[Bldg-sim] DOAS and baseline OA

Jeremy Poling Jeremy.Poling at transwestern.net
Thu Aug 18 06:39:43 PDT 2011


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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