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Golestan (Mechanical)
29 Aug 07 12:31
Please advise:
In small tenant finish-out projects in strip shopping centers.  I have used ducted retun to the RTU return.  A contractor is asking for design change into a plenum return scheme.  Last time I did this I got in trouble because the RTU could not keep up with the cooling of the space.  The air above the drop ceiling  was just too hot.  The building had R-30 roof insulation under the roof deck.  So I am not comfortable the above ceiling (as plenum) return air.

Thanks,
mechanicaldude (Mechanical)
29 Aug 07 13:02
The obvious advantage to a plenum return system is that it is cheaper to install mechanically because of less ductwork that is need. However all the electrical devices will have to be plenum rated.  The problem is if you are going with a plenum return system you must be able to guarantee that the build is constructed air tight and that there are no leaks above the ceiling space. So you will have less control over where the air travels. I never design an existing building with a plenum return unless it was already a plenum return.  
NCDesign (Mechanical)
29 Aug 07 13:34
Golestan how did you treat the plenum space in your load calculations?  Along with the information that cd122395 pointed out you also need to make sure the plumber didn't put in PVC vent pipe (check International Mechanical Code section 602).  These things will determine the answer to the question.  

Did you model it as an insulated roof, an unconditioned space, lay-in ceiling, and conditioned space?  If you did it this way your loads don't fully account for the roof load in the plenum and you are setting yourself up to have a repeat of your previous problem.  This problem is even worse if you have insulation at the roof and at the ceiling.
  
If you ran your loads as insulated roof and conditioned space, all of the roof load is accounted for.  You would just need to make sure that all materials in the plenum area are plenum rated and that the plenum is air tight.
willard3 (Mechanical)
30 Aug 07 16:38
I never use plenum returns because of the variable construction of ceilings themselves and the buck/boost transformers and etc that get put above ceilings.

If you don't really know the RA load, it's hard to size the unit with a plenum.

Plenum systems are also almost impossible to balance because what was the lowest RA pressure path to the fan today will not be the same tomorrow after ceiling tiles are removed/altered.
imok2 (Mechanical)
30 Aug 07 18:29
I found that ceiling returns are usually a disaster especially if they are a few years old. A large floor ceiling return takes a long time to balance and a very short time to unbalance. I found that most maintenance men who service these types of buildings don't understand the consequences of removing ceiling tiles and not replacing them. The farthest returns from the fan barely move any air so that the area gets very warm.
Drazen (Mechanical)
31 Aug 07 9:27
if it is really return air (not merely exhaust) and you have the mentioned stratification, it appears that plenum is not an option in case that you have no conditioned space above that ceiling - that would be waste of energy.

sunshine
Golestan (Mechanical)
31 Aug 07 10:29
Thank you all for your valuable input.  The model for the load calc I usually use is roof insulation and conditioned space all the way to the floor.  There is no insulation above the ceiling tiles. Is that a reasonable approach?

Thanks,
mechanicaldude (Mechanical)
31 Aug 07 12:15
That is how I always model it in my calculations.
atlas06 (Mechanical)
1 Sep 07 7:42
With RA plenum, the space does not see the roof load (well most of it), the coil does. With a ducted return, you will need more air flow, thus larger ducts, larger fans, etc..and you will need much more ceiling space since now you will have supply and return ducts crossing at the ceiling.
In the case of single-story malls, a return air plenum has the advantage of heating the ceiling space and keep the plumbing pipes from freezing.

MOSSMANPE (Mechanical)
4 Sep 07 16:31
RA plenums are not an automatic, but any time you can use them. They are great for mold prevention. Designing hard ducted RA systems because you think so and so is really irresponsible. If you did it for me, you would be out the door.
NCDesign (Mechanical)
4 Sep 07 16:54
I think someone decided to go trolling this afternoon.  

Back on topic..
Golestan based on how you ran your loads I would say you are more than safe from an HVAC standpoint to go with a plenum return system.  The balance and other issues brought up are valid but are probably not that big an issue if you are talking about 1400 ft2 of retail space.  

Just keep in mind some of the issues external to HVAC.  Will the electrician have to run plenum rated cable instead of MC?  Will the plumber have to use cast iron pipe instead of PVC?  Is the insulation on the underside of the roof plenum rated?  You could be changing all these other requirements by allowing a plenum return system.  Although you should save some money on the HVAC system it could cost a whole lot more if some of the other systems have already been installed and need to be reworked.    
BravoCompany (Mechanical)
4 Sep 07 19:16
See the following link:

http://uccsuiouee.org/seminars/UC%20CSU%20VAV%20Design%20Guide.pdf

The attached link is for a VAV design guide that provides some practical information.  Pages 29, 30, 31 I belive provide itemized listing of advantages vs. disadvantage.  There are some very valid points to consider.  Atlas06 makes an very valid point about the shift in load from the space to coil (if you don't understand this concept take some time to).

If you can control the right combination of construction variables return air plenums are the logical choice under the right circumstance.  If those same variables can not be controlled you would be safest with ducted design.

Anyhow good luck!
MOSSMANPE (Mechanical)
5 Sep 07 8:03
Trolling? Not intentionally, but if so it worked. The last two comments are valid - junior engineers and designers. These are the reasons why a decision to go RA plenum is "not automatic". But if I can, I will. I'm out.
huckhound (Mechanical)
11 Oct 07 13:31
my experience is, that buildings with RA plenums do have the problems mentioned above. Currently, I am working to fix a old building with sick building syndrome, ie many occupants are disatisfied with IAQ. The RA plenum is leaky, and return air flow is low, causing poor IAQ.

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