[Bldg-sim] Baseline System type exceptions

[Dahlstrom, Aaron]

If the only distinguishing feature driving the load is envelope, and the zones are on the same system in the design, I think I would keep them all on System 7 in the baseline.

I tend to use G3.1.1 exception b only when there is a good argument for why the building would be designed that way.
Very unlikely that a 7-story office building would use single zone air-cooled systems for either core or interior zones on each floor.

For additional certainty on the approach you could try an administrative request to USGBC or informal interpretation request to ASHRAE 90.1 committee.

Aaron Dahlstrom , PE, LEED(r) AP
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From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of bldgsim bldgsim
Sent: Wednesday, October 26, 2011 5:24 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Baseline System type exceptions

I have an office building that I'm trying to model per ASHRAE 90.1-2007 appendix G, with 5 core zones and 5 perimeter zones for a total of 10 zones (it has way more than 10 zones but to make it simpler we'll say 10). Because the office building is heated with natural gas and is more than 5 stories it is Baseline system 7. This office building has a lot of glass that isn't the best performance glazing. Therefore the load on the 5 perimeter zones is 50 btu/h-sqft and the load on the 5 core zones is 10 btu/h-sqft, for an average of 30 btu/h-sqft. ASHRAE 90.1-2007 G3.1.1 exception b states that:

"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 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."

Since all my spaces vary by 10 btu/h-sqft for the average (30 btu/h-sqft) I'm curious what to do. I can see a few options so far, but I want to know what your experience has been.

 *   Put all the zones in the building on system 3, because they all vary by 10 btu/h-sqft from the average.
 *   Put all the perimeter zones on system 3, making the core zones fall within a 10 btu/h-sqft average.
 *   Put all the core zones on system 3, making the perimeter zones fall within a 10 btu/h-sqft average.
 *   Put a few of either the core or perimeter zones on system 3 until the average of the others falls within range, but how do I pick which zones?
Any ideas?


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