[Bldg-sim] DOAS and baseline OA

Paul Riemer Paul.Riemer at dunhameng.com
Tue Aug 16 08:40:47 PDT 2011

A LEED reviewer has acknowledged improved zone effectiveness for our true displacement ventilation system with separate perimeter heating such that the supply air was always sufficiently cool.  I did it as an exceptional calculation because I knew they would pick at it.  The documentation was a bit complicated because Proposed must match Proposed, so my exception calculation was actually for a baseline' model with lower Ev values and higher resultant Vot values.  Of course, no guarantees from USGBC about future reviews.
A DOAS system could involve claiming improved E_v and/or E_z and I concede that I have not made E_v improvement claims to LEED.  But if I thought it would affect my building's rating then I definitely would state my case.  Let's remember that while the Ch 11 ECB method couples each proposed system to a baseline system, Appendix G does not.  I have an Appendix G LEED project where we used one AHU to serve a two story building but per Appendix G it is an AHU per floor.  So one proposed AHU, two baseline AHUs and potentially three different E_v values. 
Thanks to Jason for highlighting that excerpt.  Ventilation standards are complex in their own right but now that the energy standard specifically references them it is going to get even more fascinatingly complex.

Paul Riemer, PE, LEED AP 

-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jason Glazer
Sent: Tuesday, August 16, 2011 9:04 AM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] DOAS and baseline OA

Just to give some perspective on how ventilation will likely be handled in future versions of LEED. Here is the text from the 90.1-2010 section on ventilation.



G3.1.2.6 Ventilation. Minimum ventilation system outdoor air intake flow shall be the same for the proposed and baseline building designs.


a. When modeling demand-control ventilation in the proposed design when its use is not required by Section 6.3.2(p) or Section

b. When designing systems in accordance with Standard 62.1 Section 6.2 Ventilation Rate Procedure, reduced ventilation airflow rates may be calculated for each HVAC zone in the proposed design with a zone air distribution effectiveness
(Ez) > 1.0 as defined by Table 6-2 in Standard 62.1. 
Baseline ventilation airflow rates in those zones shall be calculated using the proposed design Ventilation Rate Procedure calculation with the following change only. Zone air distribution effectiveness shall be changed to (Ez)=1.0 in each zone having a zone air distribution effectiveness (Ez)>1.0. Proposed design and baseline design Ventilation Rate Procedure calculations, as described in Standard 62.1, shall be submitted to the rating authority to claim credit for this exception.

c. If the minimum outdoor air intake flow in the proposed design is provided in excess of the amount required by the rating authority or building official then the baseline building design shall be modeled to reflect the greater of that required by the rating authority or building official and will be less than the proposed design.

On 8/16/2011 8:56 AM, Anne Juran wrote:
> Paul - I took the same approach, and my LEED reviewer shot it down. I 
> didn't explain it, though, so that may be why.
> Have you had success with a LEED review and your approach?
> If so, did you have to explain it or did you just provide the calcs 
> that prove it? Thanks!
> *From:*Paul Riemer [mailto:Paul.Riemer at dunhameng.com]
> *Sent:* Tuesday, August 16, 2011 9:38 AM
> *To:* 'Michael A. Eustice'; 'Jim Dirkes'; 'Anne Juran'; 
> bldg-sim at lists.onebuilding.org
> *Subject:* RE: [Bldg-sim] DOAS and baseline OA
> Michael alludes to the system ventilation efficiency (E_v).
> Per ASHRAE 62, It and the zone air distribution effectiveness (E_z) 
> vary based on system configuration.
> When ASHRAE 90.1 says the minimum outdoor air must match I take that 
> to mean the base R_p and R_a values not necessarily the resultant V_ot 
> values. Similarly when LEED says DCV must be compared against the 
> minimum ASHRAE values I take that to mean R_p and R_a values as well.
> I assert that it is appropriate to calculate separate E_v and E_z 
> values for the each system in baseline and proposed models, which can 
> lead to ASHRAE 62 multi-zone calculations for the baseline systems.
> *Paul Riemer, PE, LEED AP BD+C*
> Associate / Mechanical
> *From:*bldg-sim-bounces at lists.onebuilding.org
> [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Michael 
> A. Eustice
> *Sent:* Tuesday, August 16, 2011 8:19 AM
> *To:* 'Jim Dirkes'; 'Anne Juran'; bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] DOAS and baseline OA
> The critical zone fraction as explained by ASHRAE 62.1 will, in most 
> cases, increase outside air intake in a VAV system.
> This is due to the need for the entire system to increase its outside 
> air fraction if one zone's airflow dips to its minimum VAV airflow. To 
> maintain the ventilation fraction in the one zone that reduced 
> airflow, the entire system ingests large amounts of outside air to 
> make sure it gets enough air to that particular zone.
> It's a bit more complicated than that, but that's the simplest 
> explanation. There are ways around it, but it takes time to 'correct' 
> for small, high ventilation-fraction areas that tend to drive the 
> system towards high ventilation rates when they modulate to minimum 
> position.
> Hope this makes sense.
> Michael
> *From:* bldg-sim-bounces at lists.onebuilding.org
> [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Jim 
> Dirkes
> *Sent:* Tuesday, August 16, 2011 9:12 AM
> *To:* 'Anne Juran'; bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] DOAS and baseline OA
> 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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