Probably the best way is to visually locate a crack. Many cracks are closed at room temperature and only open when the unit is fired and the heat exchanger metal is hot. However, particularly in modern 80% and 90% forced air furnaces, a thorough visual inspection is time consuming and difficult at best.
To test for cracks using a combustion analyzer, simply watch the 02/C02 readings and the CO reading when the blower comes on - usually several minutes after the burner(s) ignite.
Typically, the 02/C02 or CO readings will stabilize within 30 to 60 seconds after the burner comes on. If a crack is present, when the blower comes on air (at 20.9 percent 02) may be blown through the crack in sufficient quantities to raise the 02 (or decrease the C02) reading on the combustion analyser.
Using a combustion analyzer to test for cracks in a heat exchanger also has limitations, there are some distinct advantages. Good Combustion Analyzer are expensive about $1,200 to $1,800. At JC Heating, we have the proper tools to do a complete combustion test.
The dangers of cracked heat exchangers
Cracked heat exchangers can let dangerous levels of flue gases, smoke, and carbon monoxide gases to enter your home causing injury and or death.
Contact our Service Department @215-945-4833 if you suspect a heat exchanger problem, the life you save may be your own.
Images of Defective Heat Exchangers