Complicated school baseline system type question

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I am modeling a school that is 156,000 SF under 90.1-2007. The majority of
the school is treated with electric heat pumps but about 26,000 SF of the
building is heated with natural gas furnaces.

So the approach I would take is that the predominant condition is electric
heating, so that should be the main baseline system type. Then per
G3.1.1(a) the natural gas systems are large enough to have their own
baseline system.

My first question is do you think the main baseline should be based on the
whole school area of 156,000 SF, or the 130,000 SF. This is the difference
between system type 6 or 8.

Then, the 26,000 SF is large enough to qualify for the system type 5. But
most of these spaces are large single zone systems (i.e. cafeteria,
gymnasium, ect). And some could qualify for the single zone exceptions.
Which would bring the area under 25,000 SF. How would you account for this

And in general if spaces qualify for exceptions in G.3.1.1(b) or G3.1.1(c)
do you think the single zone systems should match the baseline energy
source or the proposed building energy source. For example if you have a
school that is heated by natural gas, but you have a small server room that
is conditioned with an electric heat pump. Should the baseline system in
that single zone system be electric? Or an electrically heated school with
natural gas in the kitchen. Should the kitchen single zone system be
natural gas like the proposed or electric because that is the baseline fuel

I couldn't find any clarifications to this issue, but if there has been any
official guidance please let me know.

Thanks in advance,

Steve Jacobs's picture
Joined: 2012-11-12
Reputation: 600

Hi Steve!

1. I think the details of the ?main? baseline system should be determined using the corresponding building area, not the gross building area.
2. I personally would not pursue more than two baseline system types generally, as a matter of poor potential ?gains? (if that is the right word) vs. time invested (primarily in documentation). This stance is however, like many things, flexible and adaptable to the situation at hand. I would encourage other energy modelers of all experience/skill levels to (pro)actively take stock of their time/resource limitations, and invest your efforts where it makes the most difference. In any case, if an exception pulls a space below the previously ?cleared? 20ksf threshold, I would consider such exceptions to be ?downstream/secondary? to establishing the main/secondary baseline types, so I would not care much as a model developer or reviewer? but that caveat is to highlight that this, like many things, is ultimately interpretable (your reviewer/AHJ is going to have the final say).
3. With respect to ?official? guidance on ?interpretable? situations, there are a couple places I am used to considering as strong ?citable? sources:
* ASHRAE maintains discussions and addenda specific to each standard/year here*
* LEED maintains a similar database of interpretations on their website, typically (but not always) with queries/answers tailored specifically with the context/application of LEED projects in mind. For projects outside of the LEED sphere, this remains a relatively ?citable? source requiring little introduction for experienced reviewers.
* Discussions opened and documented/archived here on have served me well as citable sources as well, in matters of a relatively technical nature. Seeing 5 out of 6 practicing experts agree on something easy to argue 2 ways is something many reviewers will recognize as a rarity. This is the ?messiest? source to compose/cite clearly, but a PDF inclusive of original thread/signature formatting and images has been my go-to for a few cases. If ever anyone needs a specific thread in the archives reconstructed with full formatting/attachments, please just make the request here and I or others can often bring that to the table!

Philosophically (and constructively) I would treat none of these sources as ?absolute truth? or cause to treat any AHJ/reviewer with aggression/belligerence. Standards/Codes should ideally be clear, minimizing the need for interpretation, but ALWAYS remain subject to interpretation in order to fit the case at hand if and when the intent/application to the real world may not align with what the authors? intended. In a world where all building codes were applied blindly and without active consideration/reason/understanding, construction projects would in broad strokes suffer new levels of inefficiencies in design, analysis, construction, and even endanger the safety/health of the public. Our ability to understand and *interpret* codes/standards makes our time and skillsets much more valuable. Let?s not run away from that =).

I hope this is helpful!


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Nicholas Caton2's picture
Joined: 2019-03-25
Reputation: 0

Hi Steve,

Here are my responses:

1. I think that the main baseline system should be based on the whole conditioned school area (156,000 sf). Therefore, use System Type 8. The primary system should be based on the whole building and predominant heating source condition.

2. This question is tricky and I?m not sure if there is an official stance on how to handle this one. I would suggest going with a System type 5 and then System Type 3 for anything that meets exceptions b or c.

3. The single zone systems should match the baseline energy source or the predominant building heat source. In this case though, when you apply exception a to the portion of the building that is heated with natural gas, then the predominant heating source for that portion is natural gas. But to answer your specific examples, the small server room would be conditioned with a DX natural gas furnace and the kitchen should be an electric heat pump unless it is greater than 20,000 sf. Also note that if there is a dedicated makeup air unit for the kitchen range hood that is natural gas, then that should be modeled the same in the Baseline and Proposed based on the proposed design. This is because the makeup air unit serves a process load (range top cooking heat/smoke removal) and not an occupant load. Additionally, even though you might think that the server room system is also only serving process loads, people do need to occupy the server room space to maintain and update the servers, so it isn?t the same as the makeup air unit which is only serving the ventilation used to makeup the exhaust from the range hood.


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