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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?

Thanks!

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?

Thanks!