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  1. #1
    Join Date
    Aug 2007
    Posts
    29

    'Nuther H. O. question....

    ....Hello, again. Finally got the old 80%, 24 year old furnace replaced this winter with a 95%, 2 stage valve, outside air intake, non-variable speed blower. House heated just fine. Gas bills not too bad either. In learning some things from both the installer and here on this forum; a question came to mind: My old furnace got it's combustion air from the basement. The "shutters" were used to regulate air to the burners, adjusted for optimal burn, right? Well, in looking through the "window" of my new unit as it's running I ask: Since the combustion air is coming from outside, and there's no "shutter" on the PVC air intake pipe---how the heck does the right amount of air get to the burners? I mean, the only thing I can figure (now remember, I'm a home owner) is that the induced draft blower controls the intake air based on the amount of "outflow" air it lets out. ??? What don't I understand? I guess I could call the installer and ask---but I know I drove him nuts with some of my H.O. questions while he was here. Might one of you pro's enlighten me, please? Can't help my curiosity. Thank you.

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,087
    Nothing to adjust and the inducer assures the right amount of air being pulled in to mix with the gas.

  3. #3
    Join Date
    Aug 2007
    Posts
    29

    Ok..

    Now I feel better. So, the induced draft blower is responsible for removing burned gas fumes and controlling combustion air

  4. #4
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    763
    Sort of. The inducer is to overcome the restriction of the heat exchanger and pull the products of combustion through and out. The air for primary combustion is drawn in or 'entrained' by the gas shooting in from the orifice and through the burner venturi. Any extra air required is for secondary combustion/dilution. The vent piping is sized to provide the air required for the given btu input.

    Clear as mud? :
    Where are you? Are you done yet? I got ONE more call for you.....

  5. #5
    Join Date
    Aug 2007
    Posts
    29

    the vent piping...

    Quote Originally Posted by gasguy View Post
    Sort of. The inducer is to overcome the restriction of the heat exchanger and pull the products of combustion through and out. The air for primary combustion is drawn in or 'entrained' by the gas shooting in from the orifice and through the burner venturi. Any extra air required is for secondary combustion/dilution.
    The vent piping is sized to provide the air required for the given btu input.

    Clear as mud? :
    So, even though the "vent" is 2" coming out of the top of the furnace; then immediately transitions to 3" ( 3" all the way to the roof)- that's OK? I specifically remember the installer saying that the size of the furnace called for 3" PVC intake and exhaust--but just now realized that both the intake/exhaust pipes change from 2" to 3" right at the top of the furnace. It shouldn't be 2" all the way? (Based on my uneducated view of the 2" openings on the furnace) Why doesn't the furnace have 3" openings on top? Please bear with me on understanding this. Thank you.

  6. #6
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    763
    Short answer: so they have to make less parts!

    It is correct to come off the 2" fittings in the furnace, then transition to 3" outside the cabinet. They are tested and certified to run that way. Is your furnace 100kbtu/hr or larger?
    Where are you? Are you done yet? I got ONE more call for you.....

  7. #7
    Join Date
    Aug 2007
    Posts
    29

    no...

    ...it's a 90,000.

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