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  1. #1
    Join Date
    Jul 2009
    Location
    new york
    Posts
    666

    Need some calculation for combustion air and vent sizing

    I'm at a loss here. I'm installing a 150,000 btu furnace with 4 90 degree elbows and have forgot the calculation when you have more than two. Do you multiple .90 by 150,000 btu or is is by 10%. Also i have a room 20x30x8= 4800cu.ft AND 150,000 FURNACE 150,000/1,000x50=7,5000 CUFT. So I need to bring in 2,700 cuft of fresh air and can not remember the calculation. Also when furnace is derated and orifice size is not on chart do you go up or down? Example the orfice size comes out to 27.30 and there is a 26.78 and a 28.23 which would you choose.

  2. #2
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    5,725
    You need to buy a code book. Not all municipalities use the UMC.
    "What Fools these mortals be"....Puck

  3. #3
    Join Date
    Feb 2006
    Location
    Tn.
    Posts
    1,344
    Quote Originally Posted by hvacker View Post
    You need to buy a code book. Not all municipalities use the UMC.
    Good suggestion

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