Peter,
I think as Vikram mentioned, it depends on the total exhaust (cumulative exhaust
of all the fume hoods). If the total exhaust exceeds 15,000cfm, then the
baseline needs to have AT LEAST one of the following:

1. VAV hood exhaust till a 50% min turn down over the total CFM. Hence you could
only take advantage of a lower VAV set point on the actual equipment.
2. Make-up air. Separate make-up air with supply temperature very close to room
temperatures.
3. Heat recovery . If the building has both VAV and HR, then you should be able
to take full benefits of heat recovery or vise-versa.

Hence if your total fume hood exhaust is lower than 15,000 cfm, then you should
be able to take advantage of VAV fume hood exhaust over constant volume
exhaust.

Does this make sense?

Gerald