Best method for finding crack heat exchangers

Dairon421 · March 15

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?

HVACNUT · March 15

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.

Jamie Hall · March 15

Thing is, you don't have to find the actual crack -- just know that it's there.

SlamDunk · March 15

Excess air.

EBEBRATT-Ed · March 15

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

mattmia2 · March 15

This is definitely much less of a science than I was led to believe...

ayetchvacker · March 15

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!

STEVEusaPA · March 15

Here's a simple visual find, just today. You can see the difference when the fan comes on and off.

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

EdTheHeaterMan · March 15

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)

Best method for finding crack heat exchangers

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