One way is to observe the flame pattern of the burner as the room air blower turns on. If there is a noticeable crack, the blower will disrupt the flame and cause it to yellow. Another way is to insert a smoke stick in the burner and light it. Then turn on the blower and inspect the registers for colored smoke. This is more of a messier way to do it but a real confirmation of a crack. Package units don't have much of a vent so it is very easy to run a long handled brush thru it. You can also check for Carbon Monoxide in the duct system with a tester. Then again, a good flashlight may discover the issue if you look at all seams.
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One way is to observe the flame pattern of the burner as the room air blower turns on. If there is a noticeable crack, the blower will disrupt the flame and cause it to yellow. Another way is to insert a smoke stick in the burner and light it. Then turn on the blower and inspect the registers for colored smoke. This is more of a messier way to do it but a real confirmation of a crack. Package units don't have much of a vent so it is very easy to run a long handled brush thru it. You can also check for Carbon Monoxide in the duct system with a tester. Then again, a good flashlight may discover the issue if you look at all seams.