ASHRAE 90.1 baseline HVAC system interpretation

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Hi all,

I am puzzled with the interpretation of Ashrae 90.1-2007 G3.1.1 where is
says each floor shall be modeled with a separate HVAC system.

My proposed building is a 4 story office building with 3 floors office and
a first floor consisted of cafeteria, kitchen and gym. Office floors have
one system per floor and the first floor has three systems for each
specific area. My baseline is using system 6 for the typical office floors.
I am trying to interpret the G3.1.1 for the first floor. If I combine all
those areas into one system their occupancy schedules, HVAC schedules, OA
rates etc are all different and if I use one system, the schedules will not
match the proposed. The kitchen qualifies for exception b due to thermal
loads, so I will use a single zone system for that area. I am wondering how
I should approach the gym and cafeteria. Can I use two systems in the
baseline for those areas? They do not fall into exception b since their
schedules and loads do not differ significantly from other spaces (office
spaces) but they do differ from each other and using one system for those
two different usage areas brings the dilemma of using a different schedule
from the proposed building.

Any insights will be greatly appreciated.


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My usual interpretation in this type of situation is that the caf? will have occupants when the gym is unoccupied and I apply the exception ? one system for the Gym and one system for the caf?.

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The simplest approach (in the interest of your own time) may be to define for the cafeteria/gym a ?combined? system schedule that ensures the operating hours and ventilation delivery is satisfied for both areas.

Otherwise, I think you should be able to safely interpret G3.1.1.b exceptions when the occupancy/process loads/schedules differ significantly from the rest of the same floor. That isn?t the exact wording in 90.1, but if I were your reviewer I think this fits the spirit / intent for such exceptions. Worst case scenario is your reviewer disagrees, and you?re later pushed into the first suggestion to combine those cases, resulting in a larger baseline system with broadly worse unitary efficiencies, helping your performance rating if anything.


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