A crack in a heat exchanger can sometimes be hard to see to the trained eye, even more difficult to show the homeowner.
Here I have some images showing common cracks in furnace heat exchangers. They are often difficult to see when the furnace is installed but maybe will help you understand if you have been told your heat exchanger has a crack in it.
Below you can see several of the rings that are suppose to be holding the heat exchanger together.
