Application

Sequence of Operation
Heating/Cooling Switchover
10
Siemens Industry, Inc.
Application Note, App 6663
140-1109
2014-10-27
CTL STPT is set equal to RM STPT DIAL. The values for RM STPT MINand RM STPT
MAX will be applied to limit RM STPT DIAL before it is copied into CTL STPT.
Example in Cooling Mode
If the STPT SPAN is set to 2.0 degrees, and the DAY CLG STPT is 76°F, the room unit
slider will adjust the cooling setpoint from 74°F to 78°F.
Heating/Cooling Switchover
This application does not perform an automatic heating/cooling switchover function.
The heating elements are controlled at the CTL STPT in both cooling and heating
modes. Switchover, as required, is accomplished by PPCL in the controller or in the
field panel.
Occupied and Unoccupied Modes
The occupied/unoccupied status of the space is determined by the status of
OCC.UNOCC. The control of this point differs depending on whether the controller is
monitoring the status of a wall switch or if the controller is connected to a field panel.
When a wall switch is physically connected to the termination strip on the controller at
DI 2 and WALL SWITCH = YES, the controller monitors the status of DI 2. When the
status of DI 2 is ON (the switch is closed), OCC.UNOCC will be set to OCC indicating
that the controller is in occupied mode. When the status of DI 2 is OFF (the switch is
open), OCC.UNOCC will be set to UNOCC indicating that the controller is in
unoccupied mode.
When WALL SWITCH = NO, the controller does not monitor the status of the wall
switch, even if one is connected to it. In this case, if the controller is operating stand-
alone, the controller stays in occupied mode all the time. If the controller is operating
with centralized control (connected to a field panel), the field panel can send an
operator or PPCL command to override the status of OCC.UNOCC.
Unoccupied Mode Override Switch
If an override switch is present on the room temperature sensor and a value (in hours)
other than zero has been entered into OVRD TIME, then by pressing the override
switch, a room occupant can reset the controller to occupied mode for the length of
time set in OVRD TIME. The status of UNOCC OVRD changes to OCC and remains
there until OVRD TIME elapses, at which point UNOCC OVRD changes back to
UNOCC and the controller returns to unoccupied mode.
NOTE:
Only during unoccupied mode (MODE = Unoccupied) can a room sensor’s override
switch set the controller to occupied mode; if MODE equals anything other than
Unoccupied, UNOCC OVRD will equal UNOCC.
Control Loops
Flow Loop – maintains the point FLOW STPT by modulating the supply air damper
point, DMPR COMD. The flow loop maintains the airflow at either OCC FLOW or
UNOCC FLOW depending on the value of OCC.UNOCC.