Strange ductwork - Supply connected to return???
I have an older home that has had many previous owners, and I am trying to understand a strange run of ductwork at my furnace. There is a circular duct (near top of photo) about 10" in diameter that connects directly to the return plenum of my furnace. The strange thing about it is that is comes right off a supply duct about 5 or 6 feet away to the left. What could possibly be the purpose of this? My only guess is that either the return was starved or to heat/cool the return air before it enters the furnace. I am no HVAC professional, but those would not be legitimate reasons, right? If so, what could it be possibly for? It also has a damper on it, and I accidentally nudged it, so not sure to leave it open or closed. Any advice would be much appreciated! Thanks.