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  1. #1
    Ed Bliss's Avatar
    Ed Bliss Guest

    Default Combustion Air and M1703.2.1

    Yesterday I observed two 8x16 ducts supplying combustion air from outdoors for appliances totaling 402,000 BTUs. My question is that since these ducts run horizontally for about three feet before becoming vertical, should the 1/2000 for the horizontal or the 1/4000 for the vertical be used to calculate the proper sizing?

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  2. #2
    James Duffin's Avatar
    James Duffin Guest

    Default Re: Combustion Air and M1703.2.1

    I would go with the 1/2000 or the 1/3000 that is allowed with the one permanent opening method. (G304.6.2 One-permanent-opening method)



  3. #3
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    Default Re: Combustion Air and M1703.2.1

    Quote Originally Posted by Ed Bliss View Post
    My question is that since these ducts run horizontally for about three feet before becoming vertical, should the 1/2000 for the horizontal or the 1/4000 for the vertical be used to calculate the proper sizing?
    First, if you were to use one of them, the most restrictive would apply as that is the basic premise for conflicting code requirements/allowances.

    However, ... there is no section which 'allows for' combining horizontal and vertical ducts in one duct run, thus *neither* can really be applied.

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
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  4. #4
    David Bell's Avatar
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    Default Re: Combustion Air and M1703.2.1

    Any change in direction of combustion air ducting would fall under 304.8 Engineered installations, or 304.9 mechanical combustion air supply. These are from IFGC. Engineered systems must be approved, mechanical systems require .35 cubic feet per minute per 1000 BTH/h of outdoor air.


  5. #5
    James Duffin's Avatar
    James Duffin Guest

    Default Re: Combustion Air and M1703.2.1

    It looks like the fuel gas code says the duct can be either vertical or horizontal with no mention of a change of direction being a problem. This is the code I use for combustion air.

    G304.6.2 One-permanent-opening method.
    One permanent
    opening, commencing within 12 inches (305 mm) of
    the top of the enclosure, shall be provided. The appliance
    shall have clearances of at least 1 inch (25 mm) from the
    sides and back and 6 inches (152 mm) from the front of the
    appliance. The opening shall directly communicate with the
    outdoors or through a vertical or horizontal duct to the outdoors,
    or spaces that freely communicate with the outdoors
    (see Figure 304.6.2) and shall have a minimum free area of 1
    square inch per 3,000 Btu/h (734 mm2/kW) ofthe total input
    rating of all appliances located in the enclosure and not less

    than the sum ofthe areas of all vent connectors in the space.



  6. #6
    David Bell's Avatar
    David Bell Guest

    Default Re: Combustion Air and M1703.2.1

    The opening shall directly communicate with the
    outdoors or through a vertical or horizontal duct


    One or the other,,not both

    From GE website,
    If air is supplied from outdoors, directly
    or through vertical ducts, there must be
    two openings located as specified above
    and each must have a minimum net free
    area of not less than one square inch per
    4000 Btuh of the total input rating of
    all the appliances in the enclosure.
    If horizontal ducts are used to
    communicate with the outdoors, each
    opening must have a minimum net free
    area of not less than one square inch per
    2000 Btuh of the total input rating of all
    the appliances in the enclosure. If ducts
    are used, the minimum dimensions of
    rectangular air ducts shall not be
    less than
    oth

    Last edited by David Bell; 04-03-2010 at 06:33 PM.

  7. #7
    James Duffin's Avatar
    James Duffin Guest

    Default Re: Combustion Air and M1703.2.1

    Our local mechanical inspector, the NCHILB, and the NCDOI all got involved when I asked this question and I was told to go to this section in the Mechanical Code for the answer to my question. My problem was I was convinced that you needed two combustion air vents but I was proven wrong by the fuel gas code. Your state may be different.

    701.1 Scope. The provisions of this chapter shall govern the
    requirements for combustion and dilution air for fuel-burning
    appliances other than gas-fired appliances. The requirements
    for combustion and dilution air for gas-fired appliances shall be
    in accordance with the
    International Fuel Gas Code.



  8. #8
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    Default Re: Combustion Air and M1703.2.1

    James,

    I will change the highlighting on what you posted, then explain the problem.

    Quote Originally Posted by James Duffin View Post
    It looks like the fuel gas code says the duct can be either vertical or horizontal with no mention of a change of direction being a problem. This is the code I use for combustion air.



    G304.6.2 One-permanent-opening method.
    One permanent
    opening, commencing within 12 inches (305 mm) of
    the top of the enclosure, shall be provided. The appliance
    shall have clearances of at least 1 inch (25 mm) from the
    sides and back and 6 inches (152 mm) from the front of the
    appliance. The opening shall directly communicate with the
    outdoors or through a vertical or horizontal duct to the outdoors,
    or spaces that freely communicate with the outdoors
    (see Figure 304.6.2) and shall have a minimum free area of 1
    square inch per 3,000 Btu/h (734 mm2/kW) ofthe total input
    rating of all appliances located in the enclosure and not less
    than the sum ofthe areas of all vent connectors in the space.



    "OR" does not carry the same meaning as "AND"

    Yes, you can have vertical ducts - use the applicable code section.

    Yes, you can have horizontal ducts - use the applicable code section.

    Yes, you can have vertical AND horizontal sections in the same duct - use the applicable code section, which would mean this would need to be engineered, that you are not allowed to apply either the vertical or the horizontal code section (been there, said that before).


    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

  9. #9
    James Duffin's Avatar
    James Duffin Guest

    Default Re: Combustion Air and M1703.2.1

    Since I have been told otherwise by the NC folks I'll go with their interpretation (been there, said that before). And I agree with them. If the code said and you would have to run both vertical and horizontal ducts to meet the code but it says or you can run either or both. Thanks for the input though.

    Last edited by James Duffin; 04-04-2010 at 07:01 PM.

  10. #10
    Join Date
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    Default Re: Combustion Air and M1703.2.1

    Quote Originally Posted by James Duffin View Post
    but it says or you can run either or both. Thanks for the input though.
    James,

    You are correct as far as you are taking your thinking ...

    "but it says or you can run either or both" - yes, you can run one duct horizontally, or, you can run one duct vertically, but you are not allowed to run one duct horizontally and vertically ... the duct must all be the same direction for the length of the duct.

    You could, using what you are thinking, run one duct vertically 5', horizontally 5', then vertically 5' more, then horizontally 5' more ... right?

    Jerry Peck, Construction / Litigation Consultant
    Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )
    www.AskCodeMan.com

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