I recently had this question sent to me —“What are your thoughts regarding plenum returns verses duct returns. One of the obvious is cost but how about system performance.”
Great question! In my opinion, I have always said that a “closed duct system” is always the best. With a closed duct system, you can deliver the proper amount of air to any given space and you can also remove (return) air from any given space. This adds to both the comfort produced by the system as well as the efficiency of a system.
Think about it in reverse. How efficient would a “plenum supply” be? They are done all the time with concentric diffuser coming down off a roof top unit or just having registers cut into a plenum on an air handler.It really comes down to the application and the space being heated or cooled.
If the space is small both a plenum return or concentric supply can be very applicable. Now, when you have a large open space plenum returns will probably work fairly well. However, when you have multiple small spaces as well as open spaces, in my opinion, a plenum return is not a good application. Besides not being able to control return air from the enclosed spaces, there is always the possibility of noise transmission from one part of the occupied space to another.
So, as I stated at the start, in my opinion, I have always believed in closed duct systems for maximum performance and efficiency.
All comment and opinions welcome on this and I will follow up with another post based on your comments and/or suggestions that I receive.