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  1. #1
    Robert Olson's Avatar
    Robert Olson Guest

    Question Combustion Air Ducts

    I'm running into a brick wall with a municipality here in Texas regarding combustion air for a gas fired 40,000 Btu/H hot water heater. Their ordinance mandates 2 four inch pipes or tubes as they call them to run from the water heater closet into the attic and screened where the pipes terminate in the attic. I told the inspector he was three cans short of a six pack because the gas code prohibits screening and that since the screening has an open space of 70%, the four pipe only allow for 8-3/4 sq in of space (a 30% blockage). The heater requires 10" of area (1 in per 4000 Btu/H rating). Rather than argue, I cut two openings 12" x 12" each. One is in the door and the other above the door leading into the closet to use indoor air space for combustion. I have the requirements for the CF space of 50 CF per hour change in the kitchen alone. Needless to say he red tagged and wrote me a citation. If the gas code restricts screening shouldn't it be the more restrictive of codes when in conflict?
    Have any of you folks ever run into this problem or a brain dead code officer before?

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  2. #2
    Michael Schirmer's Avatar
    Michael Schirmer Guest

    Default Re: Combustion Air Ducts

    Is the location of the water heater in a utility area adjacent to a bedroom or in an area where they are trying to isolate the water heater from living space?


  3. #3
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    Default Re: Combustion Air Ducts

    Quote Originally Posted by Robert Olson View Post
    ...I have the requirements for the CF space of 50 CF per hour change in the kitchen alone....
    I'm not familiar with "50 CF per hour change". Are you referring to 50 CF per 1,000 BTU input?

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
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    Default Re: Combustion Air Ducts

    He is reffering to the method in which the minimum required volume of air is calculated.
    Using the standard method, the minimum required volume of indoor air must be at least 50 cu ft. per 1000Btu/hour.
    From the National Fuel Gas Code, if using two spaces to meet this requirement (the room the appliance is in, plus an adjoining room), there must be two openings between the rooms and, the openings to connect the spaces must each be 1 sq. inch per 1000 Btu/hr, but not less than 100 sq. in.
    For a 40K Btuh water heater, that would be two openings of at least 40 sq. in., but because of the 100 sq. in. requirement, the minimum would be 100 sq. in.
    Keep in mind that if the connecting room(s) contain other gas appliances, they must be considered as well.


  5. #5
    Robert Olson's Avatar
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    Default Re: Combustion Air Ducts

    Michael - The closet opens directly into the kitchen.
    John - You are correct that it's 50 Cf per 1000 Bty/H in that a 40000 Btu/H unit needs 2000 CF of space to allow for a minimum air change per hour (ACH) of > than .40 per 304.5 and 304.5.1.


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    Default Re: Combustion Air Ducts

    (underlining is mine)
    Quote Originally Posted by Robert Olson View Post
    I'm running into a brick wall with a municipality here in Texas regarding combustion air for a gas fired 40,000 Btu/H hot water heater. Their ordinance mandates 2 four inch pipes or tubes as they call them to run from the water heater closet into the attic and screened where the pipes terminate in the attic.

    Regardless of what the code says, and what common sense says, "their ordinance mandates" ... means you have to do that.

    Also, unless deleted out by their amendment/ordinance, you would need to provide the required combustion air to meet the fuel gas codes ... ALSO.

    This is not a case where "the most restrictive shall apply", this is a case of "both shall apply".

    The only way to beat that is to convince the building department that the local ordinance is less safe, and could lead to death, than the requirements of the code, in which case the building department would prevail upon the city and vacate the ordinance.

    The way to address that is, by mandating a less-than-meets-code requirement, the city, the building department, the building official himself, and possibly even the commissioners themselves, could end up on the hook without the city attorney defending them because their action stepped outside their authority.

    Word that carefully and present it to the city attorney, any reasonable good and aware city attorney is going to go 'Oh, I see what he is saying, and that is not a good thing, we need to correct this in short order.'

    Short of doing that, you will need to meet both requirements.

    Jerry Peck, Construction / Litigation Consultant
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  7. #7
    Robert Olson's Avatar
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    Default Re: Combustion Air Ducts

    Jerry,
    While I agree with you, members of the council, who also act as the builidng appeals board, have basically said "It's their way or the highway" and while I agree both codes must apply, it's impossible.
    I have written a letter to the city attorney and after talking with the plumbing inspector, she sees nothing wrong with the ordinance. Problem was the ordinance was written by the same plumbing inspector. I even included a grammar school equation to show the allowable areas of free space to no avail. It's really hard to talk to someone who's three puppies short of a pet shop when dealing with design specs.

    I hoping the judge will accept the testimony of two mechanical engeeiner buddies of mine when they explain that ordinances can be enacted using an arbitrary duct size that isn't large enough to do the job and that indoor combustion air is just as acceptable in this case. Thanks for the advice


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    Default Re: Combustion Air Ducts

    Question: The issue in question calls for duct/tubing run from somewhere near a combustion area through a floor/ceiling into the attic, screen over opening in attic. How does this get by fire code? Look like a direct fire path from lower floor into the attic to me.

    The above statements are expressed solely as my opinion and in all probability will conflict with someone else's.
    Stu, Fredericksburg VA

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    Default Re: Combustion Air Ducts

    Quote Originally Posted by Robert Olson View Post
    While I agree with you, members of the council, who also act as the builidng appeals board, have basically said "It's their way or the highway" and while I agree both codes must apply, it's impossible.
    It's not impossible, that is what I am saying.

    Put in the vents like you did, that meets the fuel gas code requirements, put in their pipes through the attic and screen them, that is also okay because in your meeting the combustion air requirements with the two vents you installed, there is absolutely no need for the other vents, so, you could even lay a piece of drywall over them and still meet the codes requirements, and, in fact, after the local yokels make their inspection, close off those non-functioning pipes.[/quote]

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion Air Ducts

    Quote Originally Posted by Stuart Brooks View Post
    Question: The issue in question calls for duct/tubing run from somewhere near a combustion area through a floor/ceiling into the attic, screen over opening in attic. How does this get by fire code? Look like a direct fire path from lower floor into the attic to me.

    Stuart,

    Nothing prohibits that, there are no fire codes for IRC dwellings.

    It does, however, in my opinion - which I have mentioned many times before and like will again, a violation of the energy codes.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion Air Ducts

    Quote Originally Posted by Jerry Peck View Post
    Stuart,

    Nothing prohibits that, there are no fire codes for IRC dwellings.

    It does, however, in my opinion - which I have mentioned many times before and like will again, a violation of the energy codes.
    Thanks Jerry. I was going to change Fire Code to R602.8 Fireblocking BUT the 2006 IRC leaves the hole open by stating the firestop is to be installed AROUND any pipes, vents, ducts, cables, and wires. It's alright to provide a direct open path into the attic but for heavens sake don't let anything travel around it. Why the whole house might burn down. IF the ICC had just left the opening paragraph of R602.8 I think it would be covered.

    The above statements are expressed solely as my opinion and in all probability will conflict with someone else's.
    Stu, Fredericksburg VA

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    Default Re: Combustion Air Ducts

    Quote Originally Posted by Stuart Brooks View Post
    Thanks Jerry. I was going to change Fire Code to R602.8 Fireblocking BUT the 2006 IRC leaves the hole open by stating the firestop is to be installed AROUND any pipes, vents, ducts, cables, and wires. It's alright to provide a direct open path into the attic but for heavens sake don't let anything travel around it. Why the whole house might burn down. IF the ICC had just left the opening paragraph of R602.8 I think it would be covered.
    Stuart,

    R602.8 does not apply there, you could leave the entire ceiling open (but who would want to, and that would be a major violation of the energy code).

    R602.8 does not apply because: (see underlining)

    - R602.8 Fireblocking required. Fireblocking shall be provided to cut off all concealed draft openings (both vertical and horizontal) and to form an effective fire barrier between stories, and between a top story and the roof space. Fireblocking shall be provided in wood-frame construction in the following locations.


    That is referring to, for example, the concealed stud cavity, any and all pipes going up through it need to be sealed around. The closet for the water heater is not a "concealed" area, so you could leave the ceiling out (except for the energy code and personal comfort, etc., but, for combustion air, leaving the ceiling out and opening the closet to the entire attic would "meet code" ).



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  13. #13
    RANDY NICHOLAS's Avatar
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    Default Re: Combustion Air Ducts

    Robert
    ,
    The ducts are to be extended into the attic space (above the insulation level) down to within 12 inches of the heater closet floor (near the burners of the heater).
    The screen *hardware cloth* goes over the ducts in the attic, to prevent critters and/or insulation from entering the closet.
    This will draw the attic air to the burner of the water heater.

    Close the opening in the door.
    Do not use the living space air as combustible air to the water heater.


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    Default Re: Combustion Air Ducts

    Quote Originally Posted by RANDY NICHOLAS View Post
    The screen *hardware cloth* goes over the ducts in the attic, to prevent critters and/or insulation from entering the closet.
    Randy,

    Except that it is not allowed to place screen of any type over the top of those combustion air ducts.

    The reason being that is it much better to have debris (especially insulation) fall through the ducts and into the room than to block off the combustion air.

    The fuel gas code is specifically looking at the safety aspect of the combustion air requirements and ANY screen will not only reduce and restrict the combustion air duct size, but could allow for the eventual blocking up of the ducts, resulting in improper combustion, which could lead to loss of life.

    All other factors, such as rodents, critters, etc., are not even considered by the safety aspect of requiring them to be open.

    Then, of course, there is the energy code aspect which would not allow them at all.

    The best solution, then, is to either take them directly outside through horizontal ducts (which would not need the screens) or to somehow try to use the structure's interior space as combustion make up air.

    However, with modern structures being as tight as they are now, the option to use the structure for make up air is most likely gone, due to what is now 'unusually tight construction' - which is most homes today.

    Which just complicated matters.

    Jerry Peck, Construction / Litigation Consultant
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    Default Re: Combustion Air Ducts

    Rather than write letters and involve attorneys, I would have just put in the 4" ducts with screens, and then remove the screens after the inspection was completed.

    From an overall energy efficiency, I would rather pull the combustion air from the attic than from inside the kitchen. You'll add to the negative pressure of the house, drawing unconditioned air into the house through any openings.

    Jim Robinson
    New Mexico, USA

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    Exclamation Re: Combustion Air Ducts

    You could also install a powerd MUA unit but that's tough with a WH since you need a means of sensing when its firing. Perhaps a heat sensor?

    Passive MUA does NOT work---ASHRAE published a study, which is available on the web. The hi/lo vents do not work. Those passive vents in exterior walls do not work. There are two functional means of MUA that actually work: large enough rooms and powered MUA.

    You could write up a report to the State Fire Marshal's Office and the ICC. If a local ordinance is overly restrictive or unsafe, they have the power to over rule.

    That 70% restriction by screening is being generous. As soon as it loads up with dust bunnies, all air flow will cease. If the attic insulation is blown in, that will happen that much faster.

    Keep the fire in the fireplace.

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    Default Re: Combustion Air Ducts

    Quote Originally Posted by Bob Harper View Post
    You could also install a powerd MUA unit but that's tough with a WH since you need a means of sensing when its firing. Perhaps a heat sensor?

    Passive MUA does NOT work---ASHRAE published a study, which is available on the web. The hi/lo vents do not work. Those passive vents in exterior walls do not work. There are two functional means of MUA that actually work: large enough rooms and powered MUA.
    Bob is dead on as usual.

    The water heater could be equipped with a gas pressure switch that would make a set of dry contacts to energize a fan when the gas valve makes.

    Field Controls makes a kit that has everything that is needed to do this with a "fan in the can".

    Here's a pdf of that ASHRAE report Bob mentioned.

    We assume way too much when it involves air movement, like that it can read code books.

    Measured Performance more than just a buzzword

  18. #18
    Harry Settle's Avatar
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    Default Re: Combustion Air Ducts

    First off, everything depends on what code, if any, the state or municipality has adopted. ie International or National Fuel Gas Code. Many municipalities have not adopted anything but their own rules.

    Secondly, they can have rules equal to or stricter than code, but not looser than code.

    Thirdly, utilities, such as the gas company can adopt and use code even if the municipality hasn't. They don't have to allow subcode conditions to exist even if the municipality insists. In many areas, the utility companies are the only entities enforcing any real code.

    Fourthly, and I am on shaky ground here, as an HI, you make reports on real or perceived violations as you see them, but you don't enforce code. (I am definately ready to be schooled on this last statement)


  19. #19
    Christopher Gorton's Avatar
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    Default Re: Combustion Air Ducts

    If you interpret the screen as required, how about extending the combustion air chase up 2 feet and putting a 90 degree bend in there and then the screen?
    You then have your animal screen and nothing should fall down the intake hole.
    No horizontal screen on air intakes should be permitted. (opinion)


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    Default Re: Combustion Air Ducts

    On what grounds did they issue you a citation?

    Dylan Whitehead

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    Default Re: Combustion Air Ducts

    Have you considered contacting the Texas State Board of Plumbing to get there thoughts on this? Sometimes they if there is a major issue they will also contact the Inspectors to discuss the issue.

    Dylan Whitehead

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