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Do you actually check this when modelling for LEED? Ashrae 90.1-2007 - G3.1.1 Exception b.

asked 2018-03-23 08:39:19 -0500

updated 2018-03-25 12:16:42 -0500

Hello all,

I am working in a LEED Ashrae 90.1-2007 App G model of a laboratory building. Baseline has many zones (300+) inherited from proposed zoning.

Main HVAC system is Ashrae's System 6.

The problem lies in the G3.1.1 Exception b:

"If the baseline HVAC system type is 5, 6, 7, or 8, 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 or schedules that differ by more than 40 equivalent fullload hours per week from other spaces served by the system are considered to differ significantly".

I've found out that 100+ of the spaces fit in that exception but I am not particularly willing to model 100+ PSZ-HP single zone systems...

How many of you check this 10 Btu/h·ft2 limit is surpassed? I think this can happen more often than expected.

I see some additional issues with this exception:

  • You could get a high number of zones in that exception when you have zones in different orientations and glazing fractions (it may not be easy to find out which ones are the exception there).
  • In the above case, you could get that different zones fit into the exception when the building is rotated for each of LEED required angle. Even more since Baseline removes all external shading. Autosizing is applied for every rotation but would you need to apply this exception for each rotation? I don't think so, which makes this exception questionable.
  • The text mentions occupancy or process loads or schedules. It seems as if this distinction could be made at proposed building level (occupancy, process or schedules are equivalent in proposed and baseline), which would have more sense in my opinion, but I am not sure if a reviewer would think the same...

In short, have you ever found a case like this? What did you or would you do?


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answered 2018-03-23 14:23:16 -0500

To answer your question - I had some projects in the past that had several zones (over 100+) that fit these exception. I created separate systems for all these zones.

(FYI, I did receive a review comment on one of these projects where the LEED reviewer asked for a list of all zones and the baseline systems serving them to ensure that they adhered to this exception)

The general idea of having a separate system for these zones is to ensure that the baseline system (5,6,7, or 8) that is modeled for the entire floor is not at disadvantage of higher energy usage just due to a few zones. One way to reduce the number of zones that differ in peak thermal loads is to zone the floor plan more carefully. I lump multiple rooms together according to requirements in Table G3.1.7 to reduce the overall number of thermal zones but again, this may not always be possible.

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answered 2018-08-23 17:57:33 -0500

Brett M gravatar image

If large areas (over 20,000 SF) meet this definition the corresponding thermal zones can be grouped together into a separate system type per exception a:

"Use additional system type(s) for nonpredominant conditions (i.e., residential/nonresidential or heating source) if those conditions apply to more than 20,000 ft2 of conditioned floor area."

Exception a. should dramatically reduce the total amount of zones that require system type 3 or 4.

We regularly perform energy modeling for laboratory buildings (and design them) and commonly all of the laboratory spaces are on a single system type (as they often have specific ventilation requirement and schedule requirements) separate from the non-lab spaces such as offices/conference rooms, etc. You may have a two systems of type 5/6/7/8s on a single floor along with several system types 3 or 4 for small rooms that have additional specialty loads or schedule requirements (a cold environment room as an example).

I have not dived into how the building rotations impacts this exercise as I interpret the requirement only to apply to the base rotation. A dedicated system could be provided to a particular facade spanning multiple floors per exception a. if the zones on that facade were dramatically different than the predominant space type in the rest of the building.

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Asked: 2018-03-23 08:39:19 -0500

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Last updated: Mar 23 '18