[Bldg-sim] DOAS and baseline OA

Patrick J. O'Leary, Jr. poleary1969 at gmail.com
Thu Aug 18 07:33:13 PDT 2011

note that 90.1 requires the <i>minimum</i> outdoor ventilation rates be 
the same in both the proposed and baseline models.  this is to prevent 
gaming of the baseline system sizes by adding capacity/increasing unit 
sizes which will increase baseline energy cost.

though the baseline ventilation rate(s) can be the same or less than the 
proposed they can't be more.

when would the baseline be the same?  when you only provide the minimum 
required outside air per ashrae 62.

when would they be different?  when you provide more outside air in the 
proposed design than is required by ashrae 62.
e.g.1  providing 30% more outdoor air than required to obtain 1 leed 
point via ieqc2 (increased ventilation)   (or using the international 
mechanical code ventilation rates if your local jurisdiction does not 
accept ashrae 62)
e.g.2  using evap cooling systems sized on air change rate by volume & 
100% outdoor air.  ashrae 62 may require only 2,000 cfm but if you 
provide 20,000 cfm (based on space volume) then the proposed would have 
20,000 cfm outdoor air and the baseline would have 2,000 cfm.  in a case 
such as this using 20,000 cfm in the baseline would require a very 
oversized unit that would use a lot more energy to condition the outdoor 
air and would reward you with more points under eac1/eap2.

On 8/18/11 7:03 AM, Anne Juran wrote:
> 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 
>, 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> 
> [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 
> <mailto: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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