Plenums necessary for LEED model?

asked 2015-07-16 13:14:53 -0500

jkjenner

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updated 2015-07-16 13:40:10 -0500

MatthewSteen

10077 ●18 ●25 https://www.nrel.gov/b...

I'm preparing an energy model to be used for a LEED 2009 EAp1/c1 submission. The building I'm modelling is a single storey school, which has a mix of exposed ceilings and drop ceilings, as well as a few spaces (e.g. the gymnasium) with higher ceiling heights than the rest of the building.

I'm using eQuest, and without getting into too much detail I will just say that based on my experience with eQuest, my life will be a lot easier if I just model the entire building as exposed ceilings--in other words, remove the ceiling plenums. It seems to me this would have a very marginal effect on the model results, as I wouldn't be changing the exterior surface area, but I wanted to see if anyone had any experience with this.

Basically I'm wondering if removing the ceiling plenums in the manner described above:

Would unduly affect the accuracy of my simulation results
Would be kosher for LEED

Any insights would be much appreciated!

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Welcome @jkjenner. I edited your tags to be more general and changed plural to singular.

MatthewSteen ( 2015-07-16 13:40:51 -0500 )edit

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answered 2015-07-16 13:38:13 -0500

nfonner
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updated 2015-07-16 13:39:43 -0500

I presume that in doing so (eliminating the plenums) the consumption for both the baseline and proposed cases will be noticeably higher since you will have to make the conditioned zone volume larger to account for the exterior wall area contained in those plenums. (Exterior wall area and Window-to-Wall Ratio are defined to the bottom of roof deck not top of ceiling.) This could be a drastic change depending on how deep those plenums are, especially when dealing in terms of a simulation platform that treats a zone as uniformly conditioned (the zone volume is a uniform air temperature).

The increased consumption comes from this additional volume, but also comes from increased ventilation if one is not careful to specify the OA rate per zone based on what IEQp1 is saying. Again this is a comparison of modeling net vs. gross. I've seen GBCI call out modelers when the OA varies too much (too big and too small) from that scheduled on the MEP sheets and IEQp1.

I'm sure there are other considerations I am missing, but those are the big things off the top of my head that a reviewer will likely catch and would impact the accuracy of the model compared to one with plenums added. In my opinion, the issues above are the likely reason that most programs explicitly have plenums as a modeling feature.

Hope this helps some,

Nic

P.S. I agree that they are a huge pain in the eQuest.

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Thanks @nfonner. My main uncertainty relates to your first paragraph: the exterior wall area would be the same in either case (plenums or no plenums), and because the thermal resistance of the ceiling is negligible, at a steady state the temperature of the plenum and the conditioned space below it would be more-or-less equal. So it seems to me that by removing the plenums, all you're removing is essentially a bit of thermal capacitance, which might not have all that large of an effect...

jkjenner ( 2015-07-16 14:45:58 -0500 )edit

I would be interested to see just how close the air temperature of the conditioned space is to the plenum space. Every time I do a model in E+ it is worth modeling the plenums, but that's another story really.

Also, I would have to look into this, but I wonder if infiltration through those plenum walls is passed to the conditioned space below. In other words, modeling the plenum would mean reducing the air infiltration rate into conditioned spaces.