Well, this is contrary to what the installers told me here when my 18
year old 65% furnace died and I was forced to replace it with an 80%
(the cost delta between 65% and 80% will NEVER, NEVER be recovered in
the lifetime of this furnace even if gas prices TRIPLE!!!)

I had stuffed fiberglass insulation around the plenum of this up flow
furnace. They removed it and said that combustion air comes from the
attic. Very little air comes thru the crack at the bottom of the door
to the closet where the furnace is installed.

90% furnaces are condensing and the flue must be PVC and routed
HORIZONTALLY outside. The water that condenses and drips outside is
caustic, and limestone or marble chips need to be placed under the drip
zone to protect neighboring plants. 90% furnaces should NEVER be vented
out a chimney.

ahh my other home exhausted the 90+ furnace above grass, it wasnt lush
green but it survived

they sold you a 80 furnace and promoted it as such.

but you lost 15% efficency For the life of the furnace, how much did
you save ?$500?

With rising gas costs that payback will occur and if you sell a 90+
furnace is a sales advantage