Application

Sequence of Operation
Control Loops
9
Siemens Industry, Inc. Application Note, App 2033 140-1023
2015-08-07
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.
To enhance stable flow control, an advanced algorithm is used to calculate a
controllable setpoint as the value approaches zero cfm (lps).
FLOW is the input value for the flow loop. It is calculated as a percentage based on
where AIR VOLUME is between 0 cfm (LPS) and OCC FLOW. In the following text,
this percentage is referred to as % flow.
 If AIR VOLUME equals 0 cfm (LPS), then FLOW is 0% flow.
 If AIR VOLUME equals OCC FLOW, then FLOW is 100% flow.
The FLOW STPT percentage that corresponds to UNOCC FLOW is calculated as:
(UNOCC FLOW ÷ OCC FLOW) × 100% flow.
Example
If UNOCC FLOW = 250 cfm, and if OCC FLOW = 1000 cfm
then, in unoccupied mode the FLOW STPT
= (250 cfm ÷ 1000 cfm) × 100% flow
= 0.25 × 100% flow
= 25% flow
Since 250 cfm is 25% of 1000 cfm, the flow setpoint in unoccupied mode is 25%.
UNOCC FLOW can be set less than or equal to, but not greater than OCC FLOW.
Temperature Loop – The temperature loop will modulate HTG LOOPOUT and control
the heating elements in order to maintain the room temperature in both heating and
cooling modes.
Hot Water Reheat
The temperature loop modulates the heating valve in order to maintain the room
temperature setpoint. The reheat valve will be modulated whenever necessary to
maintain the room temperature regardless of the status of HEAT.COOL.
Calibration
Calibration of the controller’s internal air velocity sensor(s) is periodically required to
maintain accurate air velocity readings. CAL SETUP is set with the desired calibration
option during controller startup.
Depending on the value of CAL SETUP, calibration may be set to take place
automatically or manually. If CAL AIR = YES, calibration is in progress.
 For a controller used without an Autozero Module (CAL MODULE = NO), the
damper is commanded closed to get a zero airflow reading during calibration.
 For a controller used with an Autozero Module (CAL MODULE = YES), calibration
occurs without closing the damper.
NOTE:
The first time after startup or initialization, the controller will calibrate the dampers as
if not using Autozero Modules, although the Autozero Modules will be activated. All
subsequent calibrations will use the Autozero Modules only.