Full-quoter ransley wrote:

I've been told new furnaces heat exchangers are thinner to
transfer more heat and be more efficent, an old unit I took
out weighed alot more but was real inefficient. Stainless
Steel exchangers with a long warranty maybe 20 years are
offered by a few.

It makes sence that a thinner heat exchanger is more efficient.

It also means that the older furnaces get more efficient over time as
the walls of their heat exchanger rust away.