Application

Sequence of Operation
External Flow Values
14
Siemens Industry, Inc.
Application Note, App 6750
140-1328
2015-07-07
 During CV operation, the controller ignores TEMP CTL VOL. Instead, it calculates
SUP FLO STPT based on the value of the active supply flow minimum and the
amount of supply airflow needed to pressurize the room.
NOTE:
Regardless of the flow tracking method (STE or ETS) being used, the controller does
not let the actual supply airflow rise above the currently active supply airflow
maximum
. However, the currently active supply airflow minimum (OCC SUP MIN or
UOC SUP MIN) will be overridden if necessary to achieve desired pressurization.
External Flow Values
Airflows not connected to the controller must be taken into consideration when
pressurizing the room, including snorkels, canopies, as well as other supplies, such as
offices within the lab space controlled by constant volume controllers. Since these
inputs are not connected to the controller, the combination of their values must be
entered into OTHER SUP and OTHER EXH so the controller can properly control the
lab space.
NOTE:
If these airflow values change slowly or predictably (for example, VAV temperature
control and/or occ/unoc differences), steps can be taken using PPCL to have the
changes sent over the network to update OTHER EXH and OTHER SUP with the
new values
Ventilation – VAV Mode
During VAV operation, the ventilation works as follows:
OCC SUP MIN, the occupied supply minimum, is used to ensure that the room
receives enough supply air for proper ventilation during the occupied mode. UOC SUP
MIN is used to ensure that the room receives enough supply air for proper ventilation
during the unoccupied mode. If necessary, the application raises the general exhaust
flow to keep the supply flow from dropping below the minimum. See Calculating
Exhaust Flow Setpoint
for more information.
NOTE:
Since Application 6750 places a higher priority on pressurization than it does on
ventilation, the currently active supply minimum (OCC SUP MIN or UNOCC SUP
MIN) may be overridden to maintain negative pressurization.
Ventilation Setback in VAV Mode
Ventilation setback allows the minimum and maximum flows (air change rate) for each
VAV terminal to vary based on occupancy mode (OCC.UNOCC). This allows several
options for reducing the ventilation for unoccupied periods, including:
 Lowering the minimum supply flow, which allows a lower air change rate, but
maintains cooling control.
 Lowering the maximum flow, which limits the air change rate and reduces cooling
capacity.