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Combustion air opening

Dennis
Dennis Member Posts: 101
in THE MAIN WALL
to determine the cubic feet per minute, then get a ductsize calculator from any supply house, and determine your equivilent length and make the duct to that size. Grill size will be determined by net face area. Just make it eazy and cover it with hardware cloth with 1/2 inch openings.

Now if you want to save a lot of time just nail the window open, or if in the ghetto just bust it out, but leave the bars in place.

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Comments

  • MagicJS
    MagicJS Member Posts: 3
    Combustion air opening

    Hi, I'm hoping someone here can clarify NFPA 54 requirement for outdoor combustion air for me.

    Section 8.3.3 calls for openings in boiler room enclosure of 1 sq.in./2000,3000,4000 Btu/hr of input depending on method. The openings shall communicate directly, or by ducts, with the outdoors or spaces that freely communicate with the outdoors.

    While opening sizes in the boiler room is quite clear, what's not clear is the area required for the connecting duct and louver. Someone said to size them at 12 CFM/HP at .005" pressure drop, someone else emailed me a chart of manufacturer's recommendations, another said to use same area as required openings in the boiler room.

    Need a little guidance here on code compliance. Thanks.
  • Timco
    Timco Member Posts: 3,040


    "Metal louvers & grilles will have 75% free area." See attached sheet.

    Tim
    Just a guy running some pipes.
  • Brad White_30
    Brad White_30 Member Posts: 26
    My experience in reading

    NFPA and it's almost verbatim counterpart as the Massachusetts Fuel Gas Code, states that louvers shall be assumed at 30% free area. (Must be in case of wood louvers is my guess or in cases of unknown louver manufacturer). This also means that your louver is to be "assumed to be" at that free area unless you can demonstrate otherwise the net free area is maintained.

    This means if your duct is 75 square inches your louver gross area must be at least 75/0.30 or 250 square inches to obtain net free area.

    The 12 cfm per boiler horsepower (BHP=33,475 BTUH net) is reasonable. Cleaver-Brooks used to advocate 10 and that was borne out by stoichiometric calculations, including 20% excess air and some room for boiler room general ventilation.

    To me the best deal going is an Exhausto combustion air unit. Maintains a 0.05" positive pressure in the boiler room using a variable speed fan. And how cool is that?

    Brad
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