KMC Controls BAC-4000 series, Installation, Operation, And Application Manual

Page: 70 / 128

stored in an analog value object. See the topics Input objects on page 114 and
Value objects on page 117 for object details.
Discharge air temperature sensor
The Discharge Air Temperature (DAT) input is a required input for
economizer applications and is an option for other applications. The AppStat
is configured for a Type III thermistor sensor for DAT applications. Discharge
air temperature can be monitored as an analog input and is also stored in an
analog value object. See the topics
Input objects on page 114 and Value objects
on page 117
for object details.
The DAT input is a dual function input. It is used also as the input for the fan
status switch. See the topic
Fan status on page 68 for details.
PID control loops
A PID control loop calculates an error value from the difference between the
measured room temperature and the active setpoint. The error value is
expressed as a percentage and is typically used in a BAS controller to control
the state of an output. When the difference between the setpoint and room
temperature is large, the error is large. As the system reduces the difference
between the setpoint and space temperature, the error becomes smaller. In a
simple example, if the output of the PID loop that is controlling a modulating
valve is 50% the valve would be opened half way; if the output of the loop is
100% the valve position is fully open.
The AppStat uses up to four PID loops.
The heating and cooling PID loops are implemented in all models.
The discharge air loop controls the position of the economizer damper.
For this loop the setpoint is the measured temperature of the air that is
discharged by a roof top or heat pump unit. See
Economizer cooling for
roof top and heat pump units on page 75 .
The reheat loop is used only for heating outputs unit during
dehumidification. See the sequence description
Heat pump unit specific
functions on page 75
.
The PID loops in the AppStat are standard BACnet objects.
Valve operation for
fan coil units
Hot and chilled water valves are used for heating and cooling in the fan coil
unit versions of the AppStat. The fan coil two-pipe programming includes also
a method to determine supply water temperature.
On/Off valves
On a call for cooling or heating, the valve will open once the PID loop
controlling the valve has reached 50%. The valve will close once the loop has
Section 5: Sequences of operation KMC Controls, Inc.
70 Revision G