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  1. #79
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    Dec 2011
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    Georgia
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    18
    Well i suppose that would depend on what meter you are using. A good meter will pick up on co when the blower comes on in three different scenarios. If the blower comes on and the draft on the furnace decreases and the co rises that can indicate duct leakage (return of course). If the co rises instantly regardless of blower operation than the heat exchanger could be plugged ( had a bird get in one once... big mess.) if the blower comes on and the co rises and there is no change in draft it indicates that the heat exchanger is compromised. 90%+ obviously draft isn't and issue since the by products are positively vented. I have found rusted hx and cracked this way. Especially on package units. never forget. o2 6-9% and co below 100ppm and stable. The heat exchanger is never bad with these numbers. havn't seen a furnace operate with a bad hx with acceptable co readings

  2. #80
    Join Date
    Feb 2010
    Location
    In a boiler room
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    7,157
    So tell me how a tight crack that is not leaking yet can cause any change in CO numbers when the blower comes on?

  3. #81
    Join Date
    Dec 2011
    Location
    Georgia
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    18
    not to be argumentative but tell me what a "tight" crack is. and in what case have you found this type of crack and your monitor didn't get it? if so what numbers were you reading? what type of furnace? to me a crack of any size is going to show up if it is in the airstream. most hvac guys dont use monitors anyway so i understand the skeptics

  4. #82
    Join Date
    Feb 2010
    Location
    In a boiler room
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    7,157
    Quote Originally Posted by CAtech1986 View Post
    not to be argumentative but tell me what a "tight" crack is. and in what case have you found this type of crack and your monitor didn't get it? if so what numbers were you reading? what type of furnace? to me a crack of any size is going to show up if it is in the airstream. most hvac guys dont use monitors anyway so i understand the skeptics
    I use a CA on every furnace I work on. Many many times I have found cracks in heat exchangers that showed zero change in O2 or CO when the blower starts.

    One particular model that comes to mind is the Lennox Duracurve heat exchangers. Every time I have found them cracked I have found it visually and they are always in the back on the first curve. They are never IME leaking air, at least not enough to pick up on an analyzer.

    If you are relying on only a CA to find cracks you will miss over 80% of them.

  5. #83
    Join Date
    Dec 2011
    Location
    Georgia
    Posts
    18
    the lennox duracurve was a 80+ or natural draft?

  6. #84
    Join Date
    Feb 2010
    Location
    In a boiler room
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    Quote Originally Posted by CAtech1986 View Post
    the lennox duracurve was a 80+ or natural draft?
    You need to fill in your profile, are you a tech? I can't believe their are techs that have never seen a duracurve. They are all natural draft.

    My post above is true of all furnaces, I just used the duracurve as an example.

  7. #85
    Join Date
    Dec 2011
    Location
    Georgia
    Posts
    18
    sorry just signed up. I am a tech. Been in the field five years. Frankly there are not a lot of natural draft furnaces left in my part of the country. most have been replaced. I still see them occasionally. i do enjoy being able to test seperate burners with the CA. Mostly oil, and induced draft i have experiance with. 90+ also of course. Had a Tappan a few years ago with 4000ppm in the exhaust before the draft hood with a draft of .07" i removed the draft hood completely and sealed it off. It is now using a 6" barometric and working great still today. other than that i reallyt don't see many nowadays. i would like to know how any furnace with a legitimate crack in the heat exchanger will not cause some kind of co action. if it is natural draft than the combustion chamber is subjected to a constant negative pressue. with any kind of hole in it the blower will cause some infiltration of what is otherwise supposed to be a sealed chamber. drastic changes... maybe, maybe not. 10-15 minutes of operation should reveal some sort of problem. especially as it gets hotter. O2 will drop, co will rise a classic venting diagnosis could become confused with a slight crack i suppose.

  8. #86
    Join Date
    Feb 2010
    Location
    In a boiler room
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    7,157
    Most of the time a leak allowing air to infiltrate the hx will cause the o2 to rise and the CO to fall when the blower comes on. Only a severe crack would cause the CO to rise.

    Kudos for learning how to use a CA. I also regularly remove draft hoods and install barometric draft controls.

  9. #87
    Join Date
    Dec 2011
    Location
    Georgia
    Posts
    18
    wow! we should just start cracking heat exchangers and that will solve our carbon monoxide and venting problems!!!

  10. #88
    Join Date
    Dec 2011
    Location
    somewhere between here and there
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    478
    goodman with the rings(gmp)

    lennox with the duracurve(whisperheat)

    bryant (394 GAD)

    oh how i lovew to do a tuneup on these....next thing u know the red tag comes out, and i leave the house with a $6000 job

    o how i love these

  11. #89
    Join Date
    Jan 2011
    Location
    Boston, MA
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    459
    Quote Originally Posted by CAtech1986 View Post
    wow! we should just start cracking heat exchangers and that will solve our carbon monoxide and venting problems!!!
    uhh what?

  12. #90
    Join Date
    Dec 2011
    Location
    somewhere between here and there
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    478
    Quote Originally Posted by CAtech1986 View Post
    The best way to identify a cracked heat exchanger is with a combustion analyzer. with gas pressure set at 3.5 for natural and 10-11" w.c for lp. you'll notice a rise in co when the blower comes on. a drastic rise. you don't need to pull it out or anything. if it is rusting that indicates there has been a problem for a long time. What type of furnace is it?



    the best way to identify a cracked htxchanger, is to be knowledgible in the specific models and where they crack, as i posted earlier....maybe wont get all of them...but using a CA is a poor way to find them...IMO

    whisperheats are all(ok 95%) cracked...with no diff using the CA.....if crack aint open...

    and IMO a crack is a crack is a crack. integrety is gone and unsafe to operate.

  13. #91
    Join Date
    Dec 2011
    Location
    PA
    Posts
    98
    Curiousity getting the better of me..... how many of you use a CO meter (calibrated before entry to home) while doing clean and checks?

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