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  1. #1
    David Block's Avatar
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    Default Combustion Air Source

    I need some help determining the adequacy of a combustion air source. The applicable code would be the 2003 IRC. A gas burning water heater and furnace with combined BTU's of 110,000 located in a confined space accessible only through the garage. There were two combustion air sources noted, both located at the bottom of the space. One air source was a metal louvered grill measuring 14"x6" located in the wood platform the units were sitting on leading to the hallway in the residence. The other air source was a 4" diameter metal pipe dropping down from the attic space and terminating at the bottom of the confined space. I questioned the adequacy and setup of the combustion air sources. The builder keeps stating that the residence was given a certificate of occupancy so everything meets the code requirements rather than actually opening a code book and investigating. This builder aslo tried to tell me that an air gap is not needed at the water heater TPR valve extension so my confidence in his opinion is limited. Thanks for any help you guys can give me.

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  2. #2
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    Default Re: Combustion Air Source

    I'm sure others here will disagree with me but a home inspection is not technically exhaustive. What you are doing is crunching numbers.... I assume you're also crunching all of the calculations for floor loads and roof loads, measuring all of footings to ensure proper size given the loads you come up with, etc., etc.

    I'm not trying to give you a hard time.... but, personally, I don't go down the code road nearly this far.... ever.


  3. #3
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    Default Re: Combustion Air Source

    Not at all. I just happened to notice the setup of this combustion air source and it didn't look right to me so I recommended my clients get further evaluation to ensure it is proper. With both air sources being located at the bottom of the space, I figured it can't be right. When i called this as potentiall being incorrect, the builder went balistic and insisted everything in the home is correct but I had already found at least 5 other deficiencies that did not meet the code requirments and the city did in fact verify I was correct but this last issue seems to be eluding all of us as nobody will make a call if it is correct. Just curious if the setup is correct or not. I've also run into multiple problems in new construction in this area with missing fire walls in garages when bedrooms are located above them, electrical splices not in junction boxes, cross connections etc. My faith in the competency of the city codes inspectors has dwindled dramatically lately so I don't want to miss an important safety issue such as this.


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    Default Re: Combustion Air Source

    Sorry, just re-read the question.... aside from crunching actual numbers.... Do you mean the furance room has one opening to the attic and one to the living space? I think that's a HUGE problem with fire resistance if it's true.

    I'm not sure I know what you mean about 'in the wood platform' - does this mean there's an air exchange between both the living space and the attic, via the utility room?


  5. #5
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    Default Re: Combustion Air Source

    Sorry, let me clarify. When you enter the garage, in the corner there is a closet. There are two steel doors that lead into this closet from the garage with no access through the house to the closet. The doors appeared to be weatherstipped and sealed up pretty tight. There is a wood platform that elevates the water heater and furnace off the ground to meet the 18" requirement. There is a metal louvered grill in the top of the platform (where the units are sitting) that leads into the hallway of the house. There is also a 4" diameter metal pipe coming down from the attic space and terminates at the base of the water heater. Could this be correct? Any ballpark guesses?


  6. #6
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    Default Re: Combustion Air Source

    I'm not following the 'top of the platform' - A platform is horizontal... I don't see how it can have a top. It sounds like there's an air exchange from the house to the attic through the furance closet.


  7. #7
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    Default Re: Combustion Air Source

    David,
    IRC 2003 m1702.2 Confined Space. Where the space in which the appliance is located does not meet the criterion specified in Section M1702.1 two permanent opening going to adjacent spaces shall be provided so that the combined volume of all spaces meets the criterion. One opening shall be within 12 inches (35mm) of the bottom space, as illustrated in figure M1702.2. Each opening shall have a free area equal to a minimum of 1 square inch per 1,000 Btu/h(2201 mm 2/kw) input rating of all appliances installed within the space,but not less than 100 square inches (64415mm2)


    Figure M1702.2 shows openings at the top & bottom.

    If the appliances are direct-vent does not apply. M1701.1

    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  8. #8
    David Block's Avatar
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    Default Re: Combustion Air Source

    Now that you mention it, there does appear to be an air exchange between the living space and the attic space via the furnace closet so this can't be a correct setup that in addition to both of the air sources being located at the bottom of the space. Good point. Thanks. I can't wait to hear what the city has to say about this. I have no interest in doing any kind of calculations or get into a pissing match with a builder but the setup didn't look right so I questioned it and now we keep going back and forth and for the sake of my clients I just want to get the question resolved.


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    Default Re: Combustion Air Source

    Are they using the platform as a return air plenum for the furnace? If so, you can't punch a hole in it since it will be sucking air out of the furnace closet rather than supplying combustion air, a big no - no.

    Without doing the complete math, they are close to the required inches before "derating" to allow for the grill on the platform floor.
    I will bet that they are short after allowing that grill reduction.

    Last edited by Jim Luttrall; 03-17-2008 at 09:42 PM.
    Jim Luttrall
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    Dallas, Texas

  10. #10
    David Block's Avatar
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    Default Re: Combustion Air Source

    I don't think the platform was being used as an air plenum. The code always talking about one air source within 12" of the top and one within 12" of the bottom of the space so doesn't having both at the bottom of the space automatically throw up a red flag?


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    Default Re: Combustion Air Source

    Yep, it does to me.

    Jim Luttrall
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    Default Re: Combustion Air Source

    Quote Originally Posted by Jim Luttrall View Post
    --allowing that grill reduction.
    Metal grill = 75% of area.

    4 in dia = 12.5 sq inches

    14 x 6 = 84 x 75 % = 63 + 12.5 = 75.5 sq. inches total combustion air.

    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  13. #13
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    Default Re: Combustion Air Source

    I'm glad to see that I am getting some agreement that this is an improper setup. I want to provide the best possible service I can to my clients that and I hate being wrong. I hate opening a can of worms but in this case it seemed warranted to me. Thanks for all the input.


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    Default Re: Combustion Air Source

    I must be missing something obvious about this platform.... So, the grill is on the floor (platform) where the furance/water heater sit? How does is pass through to the hallway into the house? I was picturing a furnace room next to the hallway, now I'm picturing it sitting on top of it. Yet at the same level as the garage?

    It must be me since the others seem to have grasped it..... I think I crawled through too much rat feces today


  15. #15
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    Default Re: Combustion Air Source

    A picture would have helped on this i think. Ok, a closet located in the garage shares a common wall with that of the hallway in the home. There is a grill located 6" off the floor through that common wall connecting the furnace closet and the hallway. Now put a two foot high wood box in the closet and put the water heater and furnace on them. Now cut a hole in the top of the box so that it is open to vent provided in the common wall. That is the setup observed. I hope that makes more sense.


  16. #16
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    Default Re: Combustion Air Source

    Quote Originally Posted by Matt Fellman View Post
    ..... I think I crawled through too much rat feces today
    .
    .
    Sweet Dreams.
    .

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    Last edited by Billy Stephens; 03-17-2008 at 10:31 PM. Reason: Exp. provided by David
    It Might have Choked Artie But it ain't gone'a choke Stymie! Our Gang " The Pooch " (1932)
    Billy J. Stephens HI Service Memphis TN.

  17. #17
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    Default Re: Combustion Air Source

    I appreciate everyones input and advice. Today we had the builder, HVAC contractor and city code administrator get together to review the situation and the conclusion they came to was: The lack of a high/low combustion air setup was wrong. The air exchange between the living space and attic space was wrong and the quantity of combustion air was wrong. This was my first confrontation with the city and a builder and fortunately I was correct and get to preserve my reputation for another day. This was a learning experience that I hope I don't have to repeat. Thanks again for all the input.


  18. #18
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    Default Re: Combustion Air Source

    Job well done Pilgrim! Where are you from?


  19. #19
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    Default Re: Combustion Air Source

    First, forget the 4" pipe (I know there is an exception to this) but (unless the exception is met) the *MINIMUM* size of the air opening is 100 square inches. A 4" duct only has about 12 sq inches.

    Let me see if I understand what you are saying:

    1) There is a vent communicating the house space to the closet for combustion air, that vent is within 6" of the floor on the hall side and in the top of the 24" high platform in the closet side.

    If I have that right, then the opening in the closet side is above 12" above the floor and does not meet the requirements for one of the two openings.

    Also, regarding that opening, (and as Billy said) "14 x 6 = 84" so no need to go any further - it also is smaller than 100 sq inches.

    2) There is a 4" duct leading up into the attic, which, as stated above, has less than 100 sq inches - which does not meet the 100 sq inches.

    Okay, let's check on the opening requirements:

    From the 2006 IRC (underlining is mine)

    - G2407.5.3 (304.5.3) Indoor opening size and location.
    Openings used to connect indoor spaces shall be sized and located in accordance with Sections G2407.5.3.1 and G2407.5.3.2 (see Figure G2407.5.3).
    - - G2407.5.3.1 (304.5.3.1) Combining spaces on the same story.


    Each opening shall have a minimum free area of 1 square inch per 1,000 Btu/h (2,200 mm2/kW) of the total input rating of all appliances in the space, but not less than 100 square inches (0.06 m2). One opening shall commence within 12 inches (305 mm) of the top and one opening shall commence within 12 inches (305 mm) of the bottom of the enclosure. The minimum dimension of air openings shall be not less than 3 inches (76 mm).
    - - G2407.5.3.2 (304.5.3.2) Combining spaces in different stories. (left this off as this is not applicable to this installation)

    However, ... that installation is not combining interior spaces alone, it is also combining outdoor air ...

    - G2407.7 (304.7) Combination indoor and outdoor combustion air.


    The use of a combination of indoor and outdoor combustion air shall be in accordance with Sections G2407.7.1 through G2407.7.3.
    - - G2407.7.1 (304.7.1) Indoor openings.


    Where used, openings connecting the interior spaces shall comply with Section G2407.5.3. (Jerry's note: Go back up to what we just went through.)
    - - G2407.7.2 (304.7.2) Outdoor opening location.


    Outdoor opening(s) shall be located in accordance with Section G2407.6.
    - - G2407.7.3 (304.7.3) Outdoor opening(s) size.


    The outdoor opening(s) size shall be calculated in accordance with the following:
    - - - 1. The ratio of interior spaces shall be the available volume of all communicating spaces divided by the required volume.
    - - - 2. The outdoor size reduction factor shall be one minus the ratio of interior spaces.
    - - - 3. The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section G2407.6, multiplied by the reduction factor. The minimum dimension of air openings shall be not less than 3 inches (76 mm). (Jerry's note: IF the calculated opening size allows for 12 sq inches, then the 4" duct will meet this.)

    Which now takes us to G2407.6:
    - 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).
    - - G2407.6.1 ( 304.6.1) Two-permanent-openings method.


    Two permanent openings, one commencing within 12 inches (305 mm) of the top and one commencing within 12 inches (305 mm) of the bottom of the enclosure, shall be provided. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors.
    - - - Where directly communicating with the outdoors, or where communicating with the outdoors through vertical ducts, each opening shall have a minimum free area of 1 square inch per 4,000 Btu/h (550 mm2/kW) of total input rating of all appliances in the enclosure [see Figures G2407.6.1(1) and G2407.6.1(2)].
    - - - Where communicating with the outdoors through horizontal ducts, each opening shall have a minimum free area of not less than 1 square inch per 2,000 Btu/h (1,100 mm2/kW) of total input rating of all appliances in the enclosure [see Figure G2407.6.1(3)].
    - - G2407.6.2 (304.6.2) One-permanent-opening method. (left this off as this is not applicable to this installation)

    Let's do some math:

    Stated facts: "A gas burning water heater and furnace with combined BTU's of 110,000 located in a confined space "

    G2407.6 says "each opening shall have a minimum free area of 1 square inch per 4,000 Btu/h (550 mm2/kW) of total input rating of all appliances in the enclosure", 110,000 / 4,000 = 27.5 X 2 = 55 square inches.


    - M1702.1 Required volume.


    Where the volume of the space in which fuel-burning appliances are installed is greater than 50 cubic feet per 1,000 Btu/h (4.83 L/W) of aggregate input rating in buildings of ordinary tightness, insofar as infiltration is concerned, normal infiltration shall be regarded as adequate to provide combustion air. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors shall be considered part of the required volume.

    G2407.5.3.1 says "The ratio of interior spaces shall be the available volume of all communicating spaces divided by the required volume", and the required volume is from M1702.1 Required volume: . "greater than 50 cubic feet per 1,000 Btu/h", or 110,000 / 1,000 = 110 x 50 = 5,500 cubic feet.

    Now, assuming a typical house's 'openly communicating spaces' (no doors) may be 50% of the living area (just making up a number out of thin air here), and assuming a typical 1,500 sq ft house, leaving 750 sq ft 'openly communicating' (using my 50% guess) times 8 foot high ceilings = 6,000 cubic feet of 'openly communicating spaces', or, 6000 / 5500 = 1.1 is the ratio in G2407.5.3.1 above.

    G2407.5.3.2 says "outdoor size reduction factor shall be one minus the ratio of interior spaces", or, 1 - 1.1 = -0.1.

    G2407.5.3.3 says "minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section G2407.6, multiplied by the reduction factor", or, 55 square inches X 0.1 reduction factor, or, 55 square inches X 0.9 = 49.5 square inches for the opening to the outdoor air (or maybe it is half that as I calculated for 2 openings - I'm getting lost reading it ).

    Okay, presuming that it is only half of the above (for each opening, and there is one opening to the outdoors) then there needs to be an opening to the outdoors of 25 square inches, and the 4" round pipe gives 12 square inches ... notice that by reducing the required sized by half - it is still twice that which is what is provided by the 4" pipe (a 6" diameter pipe would have done it). Presuming *no grille or louver* on that pipe - a metal louver or grille will reduce it to 75% (give or take).

    *IF* I went through that correctly (and I doubt I did), then the interior opening is wrong (too small and too high) AND the outdoor opening is too small and too low.

    ONE of those openings needs to be high and ONE needs to be low - neither is 'high', the 4" pipe is low.



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

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    Default Re: Combustion Air Source

    Quote Originally Posted by David Banks View Post
    Job well done Pilgrim!
    Ditto.

    Took me so long to go through the code and type my post above that David had posted his results before I even finished my post.

    Jerry Peck, Construction / Litigation Consultant
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  21. #21
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    Default Re: Combustion Air Source

    Jerry, that was a fantastic and time consuming response. You must either be bored, have time to kill or really like to help other people. Regardless, I appreciate the response. What was determined is exactly at you said, the location and quantity of combustion air was incorrect eventhough the HVAC contractor and city code inspector approved the installation. What they decided to do is get rid of the attic pipe, install new larger high and low grills in the mechanical room that lead into the...I guess it was the laundry room...then in the laundry room they will install high and low grills that communicate to the main portion of the residence as this too would have been deemed a confined space. I'm glad we got the problem resolved with my clients but this was a builder who had completed approximately 40 units in the this developement assumably with the same setup. My concern is now that each one of these other units will have the exact same problems as the one I just inspected. I know its not my business but shouldn't the city now question the previous installations and maybe contact the owners? Wouldn't the builder be prudent to be proactive and maybe make repairs to units already completed now that a known problem has been disclosed and opens the door for litigation should a problem arise in the other units? Just my thoughts. Thanks for the help.


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    Default Re: Combustion Air Source

    Quote Originally Posted by David Block View Post
    but this was a builder who had completed approximately 40 units in the this developement assumably with the same setup. My concern is now that each one of these other units will have the exact same problems as the one I just inspected. I know its not my business
    Did that come up when the city people were there?

    If not, you should bring it up to them.

    If yes, then ...

    shouldn't the city now question the previous installations and maybe contact the owners?
    Yes, but not real likely to happen without some prodding from you (not prodding the city inspectors who came out, you are making them your friends, but my contacting the homeowners and doing a sales pitch for inspections, which includes something like 'One example of the items I found was blah-blah-blah and the city came out and agree, the builder is now correcting it').

    Wouldn't the builder be prudent to be proactive and maybe make repairs to units already completed now that a known problem has been disclosed and opens the door for litigation should a problem arise in the other units?
    Yes. But ...

    "be prudent to be proactive"

    The builder is going to throw more good money away, after having to fix that ONE house?

    Not likely.

    "be prudent" - Yes.

    Be responsible - Yes.

    Likely to happen - No.

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

  23. #23
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    Default Re: Combustion Air Source

    Jerry, I would have to agree with you. The builder is not likely to do anything else. This subject of other units having the same problem never came up. I think what happened was the city inspector royally screwed up so the codes administrator wanted to quickly resolve the issue at hand and step back out of the situation. I spoke to several of the cities code inspectors and none of them really had a grasp on the code except for the administrator. Everyone in the office was dragging their feet on the issue until the administrator had time to dig into the problem and come up with an answer. It really made me question the competency of their personnel and to be honest it was a little frightening. When that many people can look at a bad setup and think that its alright, that is scary. I don't even want to know what else they missed or allowed since I only came in on the tail end of the deal. I was thinking about putting together a letter and sending it to the other owners detailing the findings and offering to inspect for these same conditions in their units. Still not decided if I want to go that route. I may be a little naive but where I grew up, your word and integrity were everything. Even if you had to spend a little bit more time or money to make something right....you did it. Your quality of service was more important than your profit margin. I know we all need to make a living and reduce our liabilities but at what point do you sacrafice your principles and integrity to make a buck? Just my thoughts.


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