DOAS and baseline OA

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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!


ajuran's picture
Joined: 2011-05-12
Reputation: 0

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.

James V. Dirkes II, P.E., BEMP , LEED AP

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203

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 A. Eustice's picture
Joined: 2011-10-02
Reputation: 0

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

Paul Riemer's picture
Joined: 2011-09-30
Reputation: 0

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!

ajuran's picture
Joined: 2011-05-12
Reputation: 0

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

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.

Jason Glazer's picture
Joined: 2011-09-30
Reputation: 0

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

Paul Riemer's picture
Joined: 2011-09-30
Reputation: 0


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

Mark Sorensen's picture
Joined: 2011-10-02
Reputation: 0

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

Jeremy Poling2's picture
Joined: 2011-09-30
Reputation: 0

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

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!

ajuran's picture
Joined: 2011-05-12
Reputation: 0

note that 90.1 requires the minimum 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.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

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 A. Eustice's picture
Joined: 2011-10-02
Reputation: 0

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.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

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 Schaber's picture
Joined: 2011-09-30
Reputation: 0

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.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

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:??

Rejda, Galen D NWO's picture
Joined: 2011-10-02
Reputation: 0

If the general goal of LEED is to save energy, that is the correct
approach for the reviewers to pursue. I always submit my models using
that logic, because our models should reflect the energy penalty
associated with over-ventilating buildings.

Morgan Heater, P.E.

Morgan Heater's picture
Joined: 2011-10-02
Reputation: 0

I'm trying to model a small building?(600 square ft)?which will be used to?inspect cars.? The?worker will frequently be using the door.? Logic would say that the equipment sizing and design would?be different?from a similar building with a door which is not frequently used.
The building is so small that leaving the door open for a short time could result in a complete air change in a short time.
I use Trace 700.??Does anybody have any methods to account for high door usage to size the?equipment?? Trying to make up an energy model will be difficult also.

John Eurek's picture
Joined: 2011-10-01
Reputation: 0

Hi John!

I think the short answer is bump up your ACH for the room "higher than
normal." Trying to quantify any more specifically is probably a
fruitless endeavor - way too many variables in play...

Fundamentals 2009 has a fuzzy thing or two to say about to what extent
door infiltration ordinarily affects whole-building infiltration

Windows and doors (6 to 22%; 15%). More variation in window

leakage is seen among window types (e.g., casement versus

double-hung) than among new windows of the same type from different

manufacturers (Weidt et al. 1979). Windows that seal by

compressing the weather strip (casements, awnings) show significantly

lower leakage than windows with sliding seals.

That may help you establish a ceiling to what's likely.

There's also a longer discussion relating that "rough" ACH estimations
have to be made in design for the purposes of sizing HVAC equipment on
(page 16.29)... Examples provided for some smallish (80-150SF)
vestibules range from 0.5 to 2.0 ACH, but importantly illustrate the
process is necessarily somewhat arbitrary. The point they're making is
there are a bunch of hard to measure/estimate variables in play to
determine actual ACH, from height to hourly temperature differentials to
wind pressures.

Throw a "higher" number at it, at least until someone can source/share
some table of "better guesses" =).


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805

" I'm not well versed in Standard 62"

This explains your answer.

Tom Morris's picture
Joined: 2013-04-16
Reputation: 1