What is a cracked heat exchanger?

The left picture below is of a standard 80% efficient furnace. The right picture is of a 90% efficient furnace. It contains a small secondary heat exchanger for added efficiency.

In both cases the red and orange coloring represents the path of the flame and of the super heated exhaust gases. In both cases the blower pushes the house air around the chambers and out to the living space. Older furnaces vented their exhaust gases straight up through simple heat exchangers. Modern furnaces force the flame and gases through a serpentine shaped path (the heat exchanger) in order to increase efficiency. Should a crack develop in the heat exchanger, exhaust gas could spill into the house or house air could be forced into chamber. Either condition has the potential to be dangerous.

The first two pictures below are of cracked clamshell heat exchangers that have been removed from their furnaces.

The picture below is of a tubular heat exchanger that has not cracked yet and that's still in service. You can see discoloration where the tubes have had the most stress. Years from now those spots are likely to be where cracks form.

Most cracks that are found are of the hairline variety. But occasionally a furnace goes undiagnosed long enough that the crack has a chance to open up. That was certainly the case with the furnace below. What you're looking at on the right is the inside of the heat exchanger where the flame burns. Air from the house blows around that chamber to pick up heat. With such a large crack either the air from the house was blowing into the chamber or exhaust gas from the chamber was being sucked into the house. Either condition is bad news.

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