This was a “down-flow” unit, where the return air compartment is on top of the furnace, which was on top of the evaporator coil.
Typically, I see these types of units with the vent pipe routed around the return air compartment, but not this time.
The vent runs directly through the return, and out of the closet from there. Notice the soot at the inside of the cabinet.
Not a very good design. . .
Not allowed in this state flue can not pass thru any supply or return air.
ditto here…
That is probably true in most jurisdictions, and model codes address it as “constructed components” - plenums and such. In this case however, this is how the FAU was manufactured, in other words, this is (was) a listed system.
This entire complex had these units installed (34 years ago) and I inspected three of them yesterday. The fact that two, out of three, of these units showed evidence that the vents were “leaking” into the return air compartments, was a bit alarming.
It make changeing the blower motor a real pain, been there 