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RA Plenum vs. Ducting cont'd...

RA Plenum vs. Ducting cont'd...

RA Plenum vs. Ducting cont'd...

(OP)
thread403-196203: Plenum Return or Ducted Return?

Two additional comments I would like to add to the previously provided comments address acoustics and when two systems are routed down one shaft. First, regarding acoustics, I would recommend that a dog-leg lined return plenum connected at each floor's shaft openings as a cost effective first measure, possibly in combination with a soundlined shaft. These could be used only on an "as needed" basis dependent upon the results of acoustical analysis software or, short of that, on every floor as, "cheap insurance" against shaft borne RTU noise. Using lined RTU curbs with RA intakes offset horizontally from the shaft opening can also be used effectively. Of particular concern with RA plenum designs will be those spaces with RA grilles closest to the shaft opening where objectionable shaft born noise is most likely to occur. Remember that turning vanes provide a negatively efficient sound reflection in addition to being more statically efficient so you may well wish to eliminate them from this plenum. Second, if you are routing two different systems through a common shaft as is commonly done in multiple floor retail projects, it is important to isolate each system's return air streams from each other by fully ducting one of the system's return air that requires the least ducting and using plenum return for the other. This will asure that the RA temperature control feedback will be representative of the spaces each unit serves.

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