At this moment I am having my Bryant furnace replaced. The heat exchanger is bad and they claim it isnt replaceable. I said I wanted to keep the old furnace and was told they had to have the old one in exchange. My new furnace is a Carrier (practically the same one) The sold it to me claiming it had a 20 year warranty on the heat exchanger. If I remember right, the Bryant did as well, and now I have to buy a brand new furnace. I bought the Bryant 4 years ago. the manufacture date makes it 7 years old. Far cry from 20 years, and still no warranty.
I would have no qualms with fighting this further, but dont know where to go from here.