Its not rain that rusts the heat exchangers. Its long term exposure to
high humidity air that may have salt in it. Stainless steel DOES rust
in such conditions unless it is coated with a Terne coating.

Stainless Steel roofing in coastal regions will develop rust and fail in
under a decade if it is not properly coated. Such coatings won't work in
a gas furnace. Honda and Acura use stainless steel in the exhaust
systems in the cars they make. Nevertheless these cars do rust through
mufflers and tailpipes


{sigh]

Cars have stainless steel exhaust[type 409 btw] up to the cat because
the feds require all emissions components to last 50k miles.
If a roof was right on the ocean, it would doubtless rust, but after a
few hundred feet, the difference is negligable.

I live near the ocean[my house is a few hundred feet from salt marsh]
and nothing here rusts particularly.

It looks to me like the plating on the ss roofing is to make it less shiny


I cannot see how a furnace inside the house, drawing inside air[as all
low efficiency models would] could possibly rust from outside
conditions. It is far more likely that they never get up to temp from
short cycle and rust internally from the condensate sitting in them. or
the stack never gets above 212 and rusts out or drips back into the
chamber. 90+ models have a pvc output anyway. My shop is 20 feet from
salt water and has 2 10 yo 90percent furnaces. no worries

I am nottrying to be overcritical here, or even say you made the 'wrong'
decision, as I said, I made a similar decison based on old oil prices
at about the same time. I am just questioning the assumption that high
efficiency does not pay back, even at your usage