Why would a supply vent be routed back to the return side? See photo

[Gene South]

This house had one HVAC with two thermostats regulated by a zone controller panel. Why would a supply vent be routed back to the return side?
33020

[Vern Heiler]

This is a method used to do two things, depends on what they were after when they did it. One is to reduce the supply duct pressure and the other is to increase the sensible heat at the registers.

It is often done when a system is zoned to reduce the noise at the registers when one zone is dampered off.

[Jim Robinson]

Vern is on it. It usually shows up when there are zoned forced air systems, with dampers to open and close multiple zones. A bypass system for when not all of the dampers are open.

[Gene South]

Thanks Jim and Vern. Now...after 3 cups of coffee, I finally understand! I had my head screwed on backwards !

Appreciate you explanations !