Airflow Schedule based on Occupancy

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Hi everyone,

I have a zone that is served by a VAV system. The controls for the zone
state that when the zone is occupied, the VAV box should be fully opened
and when unoccupied, the zone supply air be reduced to 6 ACH. There are
multiple zones controlled in this manner.

To model this I am thinking of creating a min airflow schedule that matches
occupancy schedule. The minimum hourly fraction within the min airflow
schedule would be equivalent to the airflow fraction of 6 ACH.

In other words, if there are hourly fractions in the min airflow schedule
(which is a copy of the occupancy schedule) that are below the equivalent
fraction of 6 ACH, those hours would be changed to the 6 ACH fraction.

Hopefully that makes sense to you.

This method seems fine to me but a bit tedious as it will require
developing multiple min airflow schedules.

Any thoughts?

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Darryl,
First, I would use a user expression for ZONE: MIN-FLOW-RATIO for any zones that you want to control to 6 ACH minimum. Then I would create and apply a ZONE: MIN-FLOW-SCH with values of 1.0 during occupied hours and -999 for unoccupied hours. The expression below for MIN-FLOW-RATIO should give you 6 ACH . It will give you an error if a value is entered for ASSIGNED-FLOW that is less than 6 ACH.

/* This expression for ZONE:MIN-FLOW-RATIO calculates and enters
a value based on a desired minimum ACH. */

if (#P("MIN-CFM-RATIO") == unused)
then unused
else switch #SV(#P("TYPE"))
case 2 : unused
case 17 : unused
case 19 : unused
case 26 : unused
case 27 : unused
case 28 : unused
case 29 : unused
default : if (#RV(#L("AIR-CHANGES/HR")) == 0
.AND. #L("AIR-CHANGES/HR") = 6) then 1 $ 6 ACH
else if (#RV(#L("ASSIGNED-FLOW")) == 0) then
else no_default endif

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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via Equest-users's picture
Joined: 2016-07-15
Reputation: 400