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  1. #1
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    natural gas furnace, if the gas valve is at 4"w.c. - can we say that the furnace is overfiring ?

    Sorry, the furnace plate says - 3.5 "w.c.

  2. #2
    Senior Tech Guest
    You can say the gas pressure is high.

  3. #3
    Join Date
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    <<< gas pressure is higher>>> that is true.

    question is - is the furnace OVERFIRED ?

    I bet a pound of bananas with my fellow teck, that there is a difference between furnace being overfired and gas valve being out of range.

    Both have different implications

    o

    Opinions ...

  4. #4
    Join Date
    Feb 2004
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    2,458
    Could be undergassed..I seen many furnaces with spider nests blocking orifices and someone tried to bring up by burying the reg adjust. Why are you asking?

  5. #5
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    <<<Why are you asking?>>> , sure

    I am asking because there is a much heated discussion between me and one of my fellow tech. that there is a difference between "furnace overfiring" and gas valve being out of range.

  6. #6
    Join Date
    Sep 2001
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    Vancouver Canada
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    996
    If the rating is for 3.5" I would say a 4" it is overfiring. May not be burning more gas but your going to be out of wack with the air/gas mixture.
    "Go big or Go Home"

  7. #7
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    Feb 2004
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    I don't understand what you mean by out of range. Please explain if you would

  8. #8
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    Nov 2004
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    Black Adder,

    you are right, but need a litte bit more details.

    Yes, for sure you will break the "necessary" ratio of 10:1 to have a complete combustion, i.e. the mixture will be "rich". That is true.

    Putting this aside - what will be the implication for the furnace if the gas valve is at 4"w.c. and/or it is overfiried ?

    O.K. let me ask or put the question in a different way ?

    What determines if a furnace is "overfired" ?

  9. #9
    Join Date
    Nov 2000
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    Upstate, SC
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    2,919
    On natural gas, can you not clock the meter and find out how much gas is actually being used?

    Bobby

  10. #10
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    Nov 2004
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    mike3

    sorry, I did not see your post


    <<<what you mean by out of range>>>


    sure


    gas valve normally operate at 3.5"w.c. + or - 0.5"w.c. (if I am not mistaken)

    if a gas valve (on a natural gas furnace) is at 4"w.c. or higher - I call it "out of range.

    When we do maintenance calls - that is mandatory to check.

    So, if I see 4"w.c. or higher - I would say the gas valve is operating above the manifacturing specs.


    Question is: is "overfired" furnace the same as gas valve working at 4"w.c. (or higher) when the manifacturer calls for 3.5"w.c. ?

    What are the implicati0ns with both scenarios ?

  11. #11
    Join Date
    Nov 2004
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    BobbyBJr

    BINGO,

    that is my argument. To me - furnace is overfired when you clock it and it is above the manif. specs.

    Now we can talk.

    So, in my opinion, the amoung of gas flowing through the ORIFFICE determines if a furnace is overfired.

    That is expressed in B.T.U.s

    On the other hand if the gas valve is working at 4"w.c. (or higher) will not increase the B.T.U.'s thus the furnace will not be overfiring.

    However, the Temp. inside the HEX will be higher - resulting in Lord knows what potential problems - but that is not overfiring.

  12. #12
    Join Date
    Feb 2004
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    I guess two dollar answer is yes. However, what does clock test say, was a combustion analyzer used. Than the what if's. What if orifice size need 4" to provide rated input. I don't think I would adjust reg without at very least clocking meter. Also probably would check for orifice blockage. Several years ago I got tricked into thinking a water heater was overgassed by the clock test. Reduced input until flame was pretty low. Didn't look right, but left it that way. Few days later we get a gas leak call there. Seems there was, If I remember correctly a 12cfh underground leak. That meant I left the awh 12cfh undergassed. Dumb move. What I should have done and always did do except that time(I was assisting a rookie & made a rookie mistake) was to spot the meter first to determine only pilot flow existed, before clocking appliance

    [Edited by mike3 on 11-27-2005 at 12:50 AM]

  13. #13
    Join Date
    Nov 2000
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    I guess I agree with you to a point. I agree that the amount of gas being actually used is the firing rate, but I also believe the pressure could play a part in determining it. If you raise the pressure, should it not force more gas through the orifices? Wouldn't that raise the firing rate? Now, I am thinking the btu conttent of the gas would determine if we were actually overfired yet, but it could happen that way I am thinking. I have read the btu content of natural gas varies in some parts of the country, so I am thinking that could play a part in it too. So, I don't think gas pressure alone is enough to answer your question.

    Bobby

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