Plenum Return or Ducted Return?
Plenum Return or Ducted Return?
(OP)
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,
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,





RE: Plenum Return or Ducted Return?
RE: Plenum Return or Ducted Return?
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.
RE: Plenum Return or Ducted Return?
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.
RE: Plenum Return or Ducted Return?
RE: Plenum Return or Ducted Return?
RE: Plenum Return or Ducted Return?
Thanks,
RE: Plenum Return or Ducted Return?
RE: Plenum Return or Ducted Return?
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.
RE: Plenum Return or Ducted Return?
RE: Plenum Return or Ducted Return?
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.
RE: Plenum Return or Ducted Return?
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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!
RE: Plenum Return or Ducted Return?
RE: Plenum Return or Ducted Return?