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  1. #1
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    Feb 2004
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    temperature rise in furnace

    Checking out the furnace and I took some temperature readings, at the discharge, about 24" above the burners, I was reading 162F, inside the return air cabinet at the blower (with the door closed) I was reading 78F so a temperature delta of 84 degrees. Is this normal, if not too little or too much air flow?

    Thanks

  2. #2
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    Sep 2002
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    You have to take the temperature rise on the trunk line since the thermometer can pick up radiant heat from the heat exchanger.

    8F so a temperature delta of 84 degrees. Is this normal, if not too little or too much air flow?
    Check the furnace's rating plate.

    Old conventional units generally have a rated rise of 70-100F.

    New mid and high efficiency models tend to be rated at 30-60F.

    If the temp rise is above the rated, you don't have sufficient airflow and/or the furnace is overfired.
    I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.

    Death is the messiah. Everything else is irrelevant and arbitrary.

  3. #3
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    Feb 2004
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    Thanks, how far down the trunk line should I check? I did check the name plate and it says 75F max rise. Since the furnace was installed I have known they undersized the ductork, cuasing the higher static on the return. We did some mods to help this but the ultimate solution is to re-do the trunk lines. I may have my tech check the gas pressure and if needed, slghtly de-rate it.

  4. #4
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    The thermometer shouldn't be able to "see" the heat exchanger.

    I would do it a couple of feet into the trunk line.

    It might be possible to increase the blower speed.
    I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.

    Death is the messiah. Everything else is irrelevant and arbitrary.

  5. #5
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    Sep 2002
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    Since the furnace was installed I have known they undersized the ductork
    I've got the same problem - http://hvac-talk.com/vbb/showthread.php?t=919152
    I've discovered a natural law - everything gets progressively worse. Things only break down and become depleted. Life isn't worth living and everything is going to hell.

    Death is the messiah. Everything else is irrelevant and arbitrary.

  6. #6
    Join Date
    Jul 2006
    Location
    Virginia
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    sounds like a big furnace on a small duct system. a good place to start is find out how much furnace you need . also it is not a good idea to lower the gas pressure to reduce the temperature rise as you are changing the design conditions of the furnace

  7. #7
    Join Date
    Apr 2007
    Location
    Greenville, SC
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    Have your tech test manofold pressure to see if it is set correctly.

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