[Equest-users] Written explanation of the System Sizing Logic asshown on the SYSTEM Tab

[Nick Caton]

Hey John,


Sounds like a shotgun of queries... here's a shotgun response =):

 

Everything you see in the airside summary tab is the result of some
combination of inputs - nothing pops out of thin air so you should be
able to trace any one result to determine what combination of
inputs/schedules is producing different results.  Note the "totals" for
each zone are summed on the line describing the system.

 

Defaults are neither good nor bad.  Sometimes they're quite off.
Sometimes they do a better job than any assumption you'd make otherwise.
A big part of QC is reviewing to ensure the defaults are appropriate
where leveraged.

 

With a too-high cooling capacity, I'd start looking at whether your
loads are off and/or your airflows are being sized incorrectly... but
first you want to ensure your unmet hours aren't way out of whack -
that's a sign something isn't functioning or reacting to your building
quite right and needs to be addressed.  Remember unmet hours are
symptoms of another problems, and not always capacity
(heating/cooling/airflow).  You'll run yourself in circles if you try to
treat them as the primary problem.  Which spaces have them, the time of
day, and the time of year they occur are all potentially clues as to
what's really going on.  

 

Once your unmet hours are either zero or reigned in to a low comfy
figure, you're ready to get your loads in order.  Get the load calc
output reports (and ideally the person who ran them, if not yourself)
handy and open up your model.

 

Loads are in 2 categories: "ventilation" and "everything else."  You can
quickly assess whether the ventilation air loads are appropriate for
your spaces using the airside summary tab - where the number seems
significantly off for a system line, check the ventilation quantities
and # of people in associated zones.  Focus on spot checking
higher-occupancy spaces (i.e. gyms, cafeteria, conference rooms, etc.)
to ensure those are in sync with expectations/sizing.  If the quantity
of people seems right but the ventilation air results are still off -
move to checking your occupancy schedules and OA/person figures.  Note
how eQuest makes design day schedules which will by default reference
your 'normal' schedules - .  If you have an equipment schedule to
follow, you can input the zone ventilation air directly, but making 50
people breathe like 10 will still result in 40 people worth of heat load
you'll need to account for so I'd first try to get space populations
into the right ballpark to get your auto-sized ventilation quantities in
line.

 

Once ventilation quantities are being sized in the correct ballpark, and
if your auto-sized capacities are still off - you can review the other
loads incident on a given space with the LS-B report.  Again spot check
higher-load spaces for design heating/cooling conditions that seem out
of whack with your loads run.  Pay close attention to little things
applied everywhere, like plug load W/SF - small changes there can make a
big difference.  Something way off here may also lead you to
review/revise/correct your envelope/window properties or other
internals.

 

Airflow capacities are auto-sized based on only about a million
variables (including everything affecting loads), so it's hard to
generalize where to start for every model.  They do however have a
direct effect on coil capacities - so they definitely warrant attention
to get into the right ballpark.  I've found however that when the loads
are synced up correctly, airflows tend to follow - but pay attention to
the effects of elevation (grr) and your system sizing factors which have
a direct impact on CFM's produced by eQuest. 


If you're trying to assess system performance before and after
significantly revising the space equipment loads, it follows your inputs
should be stipulating carefully (not defaulting) the associated space
equipment loads and the internal heat loads they contribute to the
system.  Servers by design generally do not put out what they are rated
to take in without some diversity - give definition of those internals
some careful thought from a heat-gain perspective.  This applies
generally to all your internals - review the schedules to ensure you're
getting an appropriate degree of diversity!

 

Noting your building is 50% glass - that in and of itself warrants extra
attention towards whether your solar loads are in sync.  If not, it
follows to review the items that affect solar loads:  glazing
properties, shading (consider including stuff outside the building like
neighboring buildings & trees), etc.

 

Infiltration is something that can potentially have a great effect on
any model's heating/cooling capacities.  Don't miss that when trying to
QC/match others' loads.

 

...

 

Well there's a list =).  

 

~Nick

 

 

 

 

NICK CATON, P.E.

SENIOR ENGINEER

 

Smith & Boucher Engineers

25501 west valley parkway, suite 200

olathe, ks 66061

direct 913.344.0036

fax 913.345.0617

www.smithboucher.com 

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of John
Aulbach
Sent: Sunday, September 25, 2011 11:02 AM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Written explanation of the System Sizing Logic
asshown on the SYSTEM Tab

 

My boss is a skeptic of the data presented in the System Sizing Tab that
can be generated when you are the System screen and hit the Summary tab,
and I don't have a good explanation.  We looked at a single office floor
in the Southern California are, a floor with about 50% glass and about
29,000 sf. The System tab generated a size of 107 tons (installed
tonnage for the floor was 80 tons). Most load information was defaulted.

 

The installed tonnage was 80 tons and the unmet hours were few.

 

When we increased the airflow slightly, the load size was 80 tons.  When
we increased the total supply airflow double, the tonnage went to about
140 tons.

 

And, of course, the unmet hours varied, inconsistent of course between
SS-R and SS-F and the System Sizing tab.

 

All I am looking for is a detailed explanation of how the system sizing
tab is generated. Obviously, this is not TRACE sizing routine, but there
are some of us who must use this table to have a sense if a builidng has
enough cooling after equipment (read-SERVERS) has been added to a floor.

 

Can anyone who had had time to research this care to share their finds?

 

Thanks.

 

John Aulbach.

 

What is 

 

________________________________

From: Jun Furuta <JFuruta at energy-solution.com>
To: "equest-users at lists.onebuilding.org"
<equest-users at lists.onebuilding.org>
Sent: Tuesday, September 20, 2011 1:39 PM
Subject: [Equest-users] Modeling 2 stage cooling in a single zone
packaged unit

Hello,

 

I would like to model a packaged single zone AC with gas furnace that
has two stage cooling with using two compressors. The system has an
integrated  economizer and a constant speed fan.

 

Is there a way to indicate two compressors in the PSZ system type or am
I supposed to use the PVVT system, despite being a single zone system
and not having neither a variable speed compressor or fan? And if the
PVVT system type is to be used, any advice on configuring the system
would be greatly appreciated.

 

Thanks,

 

-Jun

 

 


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