Application

Sequence of Operation
Room Airflow Balance
10
Siemens Industry, Inc.
Application Note, App 2928
140-1302
2015-07-07
Room Airflow Balance
The difference between total supply flow and total exhaust flow is the room airflow
balance as shown in these calculations:
VOL DIFFRNC = TOTL EXHAUST – TOTL SUPPLY
-or-
VOL DIFFRNC = (HOOD VOL + GEX AIR VOL +
OTHER EXH) – (SUP AIR VOL + OTHER SUP)
The controller uses these calculations to maintain VOL DIFFRNC at the VOL DIF
STPT.
NOTE:
VOL DIFFRNC and VOL DIF STPT are positive numbers in a room that is negatively
pressurized and negative in a positively pressurized room.
Application 2928 has the ability to maintain a different volume differential setpoint
during occupied mode than during unoccupied mode. When OCC.UNOCC = OCC,
VOL DIF STPT = OCC DIF STPT. When OCC.UNOCC = UNOCC, VOL DIF STPT =
UOC DIF STPT.
Occupancy
The controller keeps track of the occupancy status of the room and uses that
information for the following purposes:
 To select minimum and maximum flow rates for each air terminal.
 To select whether the controller is operating in the VAV or CV mode.
 To determine whether the controller is using Supply Tracks Exhaust (STE)
or Exhaust Tracks Supply (ETS) flow tracking.
 To determine the value of VOL DIF STPT.
 To calculate the airflow level that triggers the ventilation alarm.
Occupancy status also affects the airflow level that triggers the ventilation alarm. The
occupancy status of the room is indicated by OCC.UNOCC. A digital room thermostat
can read OCC.UNOCC and display its value. This point cannot be overridden.
If the occupancy status of the room is set manually, it is necessary to work through the
following command options.
The controller works in the occupied mode whenever
one or more of the following
occupancy signals indicates occupancy:
 Commands from a field panel, NET OCC CMD
 Dry contact switch in the room, OCC SWIT DI2
 Push button on the thermostat, OCC BUTTON
If
all of these occupancy signals indicate vacancy, the controller works in the
unoccupied mode.
NET OCC CMD – The NET OCC CMD may be set from a field panel by OCC and
UNOCC commands to the LCTLR point. The commands may come from a time-of-day
schedule, a PPCL program, or operator commands. These commands work on the
LCTLR without unbundling.