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  1. #1
    Join Date
    Feb 2004
    Posts
    108

    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
    Join Date
    Sep 2002
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    I don't know
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    2,897
    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.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  3. #3
    Join Date
    Feb 2004
    Posts
    108
    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
    Join Date
    Sep 2002
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    I don't know
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    2,897
    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.
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  5. #5
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,897
    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
    General public's attitude towards our energy predicament: "I reject the reality of finite resource depletion and substitute it with my own; energy is infinite, we just need an alternative storage medium to run the cars on. The economy can grow indefinitely - we just need to "green" everything! Technology is energy! Peak what?"

  6. #6
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,512
    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
    Posts
    13
    Have your tech test manofold pressure to see if it is set correctly.

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