[Bldg-sim] Ventilation Problems in VAV Systems for LEED Model

[Dawne Herrick]

I agree with your idea to manually override the supply air quantities
for some spaces to make the OSA ratio for your system be less than one.
We have done this on a LEED project and believe it is acceptable as long
as your supply air quantities (and system sizes) are the same for your
baseline and proposed.  Good luck!

~Dawne Herrick, PE, LEED(r) AP 
Mechanical and Fire Protection Engineer 

CDi ENGINEERS 
(425) 672-1071 
www.cdiengineers.com 
________________________________

From: Keith Swartz [mailto:kswartz at ecw.org] 
Sent: Monday, February 11, 2008 7:54 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Ventilation Problems in VAV Systems for LEED Model



I am having some difficulties regarding ventilation in VAV systems for a
LEED energy model that some of you have probably encountered and would
like your ideas regarding how to handle them. I am using Trane TRACE
v.6.1.2.

 

1. ASHRAE 62.1-2004 requires some ventilation in storage rooms - 0
cfm/person (because it is typically unoccupied) and 0.12 cfm/sq ft
(Table 6-1). I have a school with some internal storage rooms (no
envelope exposure). The only internal load is lighting. Even if I have
the lights on, the supply air needed is less than 0.12 cfm/sq ft, so the
ventilation fraction for the room is greater than one. The multiple
space equation then causes the entire VAV system to need 100% outside
air. Then the baseline model would need energy recovery...You see how
far from reality this is going?

 

One idea I had to deal with it is to manually override the supply air to
some quantity above 0.12 cfm/sq ft instead of using the calculated
supply air flow, but I have a feeling that the LEED evaluator would not
look favorably on that. Perhaps it would be acceptable as long as I did
it the same in both the baseline and the proposed building. What
suggestions do you have for dealing with this?

 

2. ASHRAE 90.1-2004 Appendix G requires that the minimum volume
setpoints for VAV reheat boxes to be 0.4 cfm/sq ft for the baseline
(G3.1.3.13). This also causes the same problem as above - 100% outside
air for the system. Of course the minimum volume setpoints for the
proposed model would match the design, so it is only a problem for the
baseline. The critical rooms are not only storage rooms mentioned above,
but other spaces where the ventilation requirement exceeds the load
demand, such as restrooms. I have transfer air set up for each of these
spaces. Is there some trick with using transfer air to make it work out?


 

There were no interpretations for 90.1 that addressed this. The only
related CIR I found from the USGBC was dated 2-2-07 with a ruling on
2-26-07 for a lab which states in part, "Alternately, the project may
opt to model the VAV system as energy neutral and use the design minimum
turn down ratios of the proposed design." I take this to mean that I
could have the minimum settings in the baseline match the proposed, but
I would have to change the envelope and lighting for each room in the
system to match the baseline to make the system "energy neutral." We
would loose credit for the lighting savings. There were other CIRs that
stated that the 0.4 cfm/sq ft minimum setting could be disregarded in
spaces where pressures needed to be maintained (laboratories, hospitals,
etc.) Any suggestions?

 

Sincerely,

Keith Swartz, PE, LEED(r) AP

Energy Center of Wisconsin

455 Science Drive

Suite 200

Madison, WI 53711

 

Phone: 608-238-8276 ext. 123

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that seeks solutions to energy challenges.

 

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