noon air wrote;
For only an 18x18 return its gonna have
to be a real small furnace.... How about
post the make/model/serial number of
the furnace so we can find out what its
rated for on the airflow and we're not
just guessing here. Links to pics would
be helpfull too.

I just had a Trane 3 ton air handler installed and the opening going
into mine is about the same size.

Actually it's where the filter slides into for units that are installed
inside the dewelling. ie; Apts, condos, depluxes, etc., NOT houses where
the handler is in the garage. Although a house that doesn't have a
garage has to have it installed in the house.

Apparently these "newer" air handlers are getting smaller. The
foot-print of this new Trane is about 6" smaller than the 20 yr old 2
1/2 ton "Comfort Maker" air handler that I replaced.

Also, I have a heatpump and the emergency/aux heating coil is smaller
than the one in my older unit (i saw it when they pulled the old unit)
but according to the AC company it's the same size.

Bull****, it doesn't even get HOT, it gets WARM.

Good thing I live in Fl! LOL