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  1. #1
    dan orourke's Avatar
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    Default Furnace combustion air vent

    Last edited by dan orourke; 12-26-2007 at 09:56 AM.
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    Default Re: Furnace combustion air vent

    It's also too small to serve as a combustion air vent.

    100 square inched minimum net opening size.

    An 8" diameter duct has approximately 8 / 2 = 4 x 4 = 16 x 3.14 = 50 square inches - less than half the minimum size required.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Furnace combustion air vent

    Dan,

    You are correct, two separate ducts, one low, one high.

    Eric Barker, ACI
    Lake Barrington, IL

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    Default Re: Furnace combustion air vent

    100 square inched minimum net opening size.
    Jerry, I have always just accepted your statement that the minimum size combustion air vent is 100 Sq In... but after re-reading the IRC 2003, I can't find that requirement except in G2407.5.3.1 Combining spaces on the same floor which is in the indoor combustion air section.
    Are you sure that the minimum 100" rule applies to outdoor combustion air?
    Of course I am just looking at the IRC since that is the code I am familiar with.

    Comments please?

    Jim Luttrall
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    Default Re: Furnace combustion air vent

    Also on the combustion air subject, the IRC gives the option of "one-permanent-opening method" in G2407.6.2 when providing outdoor combustion air.
    It does add some stipulations as to clearances and increases the area of the vent to 1 inch per 3,000 BTU.
    Is this one opening provision unique to the IRC? I see several stand hard and fast on the two vent requirement...
    Any comments?

    Jim Luttrall
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    Default Re: Furnace combustion air vent

    Quote Originally Posted by Jim Luttrall View Post
    Jerry, I have always just accepted your statement that the minimum size combustion air vent is 100 Sq In... but after re-reading the IRC 2003, I can't find that requirement except in G2407.5.3.1 Combining spaces on the same floor which is in the indoor combustion air section.
    Are you sure that the minimum 100" rule applies to outdoor combustion air?
    Of course I am just looking at the IRC since that is the code I am familiar with.

    Comments please?


    There are several methods, of which the '1 square inch per 1,000 Btu/hr' does not apply to all methods either.

    This is where I have to go "Oops." because that (the 100 square inch minimum size) usually comes up along with the '1 square inch per 1,000 Btu/hr, and which the openings must be within 12 inches of the floor and within inches of the floor, and also be a minimum of 3 inches in dimension - and I forget to always read the entire rest of the question ... my mistake.

    You are correct - The 100 square inch minimum size does not apply to all methods, it does, however, apply to both 1) "Indoor opening size and location." "Combining spaces on the same story.", and, 2) "Combination indoor and outdoor combustion air." "Indoor air openings." the 100 square inches does not apply, only the 3 inches minimum dimension does, and the size/Btu/hr ratio also changes from method to method.

    The "One-permanent-opening method." does not apply due to other requirements within that method.

    'G2407.7 Combination indoor and outdoor combustion air' does, for 'Outdoor opening(s) size' shall be calculated based on one of three methods, with the 'Indoor openings' size based on that 100 square inches minimum and 3 inches minimum dimension. So ... the "Combination indoor and outdoor combustion air" method is not being used either.

    So far we have, for 'methods not being used':
    - G2407Combination.5.3 (304.5.3) Indoor opening size and location.
    - G2407.6 (304.6) Outdoor combustion air. / G2407.6.1 ( 304.6.1) Two-permanent-openings method.
    - G2407.6 (304.6) Outdoor combustion air. / G2407.6.2 (304.6.2) One-permanent-opening method.
    - G2407.7 (304.7) Combination indoor and outdoor combustion
    air.

    Which leaves us with:
    - G2407.8 (304.8) Engineered installations.
    - G2407.9 (304.9) Mechanical combustion air supply.


    If we make an assumption that it is not the latter of those two ...
    ... it does not meet any of the above ...
    ... then which method is it?

    Or shall we just say that it does not meet the requirements for any approved method?

    Sorry for the thread drift after answering your question.



    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Furnace combustion air vent

    Jim,

    I should clarify that my comments are based upon our local utilitys' requirements. They will allow one source - being a fully louvered door. I have heard of the one source requirements that others have posted but actually I don't know how valid they were.

    Eric Barker, ACI
    Lake Barrington, IL

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    Default Re: Furnace combustion air vent

    Eric,
    IRC section 2407.6 Outdoor combustion air. Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with section G2407.6.1 or G2407.6.2. The minimum dimension of air openings shall be not less than 3 inches.

    G2407.6.1 gives the two permanent openings method that every one is famliar with.

    G2407.6.2 One-permanent-opening method. One permanent opening, commencing within 12 inches or the top of the enclosure, shall be provided. The equipment shall have clearances of at least 1 inch from the sides and back and 6 inches from the front of the appliance. The opening shall directly communicate with the outdoors... ..and shall have a minimum free area of 1 square inch per 3,000 BTH/H of the total input rating of all equipment located in the enclosure, and not less than the sum of the areas of all vent connectors in the space.

    Now, that brings up another question. "... the sum of the areas of all vent connectors in the space." ??? Easy enough on the surface, am I missing something?

    Jim Luttrall
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    Default Re: Furnace combustion air vent

    Quote Originally Posted by Jim Luttrall View Post
    G2407.6.2 One-permanent-opening method. One permanent opening, commencing within 12 inches or the top of the enclosure, shall be provided. The equipment shall have clearances of at least 1 inch from the sides and back and 6 inches from the front of the appliance. The opening shall directly communicate with the outdoors... ..and shall have a minimum free area of 1 square inch per 3,000 BTH/H of the total input rating of all equipment located in the enclosure, and not less than the sum of the areas of all vent connectors in the space.

    Now, that brings up another question. "... the sum of the areas of all vent connectors in the space." ??? Easy enough on the surface, am I missing something?
    Jim,

    "and"

    That little bugger of a word is in there.

    - G2407.6 (304.6) Outdoor combustion air.
    Outdoor combustion air shall be provided through opening(s) to the outdoors in accordance with Section G2407.6.1 or G2407.6.2. The minimum dimension of air openings shall be not less than 3 inches (76 mm).
    - - G2407.6.2 (304.6.2) 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 G2407.6.2) and shall have a minimum free area of 1 square inch per 3,000 Btu/h (734 mm2/kW) of the total input rating of all appliances located in the enclosure and not less than the sum of the areas of all vent connectors in the space.



    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Furnace combustion air vent

    That was my point, the "and"
    ... the sum of the areas of all vent connectors in the space
    I could hardly imagine a instance in a standard installation where the BTU calculation would not exceed the area of the connectors. Is this maybe just in case a vent connector is in place for a future appliance installation?
    Just brain storming here.

    Jim Luttrall
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    Default Re: Furnace combustion air vent

    Quote Originally Posted by Jim Luttrall View Post
    Is this maybe just in case a vent connector is in place for a future appliance installation?
    If the vent connector is installed for future use, and not really connected to a vent, then it is 'not yet' a "vent connector", is it?

    The way I take that is meaning that, let's say you have two appliances, one natural drafting and one direct vent, only the natural drafting appliance needs combustion air, right?

    But, what if the direct vent appliance is replaced with a natural draft appliance later?

    Adding in "both" (that "and") of the vent connectors would mean the combustion air would need to be larger, right? Now, I am sure I will be hammered on this because I have not run across direct vent appliances and do not know for sure how they are vented and what they connect with (i.e., are they connected with "vent connectors"?). I hope that after being hammered that I will understand them better (just in case I ever see one).

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Furnace combustion air vent

    You know, looking back, the last time we discussed this some questions were left hanging:

    Combustion Air - InspectionNews - Home Inspection
    http://www.inspectionnews.net/home_i...stion-air.html


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    Default Re: Furnace combustion air vent

    For starters, does everyone agree these are the current standards?

    http://fcgov.com/nbs/pdf/basement.pdf

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    Default Re: Furnace combustion air vent

    Michael,

    That attached chart, is that yours or from somewhere else?

    It only designates "round" supply duct.

    It leaves out combustion air from indoors, same story, different stories, and combination indoor and outdoor, lumping them into one category (indoor) while each is different. You have 'outdoor 1' and 'outdoor 2', you should have 'indoor 1', 'indoor 2', and 'indoor 3'.

    Also, re-organizing the table to follow code section numbers would make it easier to look up and verify exact wording should the need arise.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Furnace combustion air vent

    Jerry,

    It's from the URL above the chart.


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    Default Re: Furnace combustion air vent

    Michael, you have a better memory than I to even remember we discussed that, much less find the threads. I just knew I still had some questions about the subject.
    Your info from Ft. Collins source looks like it matches what I posted, both are supposed to be from 2003 IRC.

    Jerry, you are on the same general track as my thoughts, that the requirement taking in the vent connectors into the minimum combustion air size calculation is to prevent future problems.

    Anyone else have an idea (maybe from the code commentaries) on the reason behind this?
    Having a very linear thought process, it helps me to know the "why" behind every rule.

    Jim Luttrall
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    Default Re: Furnace combustion air vent

    Quote Originally Posted by Jim Luttrall View Post
    Anyone else have an idea (maybe from the code commentaries) on the reason behind this?
    Jim,

    This is all the Commentary says on that section:

    Research has shown that for modern appliances, a single opening to the outdoors will perform as well as the traditional two opening method. The one opening method described in this section depends on a reduced pressure being created in the enclosure by the draft created by the venting system. This reduced pressure causes combustion air to enter the enclosure through the single opening. It is important that the opening be properly sized considering both sizing criteria: the square-inch-area-per-Btu/h ratio and the area minimum based on the sum of the areas of all vent connectors in the enclosure. This method allows for fewer openings, fewer ducts, and less objection by the owners/occupants.


    Jerry Peck, Construction / Litigation Consultant
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  18. #18
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    Default Re: Furnace combustion air vent

    Thanks Jerry.

    I am really wondering now what the reason is for the two opening method. It would give a redundancy factor, but most times when one opening is blocked, the other is also. Of course this is purely an academic exercise since both are allowed by the IRC.
    Most of what I see is a ceiling to attic vent with either one or two openings. Lots have the stud cavity used for the lower vent which may just be used to avoid having to provide more room for clearance in the mechanical closet.

    Jim Luttrall
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  19. #19
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    Default Re: Furnace combustion air vent

    ... and then there is the metal flex or even hard-pipe duct that is dropped from ceiling to lower 1/4 or 1/3 of the closet. It is routed through the ceiling immediately adjacent to the screened-in vent in the ceiling.


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