90.1 appendix G Baseline System interpretation

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I am working on a 90.1 Appendix G model of a 5-story mixed-use
building. The bottom two floors are commercial spaces, the top 3 floors
are residential. There is an underground parking garage as well, which I
have considered to be part of the residential use). The building is
heated by a combination of gas and electricity; gas use predominates. By
floor area the building is predominantly residential. However, about 62%
of the building's total energy use is associated with the commercial
parts. Because of the way that energy use breaks down I am leaning
toward considering the building to be non residential, using System 5 as
the baseline and taking exception G3.1.1 a (system 3) for the
residential spaces. If anyone has experience with such buildings and
could share their thoughts on the validity of my interpretation, I would
greatly appreciate it.

kind regards and my thanks in advance


Thermal Energy Systems Specialists, LLC
22 North Carroll Street - suite 370
Madison, WI 53703 USA

d.bradley at tess-inc.com


Bldg simulation's picture
Joined: 2016-04-13
Reputation: 400


I was just looking into a very similar scenario this morning in fact. I
have one question and some follow-up comments as well.

You stated that exception G3.1.1a can be applied, does the non-predominant
conditioned space total to be at least 20,000 sf? If so, shouldn't the
residential baseline (system #1) be used for all residential units
(according to Table G3.1.1A), and not system #3 as you stated? Also in this
case, the whole baseline building should be fossil fuel heat source - check
LI 10132 that discusses baseline heat source selection, if you're doing
this for LEED.

The next step would be to determine the commercial space baseline, based on
the floor areas of the first two levels. You might end up with System #3 or
#5 depending on the total area of the commercial space. At least that's how
I read "ASHRAE Interpretation IC 90.1-2007-05", you can search the quote to
find the document. I'm not sure if there's anything else that supercedes
that interpretation since.

To put it simply, if exception a applies, then I'd personally model the
upper 3 floors using system #1 and the lower 2 floors with either #3 or #5
based on total commercial space area. That's how I interpreted those two
referenced resources.


Bldg simulation's picture
Joined: 2016-04-13
Reputation: 400

I agree with Rut: extra caution to consider baseline system types #1 and #2 if you are handling a residential vs. non-residential split.

If this is for LEED also consider whether baseline system types #9 and #10 and the associated new exceptions to G3.1.1 might help clarify your situation (permissible under LEED v3 if you look to the EAp2 documentation workbook).

Also I?d think twice before considering your parking garage for either occupancy type, in this particular context. Carefully review the definitions for ?conditioned floor area? ? ?gross conditioned floor area? ? ?space? ? ?unconditioned space? in that order, and you might agree with me that parking garages are not meant to be counted for ?conditioned area.?

Bear in mind, once you have made a successful justification for multiple baseline system types, it doesn?t particularly matter which one was the ?predominant? case. It sounds as though you could claim a residential system type as your predominant occupancy (by area), and still invoke the same exception to apply a non-residential baseline type to the non-residential areas.

It still will come to pass however where it totally makes sense to have multiple baseline system types, but the rigid language permitting a distinction only by 20,000sf of conditioned floor area does not capture these realities. When this happens, the best advice I can give is to proactively reach out to the AHJ/reviewer/GBCI to explain your concerns and make your case. I have been both successful and unsuccessful in convincing my past reviewers that the language on the books does not always align with the intent. It comes down to interpretation, but I?d consider including the following when making your case:

1. Illustrate/demonstrate how the usage/EUI/operations/ventilation rates/whatever is substantially different

2. Detail other by-the-book reasons the shoe fits for a different baseline case (i.e. different heating source, residential vs. non-residential).

3. Show why the interpretation to this exception is important in the end results. If it?s ultimately a big deal to the final numbers, and you convey the gravity of the decision, it may help motivate an otherwise overworked/tired/ready-for-the-weekend reviewer to sharpen their pencils and give your case due consideration.

4. It also never hurts to offer a direct meeting/conversation? and to say please and thank you.

Best of luck!


Nick Caton, P.E.

Senior Energy Engineer
Energy and Sustainability Services
Schneider Electric

D 913.564.6361
M 785.410.3317
E nicholas.caton at schneider-electric.com
F 913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

Bldg simulation's picture
Joined: 2016-04-13
Reputation: 400

Hi David,

What is the total square footage of residential vs non-residential (excluding the parking garage)?

If the residential and non-residential areas are both over 20,000 square feet, having a predominant condition is a moot point. Model the residential units as System 1 or 2 and the non-residential areas according to Table 3.1 based on the total non-residential square footage and heating source. There is a LEED Interpretation out there that gives examples of various scenarios that your building may fall under. I can't remember the number (somewhere around 10130 or 11130 or something like that???). It's been updated since the original was put out. If I remember correctly, the residential corridors may be considered as either residential or as non-residential....

As for the parking garage, it must be handled independently. If it is heating only, either model it with system 9 or 10 (recommended) from App G 2010 or model it identically to the Proposed. If it is heated and cooled, use Table G3.1 based on the square footage. Under no circumstances should a System 1 or 2 be used for a parking garage!

Keep in mind that if the project is registered, it is also possible to submit a project specific inquiry to GBCI when your way forward is still unclear even after reviewing all available resources such as LEED Interpretations.

Hope this helps! Have a nice weekend!

Sheila Sagerer
Sustainability and Energy Services Engineer
Heapy Engineering

Sent from my iPhone

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