Jumper from supply plenum to return plenum

What would be the reasoning to run a 14” duct off the supply plenum straight to the return plenum? I don’t have a picture but when I was checking the temp drop I was getting the same temp. Traced the return and it just looped back to the supply. There were several other returns on the plenum that were pulling air from the house as well.

Possibly a bypass duct on a zoned system.


Thanks. That’s probably correct since it is a zoned system.