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  1. #1
    Join Date
    Mar 2010
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    Pleasant Hill, MO
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    Combustion analysis

    So this should be a simple question for you all. I just got back into doing field service for residential HVAC this year, after doing commercial food equipment for a while. When I was originally trained (10 years ago!) I was taught to check for cracked heat exchangers with the water test method. I never really agreed with it to begin with, and fortunately the company I'm working for now doesn't use that method. I never had the chance to learn checking for a crack with the analyzer, but now I have one to use and have to check on my tuneups, partly as a time filler really. Looking through the posts I've found, I think I have a decent baseline of numbers to work with now but I still haven't seen more than a couple that drop below 10% O2. Can anyone tell me at what level I should really be concerned with a crack?
    The highest I've found so far has been around 12%, but I've mainly been watching for an O2 spike when the blower kicks on and letting it run for a bit to heat up. I would much rather know a danger level to watch for instead of guess. I want to do things the right way. Thanks everyone!

  2. #2
    Join Date
    Jul 2007
    Location
    Virginia
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    Quote Originally Posted by acescc View Post
    I think I have a decent baseline of numbers to work with now but I still haven't seen more than a couple that drop below 10% O2.
    The highest I've found so far has been around 12%, but I've mainly been watching for an O2 spike when the blower kicks on and letting it run for a bit to heat up. I would much rather know a danger level to watch for instead of guess. I want to do things the right way. Thanks everyone!
    High CO or O2 is not always an indication of a compromised (cracked) HX. It is just a good reason to do a visual inspection.

    Leaky blowers or burner compartments can cause the same reaction.

    Looking at O2, CO and draft might narrow it down, but it is not foolproof.

    CO and O2 can go either up or down with a large crack or hole in the HX.

    Your systems with high O2 are more likely just under fired.
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
    Join Date
    Aug 2002
    Location
    Cincinnati, Ohio
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    Leaky return duct can also cause a change. It is CO that kills people not O2. That is the reading to be concerned with. Induced draft furnaces have large holes in the heat exchanger. That is where the burners go in. Water heaters have large holes in their heat exchanger, one at the bottom and one at the top.

    I don't believe the O2 or CO method is the most reliable way to find a crack, only holes. As rundawg said, visual is the only positive method.

    The only person I know that died from crack in a heat exchanger was an employee from Walmart hiding cocaine in the rooftop equipment.
    captain CO

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  5. #4
    Join Date
    Jan 2005
    Location
    Emerald Coast
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    Quote Originally Posted by Jim Davis View Post
    It is CO that kills
    The only person I know that died from crack in a heat exchanger was an employee from Walmart hiding cocaine in the rooftop equipment.
    .
    Thanks
    ..
    Do not attempt vast projects with
    half vast experience and ideas.
    ...

  6. #5
    Join Date
    Mar 2010
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    Pleasant Hill, MO
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    Thread Starter
    To clarify a little bit, I watch both the CO and O2 closely when the blower initially kicks on. I'm also continuing to watch for and changes in rate of increase or decrease as it's ran for a few.

    Really, I would actually be more surprised if most of these furnaces are correctly firing. The installation crew does mostly new construction, and I don't think they are actually started up and properly commissioned. At the same time though, that's out of my pay grade to make sure that's done!

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