Can someone explain what the Exceptions to 6.5.2.1 mean in general terms? Its on page 62 of 90.1. I am wondering where they are getting 0.4 cfm/sf and 300cfm.
It means that you are allowed to reheat air in, for instance, a VAV system. This is where the .4 cmf/sq ft comes from, since 90.1 specifies a vav minimum of 30% of the design air, or .4 cfm/sq ft.
Without this exception, it would make VAV with reheat impossible.
Also, because of ventilation requirements, some small spaces (think restrooms) need more outside air than they need supply air. Therefore, they require reheat (or they would be 55 degrees in summer).
It's less expensive to reheat the air in these small spaces than to supply a higher percentage of air to the whole system. (which is where exception 5 comes in as well).
It means that you are allowed to reheat air in, for instance, a VAV system. This is where the .4 cmf/sq ft comes from, since 90.1 specifies a vav minimum of 30% of the design air, or .4 cfm/sq ft.
Without this exception, it would make VAV with reheat impossible.
Also, because of ventilation requirements, some small spaces (think restrooms) need more outside air than they need supply air. Therefore, they require reheat (or they would be 55 degrees in summer).
It's less expensive to reheat the air in these small spaces than to supply a higher percentage of air to the whole system. (which is where exception 5 comes in as well).