Hello group,
I?m unsure how to deal with interstitial spaces between floors (see sketch attached) that are only indirectly heated and cooled. This is for a LEED project, so I have to document how I account for the energy in eQuest. The spaces are not occupied, and should not count toward building area gross square feet. Simply treating the spaces as unconditioned on one side of a horizontal partition won?t be very accurate.
I have considered modeling the interstitial spaces separately, exporting the hourly loads and temperature profiles, and creating some sort of internal load to represent them ? but that sounds like a long way around to get where I?m going.
Does anyone have a hint for me?
Thanks!
David R. Weigel, PE
David,
Your sketch shows that the interstitial spaces contain uninsulated return ductwork. Without knowing more about the design, I?d be tempted to model the interstitial spaces as unconditioned plenums. This is easy to do in eQUEST. The spaces would not be counted as conditioned floor area in the DOE-2 reports and I don?t see any LEED concerns. Check the definitions in ASHRAE 90.1 for space, conditioned space, semiheated space and unconditioned space, and reference Table 3.1, to confirm that the interstitial spaces meet the definition of unconditioned. LEED reviewers are familiar with these definitions and will refer to them if they question your modeling assumptions for the interstitial spaces.
Best regards,
Bill
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Thank you, Bill. That?s good advice.
I?ll try to post results when it?s done.
Best to you,
Dave
David R. Weigel, PE
Hi Dave,
Ultimately your model will require extra explanation if you don?t come into the same ballpark as the area numbers derived from others? credit templates - you can make the areas add up correctly in a number of ways, but you may have to decide how you?re handling your interstitial floors first/separately.
If you feel you can make the case that heat transfer to/from these floors and to/from the uninsulated return air paths is not consequential, then your simplest approach probably is to simply not define those floors during wizards and manually specify interstitial lighting and other internal loads/scheduling directly to your electric meter. I would plan on presenting a screenshot of those meter inputs as part of your documentation with a brief explanation of what they represent and how they were determined.
If on the other hand you are concerned about some of those heat transfers being significant enough to bother modeling (I probably would be with uninsulated ducts), model the floors as unconditioned space types (without plenums if appropriate), then once in detailed mode:
1. Ensure the floors/ceilings separating your conditioned floors are thermally tied to the interstitial floors with internal partitions of an appropriate construction type ? you?ll probably need to define something unique for these floor partitions.
2. Check/define exterior surfaces/constructions and internals for these zones (lighting/equipment/people/infiltration) ? you don?t want to define at meter if you intend to model heat transfers.
3. Go to airside systems tab: For the systems pulling air through these spaces as part of the return air path,
a. make sure you?ve chosen ?ducted? return,
b. enter something for duct air loss ratio (can be zero)
c. this allows you to define a zone from which the return air ducts will transfer heat to/from ? choose the appropriate interstitial zone, and fill in the remaining heat transfer inputs.
I can?t tell you from experience whether a LEED reviewer will make you model those floors as unconditioned spaces after the fact if you choose not to, but based on my general sense of the LEED review process the degree of rigor seems to scale up when pursuing larger numbers of points. I have never personally seen duct losses come up in review commentary.
It might be worth investigating whether there is any precedent or requirement to NOT model duct return leakage/losses in the way 90.1 says piping losses are explicitly NOT to be modeled. It might simplify your situation considering the potential challenges of applying these heat transfers to your baseline model in a fair fashion.
~Nick
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NICK CATON, P.E.
This is also very good advice, I appreciate it!
I believe I?m going to follow the plenum path if I can justify it with the rules that Bill pointed out. However, I?m also going to go ahead and model the interstitials as you suggest and evaluate the duct heat loss / heat gain carefully so that I can discuss it in detail in my documentation.
It will be interesting to see how the space behaves, and to determine the magnitude of its effect.
David R. Weigel, PE