I am modeling a school that is 150,700 SF. There is a welding lab in the
school and it falls within the exceptions of ASHRAE 90.1 2007: G3.1.1
Exception b. ("Peak thermal loads that differ by 10 btuh/sf or more from
the average of the other spaces served by the system..."). The welding
lab is 3,500 SF and I would use System 3 (PSZ-AC) for this space.
When I refer to Table G3.1.1A, one of the notes reads "Where attributes
make a building eligible for more than one baseline system type, use the
predominant condition to determine the system type for the entire
building." Does this allow me to use System 5 (Packaged VAV with Reheat)
instead of System 7 (VAV with Reheat) since the predominant condition
serves 147,200 SF (150,700-3,500)?
This is a LEED project and I would obviously rather use system 5 than
system 7 as my baseline HVAC system.
Thanks,
Otto
Hi Otto,
As per 90.1-2007 Addendum W, " Where attributes make a building eligible for
more than one *baseline *system type, use the predominant condition to
determine the system type for the entire building except as noted in
Exception a to Section G3.1.1." Since your lab < 20,000ft2, my understanding
is that you can use system 5.
Regards,
Cheney
Thanks Cheney.
Has anyone attempted to do this on a LEED project? Did the reviewer
accept this and were there any comments?
Otto,
My interpretation is that you determine your baseline system first
"based on usage, number of floors, conditioned floor area and heating
source", unless G3.1.1 exception a. applies. If, for example, your
school has 20,500 ft2 of classrooms with electric heating and the rest
of the school is gas, you could justify using both System 5 and System
6, instead of applying System 7 to the whole school. The note to Table
G3.1.1A only mentions exception a., so I don't think you can subtract
areas from other exceptions to G3.1.1 from the baseline system type
determination. If you're hoping to use System 5, carefully double-check
the conditioned floor area.
Regards,
Bill
Thanks Bill,
I think you are right. I will have to use exception "a" from G3.1.1 in
order to use system 5. The academic portion of the building has a VRF
system with the outside air conditioned by hot water from a natural gas
condensing boiler. I am going to separate the spaces that have outside
air (classrooms, offices, corridors......) and spaces that has no direct
outside air (restrooms, technology rooms.....) in the model so I can use
system 5 and 6 if the areas that are conditioned with only electric is
over 20,000 SF. This should work because the spaces that have outside
are conditioned with natural gas/electric and many of the other areas
are conditioned with only electric.