[Bldg-sim] LEED & Natural Ventilation

[David Reddy]

  Chris-

One thing to keep in mind regarding eQUEST/DOE-2's "natural ventilation" 
model, is that it is based on the Sherman-Grimsrud method, which was 
specifically developed for estimating envelope infiltration in smaller 
(1-3 story), single-family structures.  Below is a link to a copy of the 
original report outlining the method.  The report gives some good 
background on other infiltration models, and the context in which the 
model was developed (incl. comparison to measurements).  As described in 
the report and the DOE-2 help manuals, you will find that many of the 
model inputs are specifically relevant to small houses.  Finally, in the 
authors own words (p. 4), "this report introduces an infiltration model 
that sacrifices accuracy for versatility and simplicity."  Therefore, in 
my opinion, using this model, especially for larger structures, has a 
good potential for giving misleading results.

gundog.lbl.gov/dirpubs/10852_ShermanGrimsrud.pdf

Unfortunately, DOE-2's "S-G_NV" method is the only NV model _readily_ 
available in eQUEST.  By readily available, I mean you can select it and 
enter inputs within the detailed or other BDL interface, and many of the 
inputs have defaults.  However, just based on my experience in 
estimating the inputs for three of the DOE-2 "nonresidential" 
infiltration methods (AIR-CHANGE+AIR-CHANGES/HR, 
AIR-CHANGE+INF-FLOW/AREA, and CRACK), these models can be quite 
sensitive to inputs, including those in the SITE-PARAMETERS, and finding 
estimates of these inputs for large buildings in literature is 
difficult, if they even exist at all.   I have not had the opportunity 
to compare the S-G_NV model with another NV model for larger buildings 
(such as CFD), but I would guess that if they are close, it would be 
mostly luck that you picked the correct S-G_NV inputs.

I have modeled a couple nonresidential buildings that attempt to use a 
NV strategy to offset mechanical cooling needs.  However, to be 
conservative, I often only accounted for an expanded comfort range 
(similar to what Timothy described) rather than trying to use the 
DOE-2's S-G_NV method.  Although I think some buildings are 
well-designed to take advantage of NV, in my mind, unless you have some 
sort of mechanical controls to open windows, all bets are off.  Not that 
there aren't some real potential energy savings with well-designed NV 
strategy, its just that the methods for estimating the impacts w/ 
eQUEST/DOE-2 are not sophisticated enough.  Anyone out there have any 
evidence or comparative modeling that indicates otherwise?

After doing a lot of thinking about modeling NV, one idea for a 
"work-around" that accounts for NV using eQUEST, is using custom LOADS 
(infiltration) / SYSTEMS (availability) schedules and other inputs based 
on pre-processing of weather and building geometry data (may require 
some iterative runs...). I would guess that the majority of NV impacts 
are from wind-driven infiltration through windows.  Along these lines, 
below is a link 
<www.pnl.gov/main/publications/external/...reports/PNNL-18898.pdf>to a 
PNNL paper that describes how to develop E+ "wind-driven" infiltration 
model inputs (basically the same as DOE-2 "AIR-CHANGE" method) using 
local wind pressure coefficients and a design infiltration rate (leakage 
@ 75Pa).  For combination wind+stack driven air flow through windows, 
the model would likely have to get more complicated, but pre-processing 
to calculate DOE-2 inputs gives you a lot of flexibility in what you 
account for...

I have not actually had the opportunity to develop this kind of method 
yet, but I hope to get a project where the design (and budget) justifies 
this level of analysis.  With any type of work-around, I would plan to 
do some sort of validation with another model, such as CFD, or a 
calibration exercise, but these approaches for validating NV open other 
cans of worms...

Anyone have any experience with validating the NV model (or your own) 
that exists in the one of the available building energy simulation 
programs?   Any other ideas for "work-arounds"?  Has anyone gotten 
feedback from reviewing bodies (GCBI, utilities, code reviewers) on the 
use of DOE-2's "S-G_NV" method?

www.pnl.gov/main/publications/external/...reports/PNNL-18898.pdf



David Reddy

360 Analytics
Building Energy Analysis Consultants
mail:	12354 16th Ave NE, Seattle, WA 98125
office: 206.420.7918
mobile:	206.406.9856
web:	www.360-Analytics.com



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