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Combustion Ait - 1 opening or 2 openings



  • Chuck_17Chuck_17 Member Posts: 119

    The fuel gas code has two methods for openings for combustion air:
    304-11.1 Two openings, one high and one low
    304.11.2 One opening high
    I was wondering what the rationale was for chosing.
    I assume that the high and low opening method is to help control heat in the boiler room but do not really know this to be true.
    I prefer the one opening for larger rooms because it is easier to control the air (put a damper to close when not firing). I think most modern boilers do not loss as much heat to the room. Also smaller insulated piping will not contribute to the heat in a room.
    Can anyone offer any insight to this?
  • Chuck_17Chuck_17 Member Posts: 119

    Nice spelling!

    Hi and low combustion air vents has nothing to do with heat in the room. Hi and low vents are intended to keep the room from becoming negative.
  • Brian_24Brian_24 Member Posts: 76

    The high opening is intended to let carbon monoxide out and the low opening is there to let carbon dioxide out. However the true purpose has been misinterpeted in the code books. JMHO.
  • Rookie_3Rookie_3 Member Posts: 244

    If you had a sealed room with no vents allowing air to enter the room what would happen? As Steve said, negative.
    The vents are to bring in air for proper combution.

  • jim lockardjim lockard Member Posts: 1,059
    No Brian

    The high /low vents are for fresh combustion air. Remember it takes about 10 cubic feet of air to burn a cubic foot of Nat gas. The chimney/flue is where the products of combustion go, hopefully CO2 and water vapor on a clean burn. Brian CO has a spec. gravity of .97 so if CO should be present and we hope not it will just kinda hang in midair so to speak. Best Wishes J.Lockard
  • Robert O'Connor_12Robert O'Connor_12 Member Posts: 724

    Section 304.6.1 Two-permanent-openings method. of the 2003 International Fuel Gas Code dictate that the two openings located as prescribed are intended to induce a convective air current in the room by admitting cooler, denser air in the lower opening and allow the escape of warmer, less dense air through the upper opening. The farther apart the openings, the greater the temperature differential and the greater the convective force behind the current. A component of combustion air is cooling (ventilation) air for the appliance enclosure in addition to supplying combustion air. This ventilation cools the appliance and would help remove any combustion gases that spilled from the appliance.

    The one-permanent-opening method. or section 304.6.2 of the 03 IFG Code shows that research has shown that for modern a modern appliance, a single opening to the outdoors will perform as well as the traditional two opening method. The one-opening method described in the 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. The opening must 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 fewer objection by the owners/occupants.

    Robert O'Connor/NJ
  • Mark HuntMark Hunt Member Posts: 4,909
    You have to put

    arrows on the grills so the air knows which way to go.

    Sorta' like those signs Mass. requires now over vent terminations. Keeps the snow from blocking them.

    Arrows point the way. In fact, I would bet $100 dollars that none of the "leaky vents" that caused Co poisonings had arrows on them.

    Ok. Enough sarcasm.

    Is it not possible that cutting "vents" into a mechanical room could actually cause a problem?

    How about the job I did a couple years ago where code required we cut a 24" x 24" opening in the door to the basement for "combustion air"? the "code" did not consider the exhaust fans in the fried chicken joint at the top of the stairs. They caused such a depressurization that the unit would not draft but "code" said it had to be there.

    We refused. The "code" be damned in that case. Let the "code enforcers" go cut that hole.

    Mark H

    P.S. Here is a fool-proof way to prevent back drafting, "puff-back' or spillage.

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Boilerpro_3Boilerpro_3 Member Posts: 1,231
    One opening no longer allowed

    In the more recent International Codes, only two openings. I expect that two openings would sure help for natural gas leak or CO ventilation, though.

  • DaleDale Member Posts: 1,317
    laminar flow

    Turns out that the high opening allows air to flow both ways at the same time, the hotter product from the blocked flue out the top and the inlet replacement cooler air in at the bottom. Saw a nice smoke demo of this by a guy who does research for the NFPA code. That said 2 openings are a good idea. What I have seen is that if I'm lucky enough to get one opening it's usually lower.
  • Robert O'Connor_12Robert O'Connor_12 Member Posts: 724

    Which state are you in? In the Great State of New Jersey the 2003 IFG & Mechanical Codes are what is currently adopted. I particularly like the "I" codes for it, IMHO truly closes that door of interpretation that has many a mechanic scratching their heads as well as inspectors alike.

    I belive two openings are favored in any application concerning combustion air requirements but this (code section) focuses on "modern equipment" and not existing installations.

    The problem as I see it is that you can have say a fire inspector or building inspector make you seal up the envelope tighter than a drum (adhearing to the Energy Code) and then a mechanical or plumbing inspectors applauds your efforts and then makes you cut a huge hole in the side of your building for the combustion air requirements, whatsupwitdat? The devil is in the details as it pertains to adhearing to the code yet making said installation not only pleasing to the eye from the homeowners perspective but safe and mechanically sound and above all "code compliant".

    Robert O'Connor/NJ
  • jerry scharf_3jerry scharf_3 Member Posts: 419
    Your gas must be smarter than mine


    I haven't seen any indication that my gas can read. They must have some kind of mandatory reading course in your neck of the woods. Have you checked whether your heat can read as well? Could be a real revolution in insulation practices.

    My gas is dumb and lazy. Maybe I can import some of your gas and set up a gas school.

    (tongue firmly in cheek.)

  • WeezboWeezbo Member Posts: 6,232
    Lets Hear it for Reason and standards over code...

    sure 40 years ago the code was adopted anyone suggest recently we need these codes bought into This century?

    and lets start" easing "the idea forward, that our ticket is to inform and educate not the blind acceptance of code without consideration.:)
  • Chuck_17Chuck_17 Member Posts: 119

    I would use direct vent where possible to avoid the issue but: Boiler rooms are still put in basements and direct venting is difficult (esp. in bigger buildings). For more or less the same reason it is also hard to get the combustion air into the basement (of course this has been the situation for 100 years).
    In NY State the the one opening method is in the Fuel Gas Code but not the Mechanical Code.
  • Mitch_6Mitch_6 Member Posts: 549
    Enough on the ability of flue gas to read

    put a fan in the can in, presurize the room and be done with it.

    As to the job with exhaust fan, Mark, I do believe that commercial fans pulling X amount of air have to provide for make up air themselves especially due to the cooking equipment using up air for their combustion. Therefore the chicken joint should have been bringing enough fresh air to run there cooking equipment and vent hoods.

    Of course easier said than enforced.

    Mitch S.
  • Chuck_17Chuck_17 Member Posts: 119

    Just so I know - what is "fan in a can"?
    Why I would not pressurize the room - Assuming the room is tight you are essentially pressurizing the b-vent.
  • Mitch_6Mitch_6 Member Posts: 549
    Fan in can is

    a fan in a can attached to duct work running to the out side. It is wired into the burner controls and turns on when the gas valve opens. It brings in fresh air but only when you need it so you do not over cool the room like with an open duct.

    Does not really "pressurize" the room but helps with draft and combustion air better than any other item I know.

    See fan in drum is what they call it they also have other devises for fresh air some use the furnace return on hot air systems. Lots of fun stuff.

    Mitch S.
  • Larry FLarry F Member Posts: 25
    Does it prove?

    If mechanical ventilation is used, it must be proved to be moving air before initiating a burner cycle. Simply powering the fan at the same time as the fuel-burning equipment doesn't qualify as "interlocking". Does fan in a can do this? Or is it meant to be used in a duct run to boost air flow?
  • Mitch_6Mitch_6 Member Posts: 549

    the unit can be wired several ways, the wiring diagrams are very basic and need a little interpretation. But when you have a call for the gas valve to fire it goes to the fan in can first, once it proves it closes the connection to the gas valve.

    Again wiring can be an art on some systems so check your work, sometimes you have to call both the tech support of Fields and the Equipment manufacturer to get it strait.

    Mitch S.
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