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  1. #14
    Join Date
    Oct 2005
    Posts
    8
    how can salt water enter the supply air through a cracked
    heat exchanger when the blower puts the heat exchanger under a possative pressure?

  2. #15
    Join Date
    Apr 2004
    Location
    Palmdale, CA
    Posts
    199
    Originally posted by heatmehot
    Thank you all....that is a very good document. Question though....I had two seperate companies look at my furnace. One technician said my heat exchanger is leaking around the edges (water test), other technician did a combustion test and found no C02 leakage. Which test is more accurate??

    Thanks....

    There may be a crack and not have CO not CO2 difference one is Carbon Dioxide CO2 the gas in question is Carbon Monoxide CO. What will happen if you do have a crack eventually it will get big enough where you will have an unsafe situation. Why wait for that to happen?

  3. #16
    Join Date
    Oct 2005
    Posts
    27
    Well, for now there isn't a crack...just leaking on the edges of the heat exchanger. The heat exchanger is crimped all around. It doesn't seem like it is welded. By design...should there be a little leakage around the edges of every heat exchanger??




  4. #17
    Join Date
    Feb 2005
    Posts
    655

    Heat Exchanger

    You should have no leakage from any crimped exchanger.

  5. #18
    Join Date
    Oct 2005
    Posts
    27
    Gosh....I don't know who to believe now. I had one technician come over and told me that even the new furnaces heat exchanger have some leakage around the edges. Is that true?

    Thanks....


  6. #19
    Join Date
    Oct 2005
    Posts
    69
    I have had representatives from a major manufacturer inform me of the same thing. He said the little bit of leakage is no more dangerous than your wife cooking a turkey in the oven for 3-4hrs.

  7. #20
    Join Date
    Apr 2004
    Location
    Midwest
    Posts
    2,682
    I usually don’t have the time to respond to long drawn out post but I felt this subject matter was important enough for me to put in my two cents.

    The post by Mech is excellent explaining different methods available, but I would like to make some clarifications especially in regards to draft induced equipment.

    First off, not all HTX leak CO. Induced draft furnaces are under negative pressure. A crack on these HTX can cause air to be drawn into the HTX instead of combustion products entering the air stream.

    Make no mistake, all cracked HTX indicate a problem with the system!

    As shown in Mechs post there are several different test methods available such as Lithium Bromide/Salt Spray, water testing, Tracer gas, Bore scope…and so on.

    Let’s start with Lithium Bromide test, for most manufactures this is not recommended. Test involves spraying a solution directly into the burners, then checking the supply air plenum with a propane torch.
    Lithium Bromide or other chemical salt solutions are highly corrosive. If you used it, and it didn’t leak now, it probably will in the near future!

    How about internal water testing? That doesn’t even sound right does it? Filling each cell with water to see if it leaks? There are 8 approved methods of sealing a HTX including a “rigid pressed joint”. This seam is a rolled and crimped design that is gas tight construction not water tight construction. This method is used by many manufactures, and is not designed to hold water.

    External water testing is performed by spraying water or a light oil mix on the outside and looking on the inside of HTX for leakage. Same potential problems as internal water tests.

    Tracer gas method is an approved method by AGA. Typically tracer gas is 14% non-odorized methane. You inject it into the heat exchanger and check for leakage using a combustional gas leak detector. Works well for natural draft furnaces, works pretty well for 80 plus units but difficult to check secondary HTX on 90 plus units. The problem is in the testing process…

    The seam on a “rigid press joint” is certified by CSA (Canadian Standard Association) formally know as AGA and CGA. The seams are gas tight due to the construction and assembly technique of the HTX combined with the negative pressure effect of the draft inducer.

    In other words, just flushing a bunch of tracer gas into a cell can force a good HTX to fail the test. There is plenty of room for error…

    Scopes work well but takes practice navigating, can be difficult to search out tight areas.
    I still feel strongly that visual inspection on draft induced equipment can be very accurate and by many is a preferred method.

    No matter what method you prefer good understanding of the equipment and common sense will be the biggest aid in identifying problematic HTX.

    Since HTX either corrode or crack, the real question is…why?
    Live each day like it is your last, for one day you will be right!

  8. #21
    Originally posted by m1cowden
    how can salt water enter the supply air through a cracked
    heat exchanger when the blower puts the heat exchanger under a possative pressure?
    The salt test was at one time standard of it's time.

    The way you get a reaction on the halide leak detector
    is by first disabling the fan motor and allowing the furnace to fire until the limit trips and kills burners.

    Heat exchanger is now cooking hot and any cracks are going to open up.

    When the burners activete (blower still off) you spray salt water with spray bottle--generously.

    You place the halide detector probe into the plenum and watch the flame of the detector.

    Probe around the plenum, if the flame turns green on the halide detecor you have found the leak area.

    The heat of the flame escapes through the cracks carrying the the salt vapor that contacts the copper ring in the flame of the halide torch causes the color change,

    The key is the blower is off.

    But like the other posters have stated, CORROSIOPN!!!

    The most reliable method is to see the crack.

    Barring that a monoixide eletronic test is the best.

    [Edited by curry on 10-18-2005 at 04:20 PM]

  9. #22
    Join Date
    Oct 2005
    Posts
    27
    Originally posted by hvac r us 2
    How about internal water testing? That doesn’t even sound right does it? Filling each cell with water to see if it leaks? There are 8 approved methods of sealing a HTX including a “rigid pressed joint”. This seam is a rolled and crimped design that is gas tight construction not water tight construction. This method is used by many manufactures, and is not designed to hold water.
    This doesn't make any sense...."Rigid Pressed Joint..This seam is a rolled and crimped design that is gas tight construction not water tight construction." But..., if water can pass through then gas (C02) will definetely pass as well.


  10. #23
    Join Date
    Jan 2002
    Location
    Ft.Worth,Tx
    Posts
    4,584

    Old Days

    Old days we used to un-plug fan/circuit to let unit trip out on high limit.
    1)Spray bottle with 2 tablespoons of salt/6oz.water
    2) When furnace trips re-connect fan and spray into burner section or heat exchanger then remove one duct or go to nearest supply and see if propane torch turns orange if it does then you have a crack.
    3) Smoke candles work but they are meesy and not always true.
    4) Or you can pull blower with light into heat exchanger see if there are any lights after doors are on.
    5) Carbon Monoxide meter will tell you if there is to much C.O. present and then check one of the above methods.

  11. #24

    Re: Old Days

    Originally posted by aircooled53
    Old days we used to un-plug fan/circuit to let unit trip out on high limit.
    1)Spray bottle with 2 tablespoons of salt/6oz.water
    2) When furnace trips re-connect fan and spray into burner section or heat exchanger then remove one duct or go to nearest supply and see if propane torch turns orange if it does then you have a crack.
    3) Smoke candles work but they are meesy and not always true.
    4) Or you can pull blower with light into heat exchanger see if there are any lights after doors are on.
    5) Carbon Monoxide meter will tell you if there is to much C.O. present and then check one of the above methods.
    thanks for the correction on the flame color change.

    Every time I sneeze I lose another bain cell and another memory.

    Guess that color change was the latest loss.

  12. #25
    Join Date
    Apr 2004
    Location
    Midwest
    Posts
    2,682
    Originally posted by heatmehot
    Originally posted by hvac r us 2
    How about internal water testing? That doesn’t even sound right does it? Filling each cell with water to see if it leaks? There are 8 approved methods of sealing a HTX including a “rigid pressed joint”. This seam is a rolled and crimped design that is gas tight construction not water tight construction. This method is used by many manufactures, and is not designed to hold water.
    This doesn't make any sense...."Rigid Pressed Joint..This seam is a rolled and crimped design that is gas tight construction not water tight construction." But..., if water can pass through then gas (C02) will definetely pass as well.

    How would combustional by-products escape the HTX when it is under a negative pressure?

    If you fill it with water certainly it is no longer in a negative condition. Sort of like buying a water resistant watch, it’s water resistant, not water proof.

    The negative draw is being created by the inducer motor and proved by the pressure switch. Under these conditions the joint is gas tight.

    Think of like this, if you draw on one side of a straw nothing comes out the other end as long as you are drawing in on it right? But yet if you stopped you could not fill it full of water without it running out the other end.
    Live each day like it is your last, for one day you will be right!

  13. #26
    Join Date
    Feb 2004
    Location
    Ontario
    Posts
    700
    There are lots of different designs of furnaces, old and new, and no one right method to test. You have to see what you're working on before deciding the best method to go about checking it.

    One thing that's useful is experience of previous failures in this type of unit, how likely is it to fail, and where is the failure likely to be?

    Another valuable tool is experience in the sense of knowing what kind of heat exchanger you're dealing with, how the gases flow through, and how to get at it to look for flaws.

    Then I'd wonder what symptoms are you dealing with that are causing you to be concerned about the exchanger. Is it just a tune-up with no symptoms, or is something going wrong, or some other reason that has you suspicious of the exchanger?

    I found many cracks over the years, and no doubt missed a number too. But one thing is not to take indirect evidence if possible. If I think there's a crack, I like to see it visually, with a light and mirror if necessary, or draw a fingernail along it. I have used the water spray method on duracurves, and it's also good in a rusted exchanger that might have pinholes.

    I'm pretty sure I never condemned one furnace for a cracked heat exchanger over the years without knowing for absolute certain that it had a crack, and where it was. If you use an indirect method to test, then pull the furnace apart as much as necessary to verify the crack or defect.
    Question authority!

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