wrote:

... the thin-walled heat exchanger is more "efficient" because it's smaller.

Not much, I'd ween, if the dimensions of a forced air furnace heat exchanger
mostly depend on the air passages. With less metal, it would weigh less and
cost less, but those are different concerns...

... if the metal is a good conductor, eg steel with 50 Btu/h-ft-F,
with poor airfilm conductances on both sides, eg 5 Btu/h-F-ft^2,
thinner steel won't help much. How much, in this case, starting
with 0.050" steel?

Heh, don't try to obfuscate the facts by spewing a bunch of calcs as
usual, trying to cover up...

It's 300-year-old physics :-) What's the answer to this simple problem?

Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F...

Still stuck? Try 600 Btu-inch/h-ft^2-F.

... the thick steel conductance is 600/0.050 = 12,000 Btu/h-F-ft^2.

And if we halve the thickness, it becomes 24,000 Btu/h-F-ft^2. Wow!

So the steel thermal resistances are 1/12K and 1/24K h-F-ft^2/Btu.

Now what do we do with resistors in series?

Add them. So the "less efficient" heat exchanger would have
a thermal resistance of 1/5+1/12K+1/5 = 0.4000833 h-F-ft^2/Btu
vs the "more efficient" 1/5+1/24K+1/5 = 0.4000417 h-F-ft^2/Btu,
with 0.01% less thermal resistance :-)

Nick