One of the reasons for the limited life would be the longevity of the actual heat exchanger - they will eventually corrode due to the corrosive nature of the exhaust gases passing through them.
Additionally, the manufacturer probably doesn't want to have to stock parts for 16+ year old models.
An analog would be software; nothing stops a circa-1994 Windows NT machine from continuing to work, but good luck getting support.
Corrosion is unlikely to cause any problem with the heat exchanger unless it is really severe, as there are no moving parts in it.
> nothing stops a circa-1994 Windows NT machine from continuing to work
My older machines of that era have all failed due (most likely) to failed capacitors. The disk drives fail most likely due to lubrication issues. The semiconductors fail because all semiconductors fail due to the migration of the doping that occurs when they get warm.
None of my 80's machines will power up anymore. I'm not too surprised, they were only built to last a couple years, and it only takes one failure in those enormously complex machines to do the whole thing in.
I try to keep my XP machines running because those were the last to support DOS programs and I still support Digital Mars C++ generating DOS programs.
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.
Additionally, the manufacturer probably doesn't want to have to stock parts for 16+ year old models.
An analog would be software; nothing stops a circa-1994 Windows NT machine from continuing to work, but good luck getting support.