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Thread: Heat Exchangers

  1. #1
    Join Date
    Nov 2003
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    Omaha
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    6
    What is the best way to test a heat exchanger that has an induced draft motor on it. the system is under negative pressure so it would be pulling in air rather than letting carbon monoxide out.

  2. #2
    Join Date
    Apr 2003
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    Iowa
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    There are many ways. I start with a thurough visual inspection including pulling the limit to check for missing crimp rings if it's a 2 piece Hx.

    I then start the unit(with blower door on) and watch it fire. Closesly watch the flame pattern. Once the blower turns on the flame pattern should not distort from it's shape prior to fan operation. If it does this would warrant further exploration.

    If worse come to worse you can check CO output at the flue outside or pull the flue off and check it right at the unit. Some have differing opinions on what a proper Co output should be though I look for no more than 125ppm. If you hit 400ppm you have a problem, I have seen 600ppm. This clearly shows a combustion issue though it may not be a failed heat exchanger. It indicates improper combustion or impingement of the flame. It could be something as simple as lack of combustion air or even improper gas pressures. However, the high Co reading will tell you if you need to investigate deeper into the system.

    While it takes some time I have pulled heat exchangers if I feel there is something there that needed attention. Everytime I have chose to go to that extent I have always ended up finding something and condemning the unit.

    It all really does come down to experience. The more furnaces you see, the easier it'll be for your eye to catch something that isn't quite right. I for instance have found that even though Co is odorless, there is a specific odor to a badly failed heat exchanger from the improper combustion, once you smell it you'll never forget it. Just pay close attention to every unit and get practiced up, you'll find your own little tricks to finding problems.
    There are 3 ways to do anything in life; Good, Fast, Slow: You can pick any 2.

  3. #3
    Join Date
    Apr 2001
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    Western PA
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    Originally posted by i_got_ideas
    While it takes some time I have pulled heat exchangers if I feel there is something there that needed attention. Everytime I have chose to go to that extent I have always ended up finding something and condemning the unit.
    Me too. After I spend an hour pulling the darn thing, I had BETTER be bad!

    In a VERY bad heat exchanger, the flames will yellow or rollout on blower start up.

    Watch the crossovers, the little bitty flames between the burners. When the blower starts, they might lazily rise up a little, or barely flutter. If they do, crimp rings popped or cracks possible. If I see evidence of this, I take off the board and pull the blower. Then I take my mirror and flashlight and crawl on my back into the blower cabinet, and look up at the chambers. This can be painful, get a little piece of carpet, and watch those screws.

    If the unit is a Goodman GMP, I skip strait to pulling the blower. Look for crimp rings on the blower cut-off, and feel for rings on the blower shelf. Most of the GMP's I encounter are bad.

    I have also found sealed combustion 90's that make a different burner noise after the blower comes on. 90's are the worst to check, especially the ones with the little window. I'll Cycle the unit over and over, watching one burner each time. This sucks on Carriers with the 90 second fan on start up.

    The secondary heat exchanger blopcks your view from the blower cabinet. Sometimes I open the plenum, take out all those screw and fight the cover of the coil to look down from the top. If the furnace is heat only, there's no excuse not to open the plenum and look down with a mirror and a strong flashlight. Flashlights are better than a drop light, the drop light blinds me.

    Another good way I found is a product called HEAT- Heat Exchanger Analyzer Tester. I biught a bottle at R.E. Michael. The bottle says its for natural draft furnaces, but I never found one crack on a natural draft furnace. Draft induced furnaces draw the stuff in and really yellows the flames. Spray the stuff in the blower when the fan is running and watch the flames.



    I've found at least one cracked heat exchanger a day lately. I'm getting tired of explaining why they fail so soon. I'm finding cracks other techs miss every year. They're out there, just look hard and you'll sell lots.
    Jason

  4. #4
    Join Date
    Jun 2004
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    Maryland's Eastern Shore
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    Block off the exhaust and intake lines... hook up a magnehelic... turn on the blower... if that gauge needle moves, your done.

  5. #5
    Join Date
    Apr 2001
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    Western PA
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    Originally posted by cde72
    Block off the exhaust and intake lines... hook up a magnehelic... turn on the blower... if that gauge needle moves, your done.
    How do you block them? With the inducer running? Some cracks only open when the furnace is hot.
    Jason

  6. #6
    Join Date
    May 2000
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    Colorado flatland native
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    15,067

    You can also

    spray the outside of the HX with water from a weed sprayer. Wet it real good, then look for water inside the HX with a good penlight and I carry a 18"x1"x" mirror that I stick into the HX to get a good look. I also use the "If the pilot or flame flickers when the blower comes on....." test.
    My doctor gave me six months to live, but when I couldn't pay the bill he gave me six months more.
    Walter Matthau

  7. #7
    Join Date
    Mar 2003
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    North St Paul MN
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    858
    Originally posted by bja105
    [/B]
    This sucks on Carriers with the 90 second fan on start up.

    [/B][/QUOTE]Doesn't that only happen when you break the 120v connection? Why not cycle it by disconnecting W.
    I'd rather have a bottle in front of me, than a frontal lobotomy.

  8. #8
    Join Date
    Jun 2004
    Location
    Maryland's Eastern Shore
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    892
    Sorry I didn't reply back sooner... For the magnehelic test, you cut off the vent pipes and block them with tape, you attach your magnehelic any ports on the heat exchanger, you could tee off of the pressure switch, then you turn the blower on, if the pressure changes in the least bit, you have a breech in the heat exchanger... Can't really do it well with an atmospheric burner, but on sealed combustion it works great...

  9. #9
    Join Date
    Feb 2004
    Location
    Midwest
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    9,937
    As cde72 said, I've also used a manometer or magnahelic to check the inducer pressure. It works if the hole is very big. Good idea sealing off the outlet and inlet if possible.



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