Exceptions to G3.1.1, and how you model them?

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I was interested to see if anyone would share how they model the
exception to G3.1.1 in ASHRAE 90.1-2007 (or 04). This is the part that
says when you have a baseline building with system 5-8, you're supposed
to model system 3 or 4 for any spaces that "have occupancy or process
loads or schedules that differ significantly from the rest of the
building." It goes on to say that basically the peak thermal loads must
differ by 10 Btu / h-ft2 or more from the average of the other spaces,
or that schedules must differ by more than 40 full load hours per week,
for this exception to apply.

I've had reviewers question certain rooms that I've put on separate
PSZ-AC systems, and so I've sort of defaulted to putting almost all
rooms on the main systems (5-8). However, in looking at some of my
recent models, depending on interior loads and climate, the actual peak
cooling load on a square foot basis varies anywhere from 5-20 btu /
h-ft2 range for offices. So this means there is a big difference
between those models with 5 btu / h-ft2 peak and those with 20. For the
models with 5, almost every space in the building is going to be within
+/- 10 btu/h-ft2 of that average. However, those buildings that average
around 20 will have many rooms that are < 10 btu/h-ft2 peak (ie any
internal space whose light + plug load density is < 3.0 W / sq ft).

To use a simple example as a question: if your particular office
building averaged 20 btuh / h-ft2 peak cooling load, would you model a
separate PSZ-AC system for a core bathroom, whose peak cooling load is
only 3 btu/h-ft2?

There are lots of rooms where I question whether I should do a separate
PSZ-AC/HP system (storage rooms, stairs, mechanical rooms, lobbies,
etc). I can see where these meet the 10 Btu/h-ft2 delta exception.

So does everyone really model a bunch of PSZ-AC systems based
specifically on the average peak thermal loads of the building?

I'm thinking out loud, but if I had an office building that averaged on
the low side (5 Btu/h-ft2) for cooling peaks, it seems pretty easy to
have a perimeter conference room that exceeded 15 Btu / h-ft2 peak.
Seems like an exhaustive process to confirm this delta for every room in
a model to determine whether it needs its own system...

Sorry for the long email. Thanks!

James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

Hi James,

LEED CIR allows projects to demonstrate substantial energy savings for
well-designed HVAC system serving high process load spaces based on the
exception to G3.1.1. So the premise is "high occupancy or process load"
spaces, such as server rooms, natatoriums, before it comes to the benchmark
of 10 btu/h-ft2. Therefore, I guess your storage rooms, stairs, mechanical
rooms should not be modeled separately.



I guess your storage rooms, stairs, mechanical rooms, lobbies, etc. should
not be

chen yu's picture
Joined: 2011-10-02
Reputation: 200

Thanks Cheney, but I'm not sure I follow your email. I've already
determined that, for example, a core bathroom could easily have a peak
load that is 10 Btu/hr-ft2 less than the peak average of the rest of my
spaces. Same goes for stairs, storage rooms, etc. So doesn't 90.1
REQUIRE that I model these separately?

What CIR are you referencing?

James Hansen's picture
Joined: 2011-09-30
Reputation: 200


I believe this is an unfortunate choice of the word "spaces" in Appendix G. I think the intent is really that if you have a zone with significantly different loads or schedules, it should not be placed on one of the VAV systems serving other portions of the building. If the spaces you are describing such as bathroom, stairs, or storage rooms are defined as their own thermal zone, they should not be combined in a VAV system with spaces very different loads or schedules, but rather have their own single zone system. See Table G3.1 #s 7 and 8 for discussion regarding how to create thermal zones.

Michael Rosenberg

Rosenberg, Michael I's picture
Joined: 2011-09-30
Reputation: 0

Hi James,

The CIR is dated on 3/23/2007. It clarifies use of exception to G.3.1.1. to
document baseline and achieve greater energy savings from single-zone
systems in *high* process load areas.

As per G3.1.1. in Appendix G (90.1-2004), my understanding is to use
separate single-zone system for any space has very high occupancy or process
loads or schedules that differ significantly from the rest of the
building. You can understand it in the opposite way that the rooms have
significantly less occupancy or process loads. But I guess the CIR points to
the high process load areas in order to achieve greater energy savings from
single-zone systems.


chen yu's picture
Joined: 2011-10-02
Reputation: 200

I see, that makes a little more sense. So if I had a stairwell, which
does not have HIGHER process loads than the average space, and whose
occupancy schedule is basically the same as the rest of the office
space, then I would HAVE to have that stairwell on the base-building
System 5-8. I couldn't have it on its own PSZ-AC.

Mike, this goes against what you said in your email. This is sort of
why I asked the question...there doesn't seem to be a consensus. I
model stairwells, elevator shafts, bathrooms, etc as separate thermal
zones in all of my models, and if I read the letter of the code, it
would seem they need to be their own systems, yet the CIR sort of says
that this isn't the case, and reviewers agree, and don't like to see
rooms like this on separate systems.

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

Hi James,

I must chime in on this. First off, in reality I would more expect to see a
transfer grille above the door of the storage room, bathroom, etc., than I
would expect each to have their own system. I frequently model them as
unconditioned spaces attached to a HVAC system and call it good. Mostly if a
room of that type is conditioned, it is fed off the nearest system. Stairs
may be separately heated with a unit heater, which I would model in the
proposed case, but for the baseline I'd just stick to what makes
"engineering" sense following the spirit of the code.



cmg750's picture
Joined: 2010-10-05
Reputation: 0

I've always interpreted the exceptions you mention as an alternative way in
which special spaces MAY be modeled but don't HAVE to be modeled.

My experience has been mostly with labs and hospitals which have pressurization
requirements and thus typically meet one or more of the criteria for the
exception to be used.

LEED reviewers and CIRs have generally upheld the optional approach and allowed
all the pressurized spaces to be put on the main systems since using individual
separate units to model hospital rooms or labs is not in line with what could be
done in reality.

It may be helpful for you to take a look at the CIRs on the subject as related
to application to labs and hospitals. These could be rather enlightening
regarding the "spirit" and application of the exception.

Julia Beabout's picture
Joined: 2011-10-01
Reputation: 3