wrote:

mm wrote:

I am told by experts in the field that one of several ways that
high-efficiency furnaces squeeze more BTU's from their fuels is by
using thinner and thinner heat exchangers

This seems like one of the false efficiencies...

That simply isn't true. The heat is going to transfer more effectively
across the thinner material. The thickness of the metal provides
a resistance to heat flow, just as thicker insulation, wood, or
anything else would.

But metals are such good conductors that making the metal thinner won't
help much, given high resistance air layers on both sides, and thicker
metal will spread out hot spots and increase efficiency.

Nick