Return Plenum Box 1

[josephjoy]

Hi,

In our project the return is non ducted. I mean the false ceiling void will act as a return space.

Do i need to provide a return plenum for the diffusser or is it ok just to leave the return diffusser installed without a plenum.

Please provide your feedback guys

 

[willard3]

Non-ducted returns will bite you in the butt. Bad design, bad operation, cannot be balanced well.

 

[Nathon]

I'm having to design a finish out on an existing office space that uses the ceiling as a non-ducted return to a main, dampered plenum. I just stuck a large return grille in the middle of the space (not even near the plenum), without duct. Each partitioned space also gets its own smaller grille into the ceiling.

 

[tcihvacff1]

It's possible. But you need to take care of the following:
1. If the Surrounding rooms are unconditioned or if you have kitchen area nearby, then see that their must be a thermal barrier/partition in between the conditioned & unconditioned space above false ceiling.
2. Fresh air should be discharge freely to the application room (in general 15 percent of Supply air) or back to Unit.

You didn't mention in your post that the Unit is serving for a single room or to more.
If it's serving for more than one room, you must have a transfer duct fixed in the partition of two rooms so that the return air volume should be balanced.

Finally it's a ideal design to have a return air duct with plenum.

hope this info will help you.

 

[Drazen]

the main purpose of diffuser plenum is to smooth out air flow, but as you would anyhow have messy flow over the ceiling, diffuser plenum have little meaning.

you could try with balancing dampers at diffusers, though.

 

[dbill74]

When using a plenum (non-ducted) return, you do not need to put a box on top of the return grilles.

However, consider the space the return grille is serving. If the space being served is a conference room or executive office, plenum box on the grille can be used to mitigate sound transmission from that space into the ceiling plenum and adjacent spaces. You could also use a plenum box to block sight lines into the plenum space (I never used for this reason, just throwing it out there).

Back when I worked for a firm that did lots of tenant build-outs in high-rise office buildings, common practice was to just use egg-crate grille in the ceiling. For sound sensitive areas as I mentioned here, plenum box with several feet of duct board did the trick.

Main thing to watch for when using a RA plenum is everything else that goes above the ceiling. PVC D-W-V piping is not generally rated to be installed in a RA plenum. Also, need to be sure other disciplines are aware of this construction as not all IT cabling or electric wiring is rated to be installed in a RA plenum. (Electric wiring isn't usually a concern since it is often installed in metal conduits.)

 

[irfansiddiqui]

no need of any box, if it is office space, void area in the ceiling will act as your return plenum.
its fine and in practice.

for air flow balancing - you can do it with the dampers in supply duct

 

[DRWeig]

Read through this thread for more input:

http://www.eng-tips.com/viewthread.cfm?qid=345151

Best to you,

Goober Dave

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