PSZ-AC

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Sim List,

ASHRAE system 3 is, Packaged Single Zone Air Conditioner
(PSZ-AC). The description of PSZ-AC says, "Packaged rooftop air
conditioner". I assume that this means a single unit serving multiple
zones. This doesn't seem to make sense since the name contains the
phrase 'single zone'.

I have a building with baseline system 3, PSZ-AC and I'm
not sure how to apply this system. The proposed building has a variety
of systems. Should I have a system per zone or a single unit for the
building?

Thanks

Eric Youngson

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Eric,

I understand System 3 (PSZ-AC) to be a standard packaged rooftop that
may serve multiple "spaces", but only serves a single "thermal zone".
In other words, there is only one thermostat that controls the entire
unit.

Your baseline should have a separate PSZ-AC for each thermal zone
depicted by your HVAC drawings. Note that adjacent thermal zones with
similar load characteristics (light/equip gain, people count, exterior
exposure, etc.) may be lumped into a single thermal zone for modeling
purposes (i.e. less zones = simpler model).

Hope that helps.

Dan Russell, EIT

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I agree with Dan. The 90.1 User's Manual is a bit more clear on this
subject, and references that a PSZ unit is created for each thermal
zone.

Kendra Tupper's picture
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I would be leery about combining zones as the baseline cooling efficiency, fan power, and economizer should be set by each unit's tonnage, cfm, and sf respectively. Further it is not your design cfm & tonnage but rather the baseline autosized cfm & tonnage.

Also be sure you know your software because some programs will allow you to but more than one zone on a "single zone system" and then model no control or reheat on the secondary zones, either of which be outside the intent of Appendix G's system 3.

Paul Riemer's picture
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I think there's some confusion about spaces vs zones. A thermal zone can
have many spaces and be served by one packaged single zone system (PSZ).
A multizone system is one system serves each space/zone. While you can
attach more than one thermal zone to a PSZ system and not be technically
wrong to do so you will have the devil to pay because of the Control
Zone that you have to specify in eQUEST. It is almost impossible to
specify a control zone that won't create hours loads not met for the
other spaces in your output. There is a way around that, only specify a
heating/cooling schedule for the control zone and leave the others
blank, but it is far better to just create one thermal zone for each
PSZ: that meets all the requirements related to ASHRAE, LEED, god, etc.

I hope this helps. Many years and much pain have been a great teacher.

Carol

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Paul,

Thanks for the heads-up. This is good insight! In fact
this is exactly why I am questioning my interpretation of Appx G. I have
only combined common spaces such as corridors & restrooms, however I
believe I should take a closer look to make sure that these zones have
similar exposure on their exterior surfaces.

Thanks again!

Eric

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Carol,
I agree mostly but I would reverse your last statement to say to create one PSZ-AC or PSZ-HP per thermal zone. Remember in Appendix G, the modeler does not get to decide the HVAC zoning. Per Table G3.1.8, the zoning is the design zoning if it exists or a prescribed zoning of core & 15' perimeter zones by glazed orientation etc...

(If your proposed is multiple zone VAV and your baseline is going to be PSZ-AC or PSZ-HP, do a little reading and thinking before choosing the PSZ system type in eQUEST/DOE-2.2 or you might have many years and much pain as a great teacher on one project).

I have always assumed that it would be technically wrong to put more than one zone on your PSZ-AC or PSZ-HP but I guess I can not cite a code section to prove it. Can anyone else or other opinions?

Paul Riemer

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Carol,
I believe that this confusion arises, for me at least, in the
fact that eQUEST only allows each zone to be assigned to one space as
described in the geometry. This also seems like it would reduce the
flexibility to rezone for different systems when doing design phase
modeling. Though I don't have enough experience to think of a good
example, it seems that this subtle point may have some important
consequences for accurate early design phase modeling.
As for the slave zones with hours not met-I'd like to add that
I'm having trouble getting the eQUEST design airflows to match the
actual designed values as well. OA is Specified and matches closely in
both the proposed & baseline but the design airflow in the baseline is
around 10,000 CFM short! Could this be caused by a the difference in
multi & single zone systems?

Thanks

Eric

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I have submitted two projects for LEED C&S. In both cases the budget
case was assigned the PSZ-AC system. I used one system per zone
regardless of how the proposed model systems were assigned
zones. The proposed designs both had multiple zone systems. The
LEED reviewer agreed with how I modelled the baseline case.

Chris Jones, P.Eng.

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