Carrier/Bryant has a very specific procedure to test the heat exchanger on their 90% furnaces.
Originally Posted by paulo500
Ask the contractor if he has a combustion analyzer in order to do this test.
If this test fails, the heat exchanger should be replaced under the 20 year warranty.
It also must be determined that the failure was due to defects in material or workmanship, under normal use and maintenance.
The option of credit toward a new system is available too. This might be a good choice as your system is 12 years old.
Remember that labor costs, including diagnosis, are not included in the warranty.
Side Note: please do not put pricing in your posts(see site rules) - thanks
Instead of learning the tricks of the trade, learn the trade.