Baseline System type exceptions

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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?

bldgsim bldgsim's picture
Joined: 2011-10-27
Reputation: 0

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

Dahlstrom, Aaron2's picture
Joined: 2011-09-30
Reputation: 4


*1. I doubt you can understand exception b in this manner. My understanding
is that this exception should only be applied to individual rooms which may
need individual HVAC system due to its 'abnormal' thermal loads or schedules
other than the space at the same floor, for example, server rooms, telecom
rooms, small laboratory spaces, auditoriums and security rooms among office
*2. Probably you should further clarify how you can get 50 btu/h-sqft for
perimeter zones and 10 btu/h-sqft for inner zones. Is this 10 btu/h-sqft
heating peak load for inner zones?*



chen yu's picture
Joined: 2011-10-02
Reputation: 200