Folks:

I was looking for a way to write a user-defined expression to calculate the 62.1
ventilation requirements based on type of area and type of occupancy.
I posted a question to eSimForums.com and Charlie (cgronek) replied with the
following:

Paul,
You could write a switch statement that calculates the OA flow based on area
and occupancy for different activity descriptions. This would go in the OA-CFM
entry of the zone:

switch(#LR("SPACE","C-ACTIVITY-DESC"))
case "Offi" : #LR("SPACE","AREA")*0.06+#LR("SPACE","NUMBER-OF-PEOPLE")*5
case "Corr" : #LR("SPACE","AREA")*0.06+#LR("SPACE","NUMBER-OF-PEOPLE")*0
default : no_default
endswitch

This doesn't take into account anything like distribution efficiency or system
ventilation efficiency, so it may not be fully representative of the ASHRAE 62
requirements, depending on what system you're modeling.

Also, you mention that the number of occupants vary depending on the occupied
schedule of the area - this is true from a loads standpoint, but will not have
any affect on the outside air flow. You'd have to model demand control
ventilation (DCV) to account for occupancy variations.

-Charlie
Using this information, I wrote the following expression for a recent project:

In Outdoor Air under Airside HVAC Zone Parameters enter 0 in the Flow per
Person, the Air Changes and Flow per Area. Then right click on Airflow, click
on Edit/View user default. Choose User-defined default expression and enter
your expression using the format below. The length of the input string is
limited so if you try to enter too many space types an error will occur.

After the CASE enter up to the first 4 characters of the space type as defined
in the Building Shell/Space Properties Description tab.

The first part of each expression calculates the ventilation requirement based
on the area. In the first CASE below, I want .06 CFM/Ft2 for the offices. The
second part of the expression calculates the ventilation requirement per
person. In the example below, it is 5 CFM per person.

The formula needs to be input only once. You can then select each zone and
choose the User Default expression by right clicking on the Airflow field as
described above and the formula will appear.

You can also enter the distribution efficiency by dividing the entire expression
by the applicable efficiency if the expression is not too long. Or you can
divide each factor (the .06 and 5 for the first example) by the applicable
efficiency, which is usually .8 for the type systems I work on and enter .075
and 6.25 for this example.

switch (#LR("SPACE","C-ACTIVITY-DESC"))
CASE "OFC": #LR("SPACE", "AREA")*.06+#LR("SPACE","NUMBER-OF-PEOPLE")*5
CASE "FITN": #LR("SPACE", "AREA")*.06+#LR("SPACE","NUMBER-OF-PEOPLE")*20
CASE "TECH": #LR("SPACE", "AREA")*.12+#LR("SPACE","NUMBER-OF-PEOPLE")*10
CASE "DAYC": #LR("SPACE", "AREA")*.18+#LR("SPACE","NUMBER-OF-PEOPLE")*10
CASE "GAME": #LR("SPACE", "AREA")*.18+#LR("SPACE","NUMBER-OF-PEOPLE")*7.5
CASE "RR": #LR("SPACE", "AREA")*.06+#LR("SPACE","NUMBER-OF-PEOPLE")*5
CASE "MECH": #LR("SPACE", "AREA")*.06
CASE "STOR": #LR("SPACE", "AREA")*.12
CASE "CORR": #LR("SPACE", "AREA")*.06
CASE "ELEV": #LR("SPACE", "AREA")*.12
CASE "GYM": #LR("SPACE", "AREA")*.30
default : no_default
endswitch

Paul Diglio, CEM, CBCP