I have a 90% furnace and it does have an air intake tube that takes air
from the outside but my understanding is that is for the combusion
process. The air that is being heated comes from the return vents
(totally separate). I admitady dont know about HVAC here but this is
just my understanding.
I went from a 70% efficient to a 90% efficicent and I save about
$300/year on heating a 1200 sq foot home comparing 2004-2005. Now, I
also did two other things when doing this furnace replacement. I blew
in an extra 12 inches of celulose in the attic and put in a chimney
balloon for my fireplace since my damper was leaking cold air real bad.
These probubly helped the savings as well.