In addition to electric wires, and other things in the return air plenum of this HVAC set-up on this 1988 house, the builder felt it was o.k. to run the b-vent from the furnace at the 1st floor up through the return air plenum of the 2nd story unit. I guess they thought it would be o.k. as long as they put a metal sleeve around the b-vent.
Common sense tells me this has the potential to become a very dangerous situation.