    ducts, before they reach the FAU closet and become CA? I can split the OA duct in two and route them to two different locations (CA and FA for the RA).

    The other option is to re-purpose one of the two OA ducts as FA believing the other OA duct fulfills the CA requirement.


    (back of the FAU closet, blue is Outside/Fresh Air, red is Exhaust Air, green is conditioned air)

    Note the door to the FAU closet off the hallway is not sealed so I am sure there is some air exchange between the closet and inside air.

    I have reviewed the code and, due to our small size, we exceed all requirements for CA (see below).

    50K BTU FAU is the smallest they sell? We only need 15K BTU according to online calculations.

    701.4 Outdoor Air 2 permanent openings (currently have):
    Where communicating with the outdoors through horizontal ducts, each opening shall have a free area of not less than 1 square inch/2,000 Btu/hour of total input rating of all appliances in the enclosure.

    50K/2K = 25si

    701.4 Outdoor Air 1 permanent opening (Maybe use this?):
    1 square inch/3,000 Btu/hour of total input rating of all appliances in the enclosure, and not less than the sum of the areas of all vent connectors in the space.

    50K/3K = 17si
    4" exhaust vent = 13si

    *** So why not use one duct for CA and one for FA?

    701.9 Louvers, grilles and screens
    ... metal louvers and grilles will have a 75 percent free area.

    Outside louver:
    6x14 metal louver x2
    84si each x 0.75 = 63si



    7" horizontal duct x2 13' long = 38.5si each (from my neighbors so I assume I have the same size).

    Last edited by RandallIja; 05-16-2015 at 04:20 PM. Reason: clarification

  2. #2
    Join Date
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    Anderson, South Carolina, United States
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    No, you can't use the combustion air passageways to get fresh air for the return.

  3. #3
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    Thread Starter
    Can I just use one of the passageways for CA? Then use the other passageway, no longer connected to CA, for FA?

  4. #4
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    Jan 2004
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    No, you would be required to run a new combustion air inlet if you eliminated either of the 2 you have.


    Next, why would you ant to bring in bathroom air into your house. And fresh air intake beside a bathroom exhaust will bring in all the bathroom smells, plus the humid bathroom air.
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    How many times must one fix something before it is fixed?

  5. #5
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    Thread Starter
    Does not 701.4 infer that I can choose between one and two OA ducts?

    I see your point. The bathroom fan would not be run as often as the FAU/AC. So most of the time I would not be intaking the damp hot bathroom exhaust air. This situation is not ideal. But without punching holes in drywall and stucco, this is my only option (other than opening windows and doors) for FA.

    Thanks for the heads up.

  6. #6
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    Your windows aren't that tight that you should need additional fresh air.
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    How many times must one fix something before it is fixed?

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  8. #7
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    Apr 2007
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    Talk to your contractor, there is always a way to bring in fresh air, it's not as difficult as it seems when we do this all day everyday. We use the single combustion opening method frequently, but can not have fresh air opening within 10' of any exhaust openings. Good luck
    There are two ways to do things, Right and Again.

  9. #8
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    May 2015
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    Thread Starter
    Thanks for the information. The 10' rule is a deal breaker. I guess it is no designed FA for us.

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