[Bldg-sim] DOAS and baseline OA

[Rejda, Galen D NWO]

We have had reviews where 30% increase ventilation and DCV was provided the
GBCI indicated they being the persons with "Authority" would require 130 cfm
for the proposed system and 100 cfm for the baseline--minimum ventilation as
long as the baseline was the ASHRAE 62.1 minimum.  Even though you would
think they would both be 130 cfm.  Strange.

Galen Rejda, LEED® AP BD+C:   
Engineering Division, Design Branch 
CENWO-ED-DA 
1616 Capitol Avenue, Suite 9000 
Omaha, Nebraska (68102-4901) 
E-mail: galen.d.rejda at usace.army.mil 
Phone:  402.995.2113 
http://www.nwo.usace.army.mil/ 
Corporate Member USGBC, Nebraska Flatwater Chapter



-----Original Message-----
From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Reba Schaber
Sent: Friday, August 19, 2011 2:12 PM
To: Patrick J. O'Leary, Jr.; Michael A. Eustice
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] DOAS and baseline OA

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] 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
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] 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)

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] 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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