[Bldg-sim] DOAS and baseline OA

[Anne Juran]

I agree that the OA difference is extreme. I just let Trace run wild and do
the calculation so I'm sure I have some crazy factors that would be adjusted
for a "real" design.  In retrospect, I should have looked at it closer
BEFORE submitting to USGBC.  I'm sure it made them closer at it, whereas if
I was only slightly different they may have not questioned it.  Lesson
learned!

 

It doesn't look like I'll get anywhere with USGBC, though, as the reviewer
explicitly stated, "the total minimum outdoor air ventilation volume in the
Baseline model must never be greater than the Proposed model."

 

Thanks for all the input!

 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jeremy Poling
Sent: Thursday, August 18, 2011 9:40 AM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] DOAS and baseline OA

 

While I don't intend to discuss the merits of VAV systems and know I'm part
of a minority in that area, this is exactly why DOAS systems are being
explored more right now on the research side (take for example the number of
ASHRAE Journal articles on the topic over the past few years).  Without
seeing your OA calcs I would also agree that the difference is more than I
would expect between the two systems, but I'm more inclined to think the
DOAS might be a bit low if it is being used in conjunction with FCUs.  VAV
systems are typically underventilated due to two common mistakes in the
calculations: not analyzing with the correct Ez and not using the minimum
expected primary airflow for design purposes (refer to ASHRAE 62.1-2007
Section 6.2.5.1, specifically the note in that section).  When fixing these
two typical mistakes in calculations without optimizing the primary airflow
rates, I typically see OA requirements double from the incorrectly
calculated values.

 

This situation is a good example of when to do one of two things (possibly
both together)

1.       If this is a LEED project, submit a project-specific CIR

2.       Use section ASHRAE 90.1-2007 Section 2.5 Exceptional Calculation
Methodology to get around the requirement and document the energy savings
from reduced OA requirements for a DOAS system.

 

Jeremy R. Poling, PE, LEED AP+BDC

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Mark Sorensen
Sent: Wednesday, August 17, 2011 11:42 AM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] DOAS and baseline OA

 

Anne,

While LEED/ASHRAE 90.1 requires the ventilation rates to be the same in both
the Baseline and Proposed systems, the calculated difference for the two
systems is much higher than expected. Suggest taking another look at the
calculations and confirming that the critical zone for the VAV system has
been properly determined and whether appropriate factors for the zone air
distribution effectiveness (Ez) and system ventilation efficiency (Ev) have
been applied.

Mark Sorensen
Diversified Energy Services
Fruitport, Michigan
231-578-1264

On 8/16/2011 9:12 AM, Jim Dirkes wrote: 

Dear Anne,

I'm not sure why you think the OA requirement varies by system.  I'm not
well versed in Standard 62, but my basic understanding is that one of the
calculation methods is the result of building area and number of occupants.
Those are unchanged by system selection . which is why it makes sense for
Appendix G to require matching volumes.

 

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Anne Juran
Sent: Tuesday, August 16, 2011 8:33 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] DOAS and baseline OA

 

I'm curious how everyone is handling the OA for Appendix G simulations when
using a DOAS in the proposed system.  We have a design that is DOAS + FCU
with a  total OA of 8,000 CFM.  When you run the OA calcs for this same
building with a VAV system (the baseline), the total OA required to meet
Standard 62 is 39,000 CFM.  This difference in OA represents a significant
energy savings (in climate zone 4A), yet Appendix G requires the OA volumes
to match.  It does not seem "fair" to me that the proposed case cannot take
credit for design choice when it comes to OA.  I feel like Appendix G should
make an exception for DOAS.  Am I missing something?  Is there a way around
this?  Any thoughts are appreciated!

 

Anne

 

 

 
 
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