eQuest - outside air heating issues

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Hello all,

I have a question regarding outside air input. For my design building I enter my outside air at the system level, as the minimum outside air ratio, which I get from my equipment schedules. 90.1 requires that the outside airflow be the same. To do this I take my summary report from my design building and enter the values from that report into the baseline model zones. When I run my baseline model the outside airflow rates from the summary report match those of my design model, and my unmet heating hours are good in both models, but the heating gas consumption in my baseline building triples. The result is I am getting great, but unrealistic, energy savings.

Does anyone know why this is happening? Why would entering the OA at the zone level cause such increased heating for the save total CFM entered at the system level? How are you folks typically handling your outside ventilation air.

Any help would be greatly appreciated, thank you.

Scott Tomlinson, PE, LEED-AP

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Joined: 2011-09-30
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Scott,

I want to say off the bat that you actually are asking a very good and
fundamental question: What is a "best practice" for matching baseline
ventilation air rates to the proposed model?

My general processes is:

1. where I'll build a proposed model to match the
drawings/schedules, address anything that results in odd results/
warnings / unmet hours.

2. Build the baseline - clear out any zone/system level OA inputs
to autosize, then

3. For each airside system, enter the 1.15/1.25 oversizing
parameters and a "MIN-OA-RATIO" at the system level to match the
corresponding proposed design model's system ratio

4. Run both simulations and compare results for ventilation air to
determine if things are way off (and they have been) - I use the SV-A
reports and excel for this comparison

5. Tweak baseline systems' OA-RATIOs as required up or down to
make the total ventilation air sync with the proposed model's.

Following this procedure, which I document for model review, the design
system OA sums between the baseline model, proposed model, and
construction documents are typically very close to each other, if not
perfectly in sync.

Is this a procedure others are following or is there a simpler way?

To the rest of your email... Different systems will use different
amounts/types of energy for the same amount of load (OA). Your baseline
model's tripling of gas consumption isn't something I'd immediately
discount as unreasonable, but worth reviewing to find something you may
have missed - perhaps lowering CFM's are the culprit? In some cases,
the core "secret" to surprisingly outstanding proposed/baseline
performance is getting lucky with a terrible baseline... the
prescriptive requirements, by their nature, can be close to or far from
"a good idea" for any given project/site.

NICK CATON, E.I.T.

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If you are entering the min. OSA intake through the air handler or
system level then you need to zero out the OSA items in the zone level
to get the system OSA control to work, the only exception to this is if
you are doing something with demand control ventilation. This will
bring in the min. OSA through the air handler whenever the the air
handler fan is on, unless you set up a schedule to vary the min. %
amount of outside air that is to be brought in through the air handler
at different times. Anything that you set up for OSA at the zone level
will override what you put in through the system. The eQUEST program is
set up to automatically do this override If your air handler was set up
to cycle off at night when unoccupied or you had some schedule to vary
the outside air intake through the air handler, then the amount of OSA
with the system method would be reduced at times. While, unless you set
up some schedule, the zone level OSA method would continue to bring in
OSA, thus increasing the heating gas consumption. While there are many
factors on a program this complex that could be causing this gas
increase to happen, I would guess that it is this or something related
to this, as I have experienced this in the past.

David A. Bastow

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Nick,

First off, thanks for the reply.

If I am understanding you correctly, your method is getting the total OA of each model to match up, and you are doing this at the system level for both the design and baseline buildings. This differs from my method, where I am trying to get the OA rates to match up for each zone of the two models.

Your method would seem to work for a baseline system type 1, 2, 3 or 4, where each thermal block needs to be modeled as a separate system, but for a system 5, 6, 7 or 8 (where each floor of the baseline building needs to be modeled with a separate HVAC system) I would think you would have difficulty getting things to match up. When you work on these higher system types, and you are modifying the OA ratios of your baseline systems, do you do it equally for all the systems? If not, I would think this could give you some freedom the 'game' your baseline model in a way that USGBC would not approve of. I'm not saying you personally would do this, let me use my current model as an example.

My current building is a 3 story system 8 school. The design building has some AHUs (about 25% OA), some ERUs (100% OA) and some UHs (0% OA). Now, using my method of entering the OA at the zone level in the baseline building things are working out that I need energy recovery in the first floor HVAC system, but not the second or third floor systems. If I use your method of getting the total building OA rates to match (and if I am understanding it correctly), what would stop me from taking some of the OA from the first floor system and transferring it to the second are third floor systems so I can get all three systems under the requirements for energy recovery, and provide myself some artificially inflated energy savings, while keeping my total building OA rates equal? In my very limited feedback from USGBC it seems like they don't like letting the modeler have such freedoms.

Have you ever had any issues with USGBC using your method?

- Scott

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Joined: 2011-09-30
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Hey Scott,

To be frank, I haven't yet modeled any LEED projects that would use
systems 5+, so I have no direct experience to relate. That said, if I
were in an omniscient USGBC reviewer's shoes, and you presented this
method of matching ventilation rates, I would check that the design OA
rates for each floor are roughly matching the proposed values and design
documents. The practice to specify MIN-OA-RATIO's at the system level
to start, then tweaking as necessary has (for me) turned out to be the
most efficient way I've found of (1) getting the baseline quantities to
start in the right vicinity relative to the proposed, and (2) maintain
an equal distribution of the OA to each zone. As you point out, it's on
us to not bend the rules and redistribute the OA in a way that is
advantageous...

Off the top of my head, I think for baseline systems 5+, I would follow
the same basic procedure, using MIN-OA-RATIO's from the proposed to get
a starting point. For step 5, it may be appropriate to use a zonal
method to specify/tweak the resulting ventilation rates to match, it
would depend on the project-at-hand. On the other hand, MIN-OA-RATIO
may still be a viable option if the modeler will pay mind as you say to
treat each floor/system separately and not to simply match the gross
values for the building.

I have not received any USGBC commentary raising issues with this
approach - as a simple disclaimer: that's not to say it has actually
been scrutinized!

I'd love to hear others share their approaches if you have a "normal"
procedure... I'm sure there's a variety out there and we might all learn
something =).

NICK CATON, E.I.T.

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Scott, Nick,

To meet the 90.1 Appendix G requirement for identical minimum outdoor
air flowrates, I enter OUTSIDE-AIR-FLOW cfm for all zones in both the
baseline and proposed models. To get these values, I will either
calculate them based on the design drawings, or get them from the SV-A
reports. I set the Minimum OA Control Method to "Fraction of Design
Flow" so that the fraction of OA increases as a variable air volume
system ramps down.

The idea of adjusting the OA rates between systems, while creative, does
not meet my understanding of the intent of Appendix G. The proposed
model should simulate the actual design as closely as possible, using as
much information as is known about the design. Presumably, the
mechanical designer will calculate minimum OA rates for all zones per
code, or ASHRAE 62.1, or both.

To verify that OA rates are actually identical between baseline and
proposed models, I create hourly report blocks for at least one system
to track total system supply air flow rate and ratio of outside air to
total supply air.

I recently filled out the online LEED template for credit EAp2 for a
LEED 2009 project, and there was a note on one of the tables that if an
ERV is used, the modeler should "verify that outside air is modeled with
zero flow in both cases during unoccupied periods." To meet this, I
create a minimum air schedule, set to 0% during unoccupied hours and
"999" for all other hours to default to the program-calculated OA
percentage.

Regards,

William Bishop, EIT, BEMP, LEED(r) AP

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Nick,

In my humble opinion, even system 3 or 4 won't have same amount of OA as
your proposed system. For the base, each thermal zone has its own RTU;
for proposed system, each thermal zone could be a VAV box. The
ventilation equation will play differently. The proposed large VAV
system will have a larger diversity factor. I always put the same OA
quan. at room level.

Thanks!

Lan

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I typically specify cfm/sf at the zone level for both the proposed and the baseline models. This should ensure both models are operating with the same cfm.

One mistake I made when I was learning eQuest was I specified OSA of a VAV system at the system level thinking that the MIN-OUTSIDE-AIR was the min ratio to design flow, when it is actually the min ratio to heating flow. So, I was getting less outside air than expected. I would check the detailed reports or possibly a custom report to make sure both models are operating with an equal amount of OSA.

Jeremy McClanathan, LEED(r) AP

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Joined: 2011-09-30
Reputation: 200

I ?fourth? Bill?s, Lan?s, and Jeremy?s responses; design OA flow rates are dictated by zone requirements and should therefore be simulated at the zone level, regardless of system type. System OA parameters should typically be limited to economizer control, MIN-OA-SCH, and OA-FROM-SYSTEM (and ERV or DCV parameters, if applicable). Following this methodology ensures the zone and building-level OA flow rates match between models, and it ensures each zone is getting its design OA regardless of whether or not one is comparing single-zone systems to multi-zone systems.

DAKOTA KELLEY

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This is why I love these lists!

Thanks to everyone who?s responded and everyone who?ve yet to, in advance. The collective advice I?m reading makes a lot of sense to me, and is probably going to be more time-efficient to boot. I?m growing into a better energy modeler every time I go out on a limb like this =).

NICK CATON, E.I.T.

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One problem to be aware of, especially if you are creating your project
with an older version of eQUEST, is that unless you specify a
SYSTEM:MIN-AIR-SCH with the appropriate night-time values (either ~0.0
for no OA, or 0.001 for nighttime economizer operation), night cycling
of the systems (as specified by Appendix G) will include the minimum OA
flow. 90.1 requires OA dampers to be closed whenever the system runs
during unoccupied hours (there are some exceptions), and in many
climates, this will lead to excess nighttime heating energy. Also,
since the 90.1 baseline building typically has poorer envelope
performance, the baseline systems are likely to cycle more at night, and
therefore, not including this schedule will make the proposed building
incorrectly appear better. One issue with specifying nighttime
economizer operation (Schedule flag = 0.001), is that for multizone
systems, even during unoccupied hours, the economizer will be used to
maintain the MIN-SUPPLY-T or COOL-MAX-RESET-T. During heating periods,
this can lead to unnecessary reheat energy, which can be somewhat
mitigated by setting an appropriate ECONO-LOW-LIMIT temperature. I
typically prefer to just set the nighttime MIN-AIR-SCH value to some
small value (to simulate damper leakage), since most of the buildings I
have analyzed have not needed a significant amount of nighttime cooling.

I believe OA control options during night cycling were added as an
explicit wizard option in the last eQUEST release. In the wizard,
setting the Cycle Fans at Night property to "Cycle Fans (no OA at
night)" will create both a MIN-AIR-SCH with nighttime OA values to 0.0
and set the NIGHT-CYCLE-CTRL keyword. However, this reinforces
Jeremy's suggestion to check a few detailed SYSTEM and other command
hourly reports (this issue would not be apparent int he SV-A or other
summary reports) to make sure your model is behaving as expected...

David Reddy

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First of all, I usually set outdoor air rates by zone, but there is one
thing that I think I should add to this discussion:

If (in reality) you have a central ventilation system that provides constant
outdoor air, and a zone system that provides heating and cooling, as we all
know, eQuest requires that only one system be modeled. The challenge arises
with how to handle delivery of ventilation air. If (in reality), your zone
system has a fan that cycles on and off, then eQuest will only model
ventilation air for the fraction of the hour that the fan is ON. If the fan
is variable, even if the ventilation rate is set to "Fraction of Design", if
the airflow required drops below the OA fraction, you will not get full
ventilation. Therefore, for these systems, I always set my fan flow to
"variable" and manually set my minimum flow rate equal to the OA fraction.

--
Karen

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Joined: 2011-09-30
Reputation: 200

I specify the outside air for the baseline and proposed model by specifying the amount of outside air for each zone.

Then I create an outside air schedule and attach it to the HVAC system under Outdoor Air ? Minimum Air Schedule.

I have also noticed that in many cases the baseline model (systems 5 & 7) has a much higher heating consumption than the proposed model. Some of this has to do with energy recovery, properly sized equipment, efficient heating equipment, etc?..

If anyone specifies the outside air differently please provide print screens or describe in detail how you do this.

Otto

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Joined: 2011-09-30
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Folks,

I went thru and entered all my ventilation air at the zone level for both design and baseline systems and they are matching up, but unfortunately this wasn't my issue. I am still getting unreasonably high gas savings, actually I think it is my design gas use that is low, not my baseline design that is high.

I have my design model pretty much put together, so to create my baseline system I take a copy of my design building and modify this. My design AHUs are VAV with hot water reheat, and so are my baseline AHUs, so I modify 3 of the design AHUs into the baseline AHUs that will be serving each of my 3 floors (I've tried copying new AHUs and creating new AHUs from scratch at this step as well). I clear out the CFMs and coil capacities to allow autosizing, and I redesign ate all my zones to the proper baseline AHUs. It is this step that is causing problems. For some reason going from 16 smaller systems to 3 larger systems causes my gas consumption to increase by a factor of 4.

My unmet hours for both models are within reason, I'm just using a bunch less gas in the design building for some reason. Why looking thru the output report I noticed something. My baseline building isn't generating SS-P reports for some reason. I'm not sure if this is an indicator of something. Any thoughts would be appreciated.

- Scott

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Joined: 2011-09-30
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Take a look at the cooling reset controls required by ASHRAE 90.1 appendix
G. It seems that in your baseline model you may have a lot more re-heat,
especially if your AHUs are now serving core and perimeter zones where they
previously only served one or the other. You can test this by creating one
new AHU and have it serve all core zones.

--
Karen

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Joined: 2011-09-30
Reputation: 200