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  1. #1
    Join Date
    Sep 2011
    Posts
    79

    MUST READ: LP run on Nat. Gas config

    I have dealt with this twice this year. One unit "we" installed, another some other jake leg installed.

    If you run LP on a furnace set up for natural gas it will work, for a little while, apparently.

    Soot builds up and eventually rolls out on every ignition.

    Then, you can not simply convert the furnace, you must also blow out the heat exchanger.

    The way I learned, after the first time, was to disconnect the igniter, call for heat, and blow nitrogen through every tube.
    p.s. turn the gas off

    The draft motor will blow a good portion of the soot out of the house.

    Now remove the induced draft motor, clean off the squirrel cage and manifold. While the IDM is off, also blow backwards through the heat exhanger. If you put a blanket over the furnace while doing this, you'll prevent the big stuff from going EVERYWHERE.

    After this, no roll outs after the conversion. I tried to get away with not doing the H.E. cleanse during the conversion and had to come back... don't do that to yourself.

  2. #2
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,958
    I'm surprised that the furnace wouldn't immediately cycle on limit due to the extreme over firing.

  3. #3
    Join Date
    Jul 2005
    Posts
    5,153
    Quote Originally Posted by 54regcab View Post
    I'm surprised that the furnace wouldn't immediately cycle on limit due to the extreme over firing.
    Incomplete combustion keeps the temp down.

  4. #4
    Join Date
    May 2007
    Posts
    730
    I take the H.E. outside the house and flush it with water.

  5. #5
    Join Date
    Dec 2007
    Posts
    4,575
    Quote Originally Posted by hvacrmedic View Post
    Incomplete combustion keeps the temp down.
    the premiss is that the 3.5"NG setting is providing MORE gas than the 11"LP setting and causing soot production?
    The Food Stamp Program, administered by the U.S. Department of Agriculture, is proud to be distributing the greatest amount of free meals and stamps EVER.
    Meanwhile, the National Park Service, administered by the U.S. Department of the Interior, asks us to "Please Do Not Feed the Animals". Their stated reason for this policy "... the animals become dependent on handouts and will not learn to take care of themselves."
    from an excerpt by Paul Jacob in Sun City, AZ

  6. #6
    Join Date
    Sep 2011
    Posts
    79
    i guess you could take the H.E. out... sounds like a lot of work. these furnaces were working great after i got done with them, though time will :-/.

    i'd think it is incomplete combustion because the velocity of the gas pulls the air into the venturi, but with out that velocity (low gas pressure) not enough air is mixed before it burns hence the yellow flame (and soot). so it is actually running rich despite less gas being output.

  7. #7
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    3,958
    Quote Originally Posted by pacnw View Post
    the premiss is that the 3.5"NG setting is providing MORE gas than the 11"LP setting and causing soot production?
    Keep in mind the LP orifices are considerably smaller than NG.

  8. #8
    Im betting you will be back next year to clean it again. Had a friend that blew a furnace out in his bosses house that had white carpet,
    still dont know how he didnt get fired soot everywhere

  9. #9
    Join Date
    Sep 2011
    Posts
    79
    i ran the IDM and blew most out of the house... but the first time i didn't do that and it was a huge mess in the attic.

  10. #10
    Join Date
    Jul 2005
    Posts
    5,153
    Quote Originally Posted by pacnw View Post
    the premise is that the 3.5"NG setting is providing MORE gas than the 11"LP setting and causing soot production?
    Volume for volume, LP contains about 2.44 times more btu content than NG, so the same or similar volume of LP through the NG orifices, even at 3.5", would result in an input rate much higher than the furnace's input rating. It would definitely be over-fired and burning rich. At 11" the the problem would be worse. I've seen both cases, 3.5" and 11" without the orifice replacement, and they're both bad situations. Some Carrier units use 3.5" for LP, with only an orifice change-out required for conversion. The LP orifices are smaller even though the manifold pressure isn't changed. HTH.
    Last edited by hvacrmedic; 12-31-2012 at 01:24 AM.

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