I'm in contract for my first house right now, and had my inspection yesterday. When I opened the cabinet with the heater, I saw everything connected as normal, except for a flexible 5" pipe just dangling from the ceiling in the heater closet. I asked the inspector what that was for, and he said it was the cold air return, just venting into the same space as the heater. He said it went to the roof, and brought fresh air into the chamber, for the heater to use. It wasn't connected to anything, just a vent line to the outside, and it worked just fine.