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CI-v4 EAp2:Minimum energy performance

ASHRAE 90.1.2010 - Baseline System using Exception of G3.1.1

Hi,

Greetings!

We are doing the energy model for Office space which has registered under LEED ID+C v4. The overall floor area of the project space is 45,000 Sq.ft & the overall floor area is 1,20,000 Sq.ft. Conditioned area of project space is 38,000 Sq.ft & the overall conditioned area is 1,00,00 Sq.ft. This is G+4 Floors building.

As per Baseline Energy Model/Appendix G,

                The baseline system is system 6 – PVAV with PFP Boxes & we have considered one system per floor. However, we have decided to consider exception of G3.1.1

c. If the baseline HVAC system type is 5, 6, 7, 8, 9, 10, 11, 12, or 13 use separate single-zone systems conforming with the requirements of system 3 or system 4 (depending on building heating source) for any spaces that have occupancy or process loads or schedules that differ significantly from the rest of the building. Peak thermal loads that differ by 10 Btu/h·ft2 or more from the average of other spaces served by the system, or schedules that differ by more than 40 equivalent full-load hours per week from other spaces served by the system, are considered to differ significantly.

 

For the above-mentioned exception, we have calculated the peak thermal load for all the conditioned spaces by considering the following energy model inputs/methods.

  1. Baseline system is 6
  2. Auto sized capacity (Cooling 1.15 & Heating 1.25)
  3. Supply air temperature difference is 20 deg F

Our software has provided the floor area and peak thermal load in KBTU/Hr. From these, we have calculated the BTU/Hr/Sq.ft. Also, we have compared each room’s BTU/Hr/Sq.ft with an average BTU/Hr/Sq.ft of rest of spaces.

Finally, we have found that which are the spaces are differing more than 10 BTU/Hr/Sq.ft.

Can this approach be correct?

Which system to be used for identified spaces? (System 3 or 4 or same 6)

Note:

80% of conditioned spaces are office, meeting & manager cabins

10% is corridors

10% - Elec, kitchen & storerooms.

 

We look forward to your kind response.

Thanks.

2
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Profile picture for user sheffer
Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4461 thumbs up

Wed, 10/02/2019 - 15:46

If your calculations demonstrate that the exception applies then you must apply it. You must use the same fuel type so if it is a system 6 for the balance of the building then you use a system 4 for the spaces where this exception applies. So which spaces does the exception apply?

Wed, 10/02/2019 - 18:20

As per the baseline simulation, 53% of the offices are differing more than 10btu/hr/sq.ft while comparing with avg peak load of other zones. Rest of the spaces are under 10btu/hr/sq.ft. So, can I use the system 4 with CAV for 53 % of the spaces. System 6 for remaining conditioned spaces. Please let me know your reply. Is there any CIR for the same. Thanks...
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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4461 thumbs up

Wed, 10/02/2019 - 18:42

Typically you will not have this degree of difference between similar or the same space type. You generally have this difference if the spaces have vastly different temperature set points or large internal loads. If you are claiming this exception for some office areas it will most definitely be questioned by the reviewer. I am not aware of a CIR on this subject.

Wed, 10/02/2019 - 18:51

There is no highly use process equipments in the office spaces. Only plug load/EPD. The only difference between these office spaces are glazing area, orientation and location. So these factors only the reasons for the peak load. Once again please refer below, LPD, u values, shgc are as per ASHRAE. Cooling capacity and supply air are auto sized. Fresh air and no of people are as per ventilation calc. Once again thanks....
Profile picture for user sheffer
Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4461 thumbs up

Wed, 10/02/2019 - 18:57

If you are going to try and claim this you will need to provide all your calculations to demonstrate this difference to the reviewer. What you are claiming is highly unusual.
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Waleed AlGhamdi
Sustainability Enabler
Eskew+Dumez+Ripple
LEEDuser Basic Member
20 thumbs up

Wed, 10/02/2019 - 21:52

Given the location of the project in the UAE this might be the case, and in my opinion, the 10 Btuh/SF in the exceptions is kind of a loophole for an extreme climate zone like this. I recall doing an exercise for a city in the Middle East where the difference was more than 50 Btuh/SF, all due to enormous solar gain through the glazing when compared to core zones. As a sanity check I removed all glazing and the difference in peak loads between the perimeter and core zones dropped dramatically.  I'd say before taking advantage of the exception take a look at unmet load hours and box flow rates. 

Thu, 10/03/2019 - 00:49

Thank you for the valuable guidance and support...

Fri, 07/17/2026 - 08:41

We have a similar case in which we are not sure how to apply exception b.
The project is a manufacturing facility with spaces for offices, canteen and locker rooms, with these particular features:
 
  • The manufacturing facility is relatively small, it is not the typical case where the office/ common areas account for a small share of the total conditioned area.
  • The manufacturing area has also different zones with different internal loads and schedules.
With such a mix of uses and operational conditions, maybe nearly all spaces may fall under exception c (there is no a "clearly dominant" average in our building). So we were wondering if we should go beyond the literal statement of the exception and apply the underlying intent, which is to avoid oversizing and unmet hours in the baseline case. The approach would be:

- Apply the selected system per Table G3.1.1A to the office area (althought it is not the biggest in the building, is the most "homogeneous" area to apply a multizone system). In our case, the building will fall in System 6.
- Apply System 4 to all other thermal zones, provided that they are different enough to fall under exception b.

The alternative and more "normative" approach would be:
- Apply system 6 to the biggest (main) manufacturing area.
- Apply system 4 to all other thermal zones (other manufacturing, offices...), provided that they are different enough to fall under exception b.

What approach do you think would be the most appropriate? Thank you very much
Profile picture for user sheffer
Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4461 thumbs up

Fri, 07/17/2026 - 20:59

Exception c is for laboratory spaces so that does not appear to apply. It is hard to say what you should do without more information. You seem to understand the normal application of this exception. Also the word used in the notes to Table G3.1.1A is "predominant" which is very different than "clearly dominant". Predominant is just the largest area. Perhaps you could justify breaking up the manufacturing area and into other space types with different loads and schedules? Maybe then the offices are predominant? In any case be sure to write a clear narrative that quantifies the actual areas and space types in the building to justify your baseline system selection.

Mon, 07/20/2026 - 09:57

Thank you for your comments, Marcus (sorry for the mistake, I meant "exception b", not "c").

Even if we break up the manufacturing area, there will remain a larger space which could be considered as "predominant", so we will probably go to demonstrate that with an schedule, assign the System 6 to this space and apply System 4 to all othes spaces which differ from this predominant one according to exception b.Thank you again.

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