I am suspicious as to how this 90% efficiency is calculated as it
would mean that the stack plenum temperature at the furnace should be
close to ambient temperature, a practical impossibility. It didn't
occur to me to see the exhaust stack but it looked like the regular
galvanized steel. I would never accept plastic pipe (above article)
for this installation anyway.
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the 90% is safely vented using plastic pipe at just ocver ambient temp
by using a secondary heat exchanger.

plastic being non corrosive is actually safer in this application since
steel or glvanized will rot out.

heat exchangers on 90+ furnaces are stainless steel to inhibit corosion.