Are you sure about that? I did some research on this on the internet, and the house air in the exchanger is at a higher pressure than the exhaust gas, and so house air goes into the exhaust if there's a crack, not the other way around.
Possibly not the heat exchanger as a most likely cause, when all is functioning well (having just performed my own brief online research).
However, the original question concerned the longevity of gas furnaces, and possible limiting factors on their lifespan.
As such, you might care to contemplate:
* What are origins of CO?
* What are such possible origins within a house?
* What are the likely mechanisms for transit of CO from normal exhaust systems to interior space?
* What might be the long-term consequences of thermal stress, oxidation, cycling, etc., on manifolds of such spaces.
If you choose to focus myopically on heat exchangers and heat exchangers alone, that's your call. I'd suggest keeping the batteries up to date in your CO monitors, however.
