ReplyA rise in O2 when the blower starts is not always due to a cracked heat exchanger. Can also happen from other leaks in a furnace.
Never condemn a HX on the combustion results alone, you need to verify it visually.
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Is there a general consensus on how much the 02 needs to rise in the flue gases post blower turn on to point to a cracked heat exchanger?
Also, has anyone heard of someone using this test on a natural draft furnace?
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A rise in O2 when the blower starts is not always due to a cracked heat exchanger. Can also happen from other leaks in a furnace.
Never condemn a HX on the combustion results alone, you need to verify it visually.
Professional Member
I've thought about this for a while and can't come up with any other pathway for more O2 to enter the flue than the HX. Please explain.Originally Posted by Chuck
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Air leaks around the vestibule panel on non-sealed combustion units. Also gasket leaks allowing air into the burner box on sealed combustion units.
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Where there is supposed to be a silicone seal and it breaks down. I have personally never done one but hear goodmans are bad about this.
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I had a few Trane/Am Std furnaces where the screws loosened slightly in the oval ports on the heat exchanger face where the flame enters. Caused a rise in o2 when the blower started.
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To answer your original question, anytime there is more than .5% rise in o2 when the blower starts I am investigating to find out why.
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Gotcha. Thanks for the info. It made sense to me that on a non-sealed combustion furnace that you can have all sorts of ways to introduce O2 via the draft hood/box, but it was not initially clear how a sealed combustion unit would introduce 02 in any avenue than the heat exchanger.
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