I am working on the modeling for a LEED project that is a science builidng
at a university. There are 4-5 chemistry classrooms that have VAV fume
hoods, all served by the same AHU. Since this is a "teaching lab" I expect
the hoods to only be used for about 20% of the hours that the space is
occupied. For example, a typical 1 hour class would be consist of a 45
minute lecture and 15 minutes of bench-work under the hoods.

My reading of 90.1-2007 indicates that for the baseline model the hoods are
constant volume. Is this the assumption that others have also used?

Assuming this, is it acceptable to take credit for the VAV hoods running at
a mimimum flow rate except for the 20% of occupied hours that students are
actively using them?

I have seen the guidance from labs21, however this appears to more focused
on a research lab with extensive fume hood use:
http://www.i2sl.org/documents/toolkit/ashrae_appg_2007_508.pdf

Thanks for your input,
-Adam