LEED Baseline secondary HVAC system

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I am modeling a residential building under LEED 2009. With reference to
ASHRAE 90.1-2007, Exception 'a' in section G3.1.1 which states that we
should use additional system types for nonpredominant conditions if they
apply to more than 20,000 SF of conditioned space - I am confused which
system type I should use for my corridors and other non-residential
conditioned spaces, which are about 21,000 SF total, spread across 20
floors. Do I use System 7, because my non-residential spaces are located
on more than 5 floors, or use System 3 or 5 because the area is less
than 25,000 SF?

Thanks,

Ritwik Kakati

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Based on the exact wording of the Table G3.1.1A, I would say you are
System 7, which is for "Nonresidential and more than 5 floors OR >150,000
SF. However, we all can see that it is probably a pretty ridiculous
baseline system for a corridor ventilation unit.

On multifamily projects I've worked on, I've used system 5 in the past
(even though the floor number requirement would trigger system 7) and
never been called on it. That being said, usually that was on projects
with ~7 stories, so I was much closer to the threshold, so maybe the LEED
reviewer just let it slide. Even System 5 is a bit peculiar for a corridor
unit. At least in my climate they end up usually being packaged rooftop,
constant volume units.

Nathan Miller, PE, LEED AP BD+C

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I would argue that residential system type (PTAC/PTHP) should be applied to
the entire multifamily building. The heading of the first column in Table
G3.1.1A is Building Type, not Space Type. The note below the table reads
"Residential building types include dormitory, hotel, motel, and
multifamily..". If the baseline HVAC should be determined solely based on
the conditioned floor area and number of floors of residential versus
non-residential spaces, then why was the building type used as column
heading and in the note below the table? A case study in 90.1 User Manual
involves a building with 4 multifamily and 4 non-residential floors. The
4-storey 54,000 SF non-residential portion in the case study is modeled with
System 5, and the 4 residential floors are modeled with PTACs. If
corridors/stairwells in multifamily buildings are modeled with
non-residential baseline HVAC, then most multifamily buildings over 5
floors would have System 7 serving these areas. I agree with Nathan that it
doesn't makes much sense, especially since most of these spaces are
indirectly conditioned by losses through interior walls of apartments.

Maria

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It kind of falls in that grey-area of whether you get to CHOOSE to apply
the exceptions listed in G3.1.1 or whether you SHOULD apply them if they
are applicable to your project.

The emphasis the in following excerpts is mine.

G3.1.1 exception a states: "Use additional system types for
non-predominant conditions (i.e. residential/nonresidential, or heating
source).

The same footnote you reference in table G3.1.1A states "Residential
building types include dormitory, hotel, motel, and multifamily.
Residential space types include guest rooms, living quarters, private
living space, and sleeping quarters. Other building and space types are
considered nonresidential.

To me it feels like exception a says use additional system types for
non-resi spaces, such as retail or corridors because they will have
different loads and operational hours, though I can see your point as
well.

I've never had a LEED reviewer tell me to use PTAC/PTHPs for corridors,
but I'd be willing to bet others have used these systems for corridors and
also not received comment on them.

Nathan Miller, PE, LEED AP BD+C

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Nathan, Maria,

Thanks a lot for your suggestions. I agree with both of you that System
#5 or #7 is quite different from the actual gas-fired constant volume
rooftop unit that will serve the corridors. Maria also made a great
point about Table G3.1.1A having "Building Type" as the header and not
"Space Type".

I think I will go for System #7 since that's where the simplest
interpretation of Exception G31.1.1a leads to, and see what comments I
receive from USGBC about it.

Thanks again to everyone for joining in the discussion.

Ritwik Kakati

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Ritwik,

For better or worse, I agree with Maria that PTAC/PTHP should be applied to the entire multifamily building. I disagreed with a LEED reviewer's requirement that I separate a three-story dorm project into residential/nonresidential portions. I mentioned the example of the User's Manual case study that Maria described below, but I changed the baseline anyway to keep from going back and forth with the reviewer. I complied with the LEED reviewer's request by using System 3 per nonresidential zone instead of System 1. What a waste of time for negligible energy impact.

If you feel compelled to model additional system types for nonpredominant conditions, I guess you use System 3 or 5 based on the floor area.

Regards,
Bill

[Senior Energy Engineer 28Jun2012]

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Yeah, an important point is that Appendix G is meant to cover all building
stock, which is why occasionally you end up with a bizarre baseline system
that doesn't really fit. They tried to sculpt the baseline system tables
and exception notes as best they could, but this happens to be a system
type that falls in a weird place.

Nathan Miller, PE, LEED AP BD+C

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Another weird off-shoot of this discussion is that I think *technically*
if your corridors have exterior walls, the baseline building constructions
and glazing properties for these areas should be based on the
non-residential envelope requirements of Table 5.5, because like retail
spaces, these are non-resi spaces per the previously discussed table
notes.

Of course in real life most projects will have the same wall assemblies in
the corridors as they do in the immediately adjacent apartments. This
probably doesn't come up much because corridors usually don't have much
exterior wall, but I wonder if anybody has ever seen that as a reviewer
comment? I had a recent project with a lot of corridors with exterior
walls, and I used the non-resi wall and window constructions, but I
haven't head back on reviewer comments yet.

Nathan Miller, PE, LEED AP BD+C

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