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  1. #1
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    Goodman GMV8 High Excess Air Reading

    I have a 13 year old Goodman GMV81155CX gas furnace that I recently had routine furnace maintenance on. It was my first time I had used this HVAC company.

    All readings with the technician's meter were as expected, no levels were deemed out of the ordinary including O2 and CO levels. The last reading the technician took with his meter was the excess air (XAir%), and the meter read north of 170%. I am told anything over 100% excess air is considered a flag and there may be a cracked heat exchanger which may lead to increased CO levels in my home. The CO reading they read with the meter in the flue was within appropriate levels. The HVAC company is pushing me to replace my entire furnace as a result of the high excess air reading. Naturally, since this was my first time using the HVAC company, I am hesitant to do so since it is a big investment.

    Following the technician's visit, I purchased a few CO detectors that show a live readout of the CO ppm levels in my home and I have been moving them all over my home to see what the readings are. After leaving the detectors alone for several hours, all readings have all measured < 30 ppm. My first question is: does a high excess air % reading by an HVAC technician's meter always lead to elevated CO levels in a home? If so, is there a specific location in my home where I should move my CO detector to see the highest concentration to confirm there is a cracked heat exchanger? I imagine in the furnace room would be the best indication of high levels, but I didn't see any high levels there either.

    Thank you in advance for your input.

  2. #2
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    Here's how it should work. If I find something of concern regarding the integrity of the heat exchanger, I inform the customer of the info and explain why my finding is concerning and the exchanger should be pulled and physically inspected for damage. A furnace with a cracked heat exchanger doesnt always produce CO, but it has the potential to, that's why the exchanger must be visually inspected. Heat exchanger should still be under warranty if it was registered, did they check warranty status?

  3. #3
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    Quote Originally Posted by Makeitcold View Post
    Here's how it should work. If I find something of concern regarding the integrity of the heat exchanger, I inform the customer of the info and explain why my finding is concerning and the exchanger should be pulled and physically inspected for damage. A furnace with a cracked heat exchanger doesnt always produce CO, but it has the potential to, that's why the exchanger must be visually inspected. Heat exchanger should still be under warranty if it was registered, did they check warranty status?
    Thanks for your feedback, the technician did an excellent job explaining why the finding was concerning. The heat exchanger warranty was checked and is indeed still under warranty so the part is free but the labor associated with replacing the part was quoted to me for approx. $2000. The technician expressed great concern with replacing the part due to the unit's age (13 years) and recommended that I replace the whole unit instead. And... since the furnace is the same age as the AC unit, that I might as well replace that too especially since R22 freon phase out at the end of this year.

    In your opinion, as long as I am not detecting any CO levels > 30 ppm around my home, are there other reasons why I should need to replace the heat exchanger? Such as lower EFF, other safety aspects, etc.?

    Thanks again.

  4. #4
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    Quote Originally Posted by satchmo05 View Post
    All readings with the technician's meter were as expected, no levels were deemed out of the ordinary including O2 and CO levels. The last reading the technician took with his meter was the excess air (XAir%), and the meter read north of 170%. I am told anything over 100% excess air is considered a flag and there may be a cracked heat exchanger which may lead to increased CO levels in my home. .
    If your O2 is within tolerance, the excess air is within tolerance.

    Excess air is just a calculation the analyzer computes based on the amount of O2 in the flue gas sample.

    The combustion analyzer basically uses the following formula to calculate the amount of excess air:

    % Excess Air = (% O2 measured ๗ 20.9 - % O2 measured) x 100

    So do you know what the “not out of the ordinary” O2 and CO numbers were?
    Instead of learning the tricks of the trade, learn the trade.

  5. #5
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    Quote Originally Posted by satchmo05 View Post
    In your opinion, as long as I am not detecting any CO levels > 30 ppm around my home, are there other reasons why I should need to replace the heat exchanger? Such as lower EFF, other safety aspects, etc.?
    I would not want any measurable CO level in my home.

    Long term exposure to low levels of CO is worse than short periods of high levels.

    Testing should be done to identify the true source of the CO. It may even be the water heater (if it is gas).
    Instead of learning the tricks of the trade, learn the trade.

  6. #6
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    Quote Originally Posted by satchmo05 View Post
    I have a 13 year old Goodman GMV81155CX gas furnace that I recently had routine furnace maintenance on. It was my first time I had used this HVAC company.

    All readings with the technician's meter were as expected, no levels were deemed out of the ordinary including O2 and CO levels. The last reading the technician took with his meter was the excess air (XAir%), and the meter read north of 170%. I am told anything over 100% excess air is considered a flag and there may be a cracked heat exchanger which may lead to increased CO levels in my home. The CO reading they read with the meter in the flue was within appropriate levels. The HVAC company is pushing me to replace my entire furnace as a result of the high excess air reading. Naturally, since this was my first time using the HVAC company, I am hesitant to do so since it is a big investment.

    Following the technician's visit, I purchased a few CO detectors that show a live readout of the CO ppm levels in my home and I have been moving them all over my home to see what the readings are. After leaving the detectors alone for several hours, all readings have all measured < 30 ppm. My first question is: does a high excess air % reading by an HVAC technician's meter always lead to elevated CO levels in a home? If so, is there a specific location in my home where I should move my CO detector to see the highest concentration to confirm there is a cracked heat exchanger? I imagine in the furnace room would be the best indication of high levels, but I didn't see any high levels there either.

    Thank you in advance for your input.
    If there was not high levels of CO in the flue gas, I would not be worried. I would request another test with a recently calibrated analyzer.

  7. #7
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    Where does that 'logic' end I wonder? Hell; the house is at least 13 years old - better replace it now. <g>

    Maybe you should get some other quotes to replace the HX? Maybe those people won't be pricing the repair so as to sell the new install?

    The R-22 phase out should be of zero consideration for you making these decisions.

    PHM
    -------



    Quote Originally Posted by satchmo05 View Post
    Thanks for your feedback, the technician did an excellent job explaining why the finding was concerning. The heat exchanger warranty was checked and is indeed still under warranty so the part is free but the labor associated with replacing the part was quoted to me for approx. $2000. The technician expressed great concern with replacing the part due to the unit's age (13 years) and recommended that I replace the whole unit instead. And... since the furnace is the same age as the AC unit, that I might as well replace that too especially since R22 freon phase out at the end of this year.

    In your opinion, as long as I am not detecting any CO levels > 30 ppm around my home, are there other reasons why I should need to replace the heat exchanger? Such as lower EFF, other safety aspects, etc.?

    Thanks again.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  8. #8
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    Agree with PHM about the logic. Carbon Monoxide clings to blood vessels and prevents oxygen from doing so, it can take weeks after exposure has ended to get it out of your system. If I read this OSHA article correctly, it said 50ppm for less across an 8 hour day was max allowable, so 30ppm over a constant time period could build up in your system, maybe not kill you but make you feel like you've got pneumonia. Sounds like you possibly have a salestech or at best someone that doesnt know how to interpret the data. If, and it's a big if, the heat exchanger is cracked then I would want to know why before changing it, do a free manual j at www.loadcalc.net and find out if you even need that much heat, ask for the ducting static pressure to be measured to see if enough air can even go through the ducting. On the coldest day that furnace should almost never shut down when properly sized

  9. #9
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    Quote Originally Posted by ksefan View Post
    If there was not high levels of CO in the flue gas, I would not be worried. I would request another test with a recently calibrated analyzer.
    In your opinion - if the heat exchanger was cracked, would CO leak in all parts of my home or would it leak into utility room where my furnace is located? I'm trying to get a feel for where the CO would be at its highest concentration. Thanks for the reply.

  10. #10
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    Thread Starter
    Thanks for the input.

  11. #11
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    Quote Originally Posted by satchmo05 View Post
    In your opinion - if the heat exchanger was cracked, would CO leak in all parts of my home or would it leak into utility room where my furnace is located? I'm trying to get a feel for where the CO would be at its highest concentration. Thanks for the reply.
    To get measurable CO at a register, or in the space, a furnace has to be making seriously high levels of CO, or it has to have been making CO over a long period of time.

    A 100,000 btuh condensing furnace uses roughly 25 cfm for complete combustion, and this same furnace in heating should be moving 1500 cfm of supply airflow through the duct system.

    25 cfm is roughly .016% of the total supply air flow. To get a 10 ppm reading at a register, the furnace would have to be making over 600 ppm of CO, and that is if you dumped 100% of the flue gases into the supply airstream.

    A small crack in the HX may leak 2% of the 25 cfm, or .5 cfm (.0003%). The appliance would have to be making several thousand PPM of CO just to get a 10 ppm reading on a CO detector.

    Checking for CO in the duct work or space is a poor method of determining if the Heat Exchanger has been compromised.

    If you have a natural draft gas water heater, you have a better chance of getting CO in the home than with a furnace.
    Instead of learning the tricks of the trade, learn the trade.

  12. #12
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    IF you have a reading of <30 PPM CO you should have the source located. It is most likely not the furnace although it could be. Other appliances and attached garages are more likely or sometimes in large cities with large amounts of traffic the background (outside) levels can be that high as was found downtown Chicago some years ago.

    As for the furnace being bad you only shared one reading of several that would give more insight into the health of the unit. Sad to say many techs do not understand the benefits of a Combustion Analyser except as a sales tool. It may be a case where your furnace is needs to be adjusted, or the readings may be just fine. If you provide those other readings we may be able to give you better insight as to the health of your furnace.

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