Technically the primary hx is NOT covered under this warranty, only the secondary. I always replace both and the customer is aware if they reject primary cost it is on them.

I believe the replacement secondary is the exact same hx that failed, you need to confirm this. You also need to confirm the warranty as I believe the warranty ends with the initial warranty period, in your case no further warranty.

I would usually suggest new furnace over hx replacement, you do have the parts for future failure(why you have them is a whole other topic) which would be my reason for new over fix.

Your call at this point.

Here is a link(I am not sure of the validity or if this) to the Carrier settlement info:
https://www.furnaceacexperts.ca/heat-exchanger-failures-warranty-program/

 

Some additional information.

Regarding the primary heat exchanger cells, according to the Carrier HVAC contractor they will also replace under warranty both the primary AND he secondary cells at the same time as part of the warranty. I think that is pretty amazing for Carrier. Additionally, what I didn't share before is that I'm a mechanical engineer with electrical/mechanical aptitude with experience troubleshooting, cleaning and repair/maintenance on my furnace. (ie for example I had the burner nozzles out of the firebox last week and cleaned them up by removing rust plugging one of the 4 burner nozzles)

 

20 years old, replace.

 

No doubt about it, replace it. You'll get a furnace with a better design. Be sure to have a manual J load calculation performed to determine the correct size because 90% or more of the equipment we encounter is oversized.