[Equest-users] Modeling 2 stage cooling in a single zone packaged unit

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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.



Jun Furuta's picture
Joined: 2011-09-30
Reputation: 0

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?


John Aulbach.

What is

John Aulbach's picture
Joined: 2011-09-30
Reputation: 1

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

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

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

I had similar experience about oversized system in summary reports.
I now look at the total cooling loads from SS-D report in kbtu/hr and divide
it by 12 (1ton=12000 btu). I also add a factor of 15% as that cooling load
may be undersized.

This number is approximate correct to what a typical system should be based
on trace calcs.



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

I am trying to model the energy savings using the dual enthalpy function and
cannot see any reduction in cooling load for using outside air for free
cooling. Has anyone found any problems or tricks with the economizer
function in eQUEST?

I am specifying a dummy zone and am entering a 2btu/lb difference between
outside air and return air. I am not specifying any other outside air
minimum or maximum values.

Account Deleted's picture
Joined: 2011-09-30
Reputation: 0