Plenum Returns – Good or Bad?


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.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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2 Responses to Plenum Returns – Good or Bad?

  1. Robert says:

    Hello Mike. I have a building that has 2 Trane 15 ton systems. To me it always smells musty. Its a matched 15 ton split system with two 7.5 ton compressors . I noticed very little if any condensation dripping from the system out the pipe. I believe this system is cooling the building with a dry coil and not removing the humidity. The second system has condensation flowing out the pipe. Superheat runs approximately 10 to 12 degrees high. I was thinking air flow, TEV or a partially plugged evaporator coil. The systems serve an open space area with each taking care of 1/2 the space. What are your thoughts?

    • Your superheat sounds like it is ok. Have you checked the drain to make sure it isn’t plugged. It could cause the “musty” smell if the insulation is getting wet. From the little data you have provided, I would look to something like a drain problem

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