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I understand you are trying to determine if your heat exchanger is cracked. This can be done in different ways. But if you watch the burners when they light, do they shoot straight into the chambers? And do they continue to do that when the blower (fan) comes on?
If the heat exchanger was cracked, the flame would tend to "lick around" and not shoot straight into chambers. A reading of 8PPM in the exhaust would not concern me too much. There must be over 50 PPM for me to even start thinking about examining heat exchanger. That small amount could even be caused by gas pressure set to high.
It is usually not a good idea to draw intake air from the area around the furnace as you probably know. It must be fresh outside air.
OK, I was just going by your original question where you said you checked for CO in the exhaust and got 8 PPM.
The best test for a defective heat exchanger is high CO (over 50 PPM) as read in the exhaust of the unit. Other signs are flame licking out and not shooting straight into the chambers and obvious signs of sooting.
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