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Best method for finding crack heat exchangers

Dairon421
Dairon421 Member Posts: 80
in Gas Heating
Hey guys what is the best method to find a cracked heat exchanger.  I know you can look at it with a camera but it only shows a small percentage of the furnace. I heard of a powder you can use but never tried it. What are your thoughts?

Comments

  • HVACNUT
    HVACNUT Member Posts: 6,332
    I had one just yesterday. Oil fired low boy. While doing the combustion test my numbers were at 10.5% CO2, 50% excess air. When the blower motor started, CO2 immediately dropped to 8% and excess air went to 85%. Confirmed with over fire draft.
    0 ppm CO in the ducts. 
    Time for a new furnace. 
    STEVEusaPAGGross
  • Jamie Hall
    Jamie Hall Member Posts: 24,856
    Thing is, you don't have to find the actual crack -- just know that it's there.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EdTheHeaterMan
  • SlamDunk
    SlamDunk Member Posts: 1,670
    Excess air.
    EdTheHeaterMan
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 16,477
    edited March 2022
    The flame may change, the draft may change, when stopping and starting blower motor. You may get Co in ductwork.

    With a high efficiency furnace you can seal the end of the flue pipe with saran wrap and watch the saran wrap while someone starts and stops the blower if the saran wrap moves the hx is toast
    EdTheHeaterMan
  • mattmia2
    mattmia2 Member Posts: 10,933
    This is definitely much less of a science than I was led to believe...
  • ayetchvacker
    ayetchvacker Member Posts: 63
    EBEBRATT-Ed said:
    The flame may change, the draft may change, when stopping and starting blower motor. You may get Co in ductwork. With a high efficiency furnace you can seal the end of the flue pipe with saran wrap and watch the saran wrap while someone starts and stops the blower if the saran wrap moves the hx is toast
    That is genius!
    Fixer of things 
    Lead Service Technician
    HVAC/R
    ‘09Moto Guzzi V7
    ‘72CB350
    ’83Porsche944
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,505
    edited March 2022
    Here's a simple visual find, just today. You can see the difference when the fan comes on and off.
    https://youtu.be/QvNFOVffx4Y

    Of course 6.2% CO2 and 139% excess air was an obvious tip-off

    There was an error rendering this rich post.

    ayetchvackerHVACNUTmattmia2
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 9,380
    edited March 2022
    Unless I could actually see the crack, I did 4 tests.
    1. CO2 with blower on and off. must be at least 2% difference. (because the change in air pressure in the furnace room can actually change the CO2 by 1% even with a good HX.)
    2. Draft over fire with blower off and on.
    3. Draft at breach with blower off and on
    4. Smoke
    ..a. a smoke bomb in a gas heat exchanger and a smoke tester in the plenum.
    ..b. Run an oil burner extra rich to produce #10 smoke inside the HX and test the plenum for smoke.
    (both a. and b. with blower off)
    I always liked at least 2 tests to confirm the crack when not visible. (like #1 and #2 or #3 and #4 or #1 and #4)

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    ayetchvacker

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