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  1. #1
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    How close to the efficiency rating is close enough?

    I do mainly all commercial work in HVAC and have never really done any residential except for my own and friends of mine. Anyway..We got a fancy new combustion analyzer in at work that we are going to have to use weekly on all our in house equipment. I brought it home to play with to test my furnace. The first thing i noticed is that the gas pressure was at 8.75" while i am running natural gas!. The CO levels were 350-370 consistently through the exhaust. I drop the gas pressure down to the manf specs to the usual 3.5". CO is now 25-26, O2 is 13.5.. efficiency is 78%.

    Here are the differences before and after

    BEFORE:
    O2 - 6.7
    CO2 - 8.1
    CO - 348
    FLUE - 360dF
    INLET - 66dF
    NETT - 294.6dF

    EFF 81.5%
    Losses 18.5
    XAIR - 47.2

    CO/CO2 - 0.0042
    CO AIR FREE - 512
    GAS PRESSURE - 8.67

    AFTER:
    O2 - 13.5
    CO2 - 4.2
    CO - 26
    FLUE - 296dF
    INLET - 66.3dF
    NETT - 230.5dF

    EFF 78.2%
    Losses 18.5
    XAIR - 47.2

    CO/CO2 - 0.0042
    CO AIR FREE - 73
    GAS PRESSURE - 3.52

    The furnace is 15yrs old as its a 1998. Never really did much analyzing like this so thats why I am asking how close is close enough, I imagine the age of the equipment comes in to play as to how close/far away it should be. 78% sounds good to me since its 15yrs old and 80% is the actual rating.. I bought the house about 6yrs ago so its been running like this ever since and i never knew

    Thanks as always!

  2. #2
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    Jul 2007
    Location
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    First, the efficiency readings on your analyzer are not accurate, because they are based on preprogramed data into the analyzer.

    I'm assuming this is an Induced Draft Furnace? Maybe a package unit?

    Single readings make it hard to interpret what the problem is.

    A start up reading, 3 run time readings, and a shut down reading is what you need to see the whole picture of how the appliance is operating.

    Were these numbers stable?

    Did the numbers change when the blower came on?

    What was the draft reading?

    Supply air readings are good to have too.

    Your "AFTER" reading shows the unit is UNDER FIRED with 13.5% O2 and 296f flue temp, even though the CO is < 100ppm and manifold pressure is now down to 3.5"wc.

    At 15 years old, before I would not make any more adjustment, I would ensure the HX is in good shape, and the inducer motor and wheel are in good shape and operating properly.

    Once those are confirmed good, then I would adjust the manifold pressure up a little to get the O2 below 9%, and see if the CO is less than 100 ppm.
    Last edited by rundawg; 01-04-2013 at 01:43 PM. Reason: typo
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
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    NC
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    Quote Originally Posted by rundawg View Post
    First, the efficiency readings on your analyzer are not accurate, because they are based on preprogramed data into the analyzer.

    I'm assuming this is an Induced Draft Furnace? Maybe a package unit?

    Single readings make it hard to interpret what the problem is.

    A start up reading, 3 run time readings, and a shut down reading is what you need to see the whole picture of how the appliance is operating.

    Were these numbers stable?

    Did the numbers change when the blower came on?

    What was the draft reading?

    Supply air readings are good to have too.

    Your "AFTER" reading shows the unit is UNDER FIRED with 13.5% O2 and 296f flue temp, even though the CO is < 100ppm and manifold pressure is now down to 3.5"wc.

    At 15 years old, before I would not make any more adjustment, I would ensure the HX is in good shape, and the inducer motor and wheel are in good shape and operating properly.

    Once those are confirmed good, then I would adjust the manifold pressure up a little to get the O2 below 9%, and see if the CO is less than 100 ppm.

    Thanks, numbers were very stable.

    I meant to bring an additional thermo couple home today when i left early but forgot,there are a couple slots on the meter for two temp readings on. And yes it does have a draft inducer When you ask if the numbers are changing when the unit comes on, isnt that to be expected depending on how long the unit has been off between each cycle?
    I adjusted per recommendation, this is what i am at now.. I understand how all the numbers work together and such, I just have not done any of this since school about 6+yrs ago.



    CO/CO2 - 0.0042
    CO AIR FREE - 512
    GAS PRESSURE - 8.67

    AFTER:
    O2 - 9.1
    CO2 - 6.7
    CO - 26
    FLUE - 347dF
    INLET - 57.6dF
    NETT - 290.0dF

    EFF 80.2%
    Losses 19.8
    XAIR - 77.1

    GAS PRESSURE - 6.65

  4. #4
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    Quote Originally Posted by Shockwave View Post
    When you ask if the numbers are changing when the unit comes on, isnt that to be expected depending on how long the unit has been off between each cycle?
    CO and O2 readings should not change when the indoor blower comes on.

    One other thing I would do is verify the proper Orifice size is installed.

    Some will say to change the orifice size so you can keep the manifold pressure down near the manufactures recommended 3.5"wc.


    Side note:
    You have enough posts to apply for Pro membership. You will get tons of good info in the "LOCKED" Pro section of this forum if you have other questions. This is bordering on too much info for this "OPEN" forum.
    Instead of learning the tricks of the trade, learn the trade.

  5. #5
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    Jun 2003
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    What did you measure the as pressure with? I think your reading is wrong. If you had a furnace fired at 8.75 for 15 years it would be destroyed by now. On top of that, most houses don't have over 8" at the piping, so the likelyhood of 8.75 at the manifold is very low.

    I suspect your gas pressure reading may have been off, and the furnace was slightly overfired before, but now way underfired.

  6. #6
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    Quote Originally Posted by craig1 View Post
    What did you measure the as pressure with? I think your reading is wrong. If you had a furnace fired at 8.75 for 15 years it would be destroyed by now. On top of that, most houses don't have over 8" at the piping, so the likelyhood of 8.75 at the manifold is very low.

    I suspect your gas pressure reading may have been off, and the furnace was slightly overfired before, but now way underfired.
    The meter has a pressure port built into it, i am using the one off the meter.

  7. #7
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    Quote Originally Posted by craig1 View Post
    I suspect your gas pressure reading may have been off, and the furnace was slightly overfired before, but now way underfired.
    I agree with the manometer being off on manifold pressure readings - those are some odd numbers he posted.

    But, assuming (which is all we can do) his analyzer is reading correctly, he is not terribly underfired now with an O2 reading at 9.1%. 6% would be even better though.
    Instead of learning the tricks of the trade, learn the trade.

  8. #8
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    Quote Originally Posted by Shockwave View Post
    The meter has a pressure port built into it, i am using the one off the meter.
    This might be a silly question, but you are not measuring at the outdoor gas meter are you?

    You need to be measuring manifold pressure at a port on the burner manifold, or outlet pressure tap on the gas valve.
    Instead of learning the tricks of the trade, learn the trade.

  9. #9
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    Quote Originally Posted by rundawg View Post
    This might be a silly question, but you are not measuring at the outdoor gas meter are you?

    You need to be measuring manifold pressure at a port on the burner manifold, or outlet pressure tap on the gas valve.
    yeah man

    I found the problem..the damn thing was reading in HPA units, in "WC

    restarting all test now

  10. #10
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    Quote Originally Posted by Shockwave View Post
    yeah man

    I found the problem..the damn thing was reading in HPA units, in "WC
    Thank craig1, he's the one that caught the manifold pressures off the chart.
    Instead of learning the tricks of the trade, learn the trade.

  11. #11
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    I used an online calculator to convert from hPa to inWC and got the following

    before: 3.48"
    after: 1.41"

    So your initial gas pressure was perfect, and your after was underfired

  12. #12
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    This is a good example of how valuable a temp rise across the heat exchanger can be. Probably would have made the OP say hmmmm when he saw how low it was after he lowered the manifold pressure.

    The CO is too high in the first set of readings.
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