[Bldg-sim] DOAS and baseline OA

Patrick J. O'Leary, Jr. poleary1969 at gmail.com
Fri Aug 19 16:12:21 PDT 2011

fancy meeting you here reba ;-)

1. the quote from the user's manual is verbatim, and 90.1 itself also 
refers to it as "proposed and baseline designs"
2. depending on your leed review team the potential difference between 
proposed and baseline may not be caught.  that really depends (like 
anything else) on the specific reviewer's knowledge of 90.1/user's 
manual.  i've done it the same way you have over the years and not 
received a comment on it for packaged system but did have a comment on 
it for a project with both packaged and evap cooler systems in a 
firestation or two ago.  i think how non-packaged/non-vav systems might 
be more of a key in how to interpret it.

3. neither the handbook, or standard, to my knowledge specifically calls 
out ashrae 62 calculations as the minimum in the actual text of the 
relevant section.  if you were modeling to the 2000/3/6/9 international 
mechanical code i would think it would apply the same way.  specifically 
for leed purposes though the baseline is ashrae 62 so i will comment 
around its requirements as it also impacts ieqc2.  but i can see how the 
"minimum designed" might be interpreted differently than the "minimum 
calculated" by ashrae 62.

thinking out loud for a minute, as engineers designing a system how do 
we approach the outside air calculations and how do we show compliance 
for plan review or leed purposes?  typically we first look at the 
project, what type of zoning we want, what systems we're planning on 
using, and then we calculate what the minimum outdoor air rate is 
required by whatever code we are using.  we typically use ashrae 62 so 
we can reduce the outdoor air compared to the international mechanical 
code, save some energy, and possibly downsize smaller packaged units 
(when applicable) and since it is the basis for leed compliance points 
it is easy to use the ashrae/leed provided spreadsheet -so we've killed 
2 birds with one stone.  calculated minimum outdoor air per 62, and have 
a speadsheet to show leed/plan review compliance for the minimum outdoor 
air rate.

so now we have a minimum calculated outdoor air rate, per ashrae 62.   
in packaged/vav systems does this number constitute the minimum outdoor 
air rate we are designing to? i think we would agree if 100 cfm were 
required, and we provided 100 cfm, then the minimum design rate is the 
same as the provided design rate.  but if 100 cfm were required and we 
were to provide 130 cfm because we are designing the system to obtain 
one leed point from ieqc2 are we designing to 130 cfm as a minimum?  or 
designing to 100 cfm as a minimum because that is the requirement we 
start with to get to 130 cfm as 30% more?

but the key for intent, i think, is more obviously when you're using 
non-mechanically cooled systems like evap coolers.  evap coolers are 
sized (typically) on volume to create an air change rate of 2.5 to 3 
minutes.  going back to my previous email, if you use ashrae 62 
calculations and find you are only required to provide 2,000 cfm of 
outdoor air but you are designing an evap system that "requires" 100% of 
outdoor air at 20,000 cfm based on the volume of the space and the air 
change rate, what is the minimum you are designing to?  you know you're 
designing to the 20,000 cfm to have a properly sized evap cooler.  but 
if you use 20,000 cfm as the outdoor air rate in your baseline 
simulation the equipment size in the simulation is now a 50-ton unit 
just in airflow capacity.  since 90.1 requires the baseline units to be 
sized per load this is now making the baseline load the outdoor air rate 
and not a reflection of the heat gain through the building envelope and 
from equipment/people.

so imho the intent is to size the baseline on the ashrae 62 calculations 
for the minimum required outdoor air rate.

i received a response from the usgbc support desk on this before & will 
look it up again to see what their exact language was - it concerned the 
use of evap coolers and outdoor air rates but it was what pointed me in 
the direction that the baseline outdoor air rate could be lower than the 
proposed based on what was required.

On 8/19/11 12:12 PM, Reba Schaber wrote:
> Patrick,
> I clearly understand that you can't change the outside air rate just 
> because the system changes.  I'd like to explore the correct 
> ventilation rate that should be used in both buildings.
> ASHRAE 90.1-2007, section G3.1.2.5 states, Minimum outdoor air 
> ventilation rates shall be the same for both the proposed and baseline 
> building designs."
> LEED 2009 reference guide, page 272 says, "Outdoor ventilation rates 
> should be identical to proposed case."
> When I've increased the ventilation rate by 30% for IEQc2, I have used 
> the increased ventilation rate in both baseline and proposed.  I have 
> not received any comments against that.  Has anyone else?  Even though 
> the ASHRAE 62.1 calc may say the minimum ventilation required is 100 
> cfm, if I my drawings say "set the outside air damper minimum position 
> to 130cfm," 130 cfm is then the minimum ventilation rate.
> Is the quote from user's manual verbatim? "...the minimum ventilation 
> rates designed for the proposed building must also be modeled the 
> same..."  It uses the word designed.
> Where does it say the 62.1 _calculated_ rate has to be used?  Thanks 
> for your feedback.
> Reba
> *From:* bldg-sim-bounces at lists.onebuilding.org 
> [mailto:bldg-sim-bounces at lists.onebuilding.org] *On Behalf Of *Patrick 
> J. O'Leary, Jr.
> *Sent:* Thursday, August 18, 2011 9:46 AM
> *To:* Michael A. Eustice
> *Cc:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] DOAS and baseline OA
> in reality, i would agree with you.  if you were attempting to compare 
> energy simulations for two different types of systems and were 
> comparing the difference in outdoor air loads on system size/capacity 
> (and costs of different systems) using the minimum per the different 
> system types would be beneficial.
> in leedality though the approach is a little different.  90.1 app g 
> requires the minimum outdoor ventilation be the same to help prevent 
> gaming the energy simulations, and is "enforced" by the leed 
> reviewer's commenting on it.   the outdoor ventilation rate is the 
> minimum designed outdoor ventilation rate.  so if the design is for a 
> DOAS system then the minimum outdoor air rate for that system type is 
> to be used in the baseline system.  same as if it the design were a 
> packaged system but the baseline were VAV based on SF of the building 
> (it happens) - the minimum oa rate would be what is designed for the 
> packaged system.
> except for the exception noted on page 182 when using dcv in the 
> proposed if it is not required in the baseline.
> look in the user's manual on page g-26, right column in the 1st 
> ventilation paragraph, "outdoor air ventilation can be a major 
> contributor to building energy consumption, but i is not considered an 
> opportunity for energy savings under the performance rating method.  
> the minimum ventilation rates designed for the proposed building (not 
> counting extra ventilation for economizer cooling) must also be 
> modeled the same in both the baseline building and the proposed building."
> in other words, in order to comply with the energy simulation 
> requirements of 90.1 appendix g (the performance rating method) the 
> minimum outdoor air rate has to be the same in both the proposed and 
> baseline energy simulations (models).  so yes, outside of the one 
> exception for dcv systems, the baseline outdoor air rate is the same 
> as the proposed if you are only providing the minimum required per 
> ashrae 62 regardless of what system type the baseline building is 
> required to use.
> On 8/18/11 7:47 AM, Michael A. Eustice wrote:
> Unless I'm misunderstanding something, your sentence:
> 'when would the baseline be the same?  when you only provide the 
> minimum required outside air per ashrae 62.'
> Seems untrue to me.  Just so everyone is clear, under exact same 
> conditions (occupancy, sqft, bldg type etc.), with the only difference 
> being system type (VAV versus DOAS), the minimum required ventilation 
> cfm required by ASHRAE 62.1 will be different.  The VAV system will 
> almost always demand a higher minimum ventilation cfm, sometimes 
> significantly more, since one zone can drive the system towards 100% 
> outside air.
> Michael
> *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 *Patrick 
> J. O'Leary, Jr.
> *Sent:* Thursday, August 18, 2011 10:33 AM
> *To:* Anne Juran
> *Cc:* bldg-sim at lists.onebuilding.org 
> <mailto:bldg-sim at lists.onebuilding.org>
> *Subject:* Re: [Bldg-sim] DOAS and baseline OA
> 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> 
> [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 
> <mailto: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)
> If this is a LEED project, submit a project-specific CIR
> 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> 
> [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 
> <mailto: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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