Application

Sequence of Operation
BTU Calculations
33
Siemens Industry, Inc.
BACnet LCM-OAVS, Application 6760
140-1326
2015-07-07
As the demand for heating decreases (TEMP LOOPOUT drops), DISCH STPT
eventually reaches DISCH MIN. If TEMP LOOPOUT drops further, then the value of
TEMP CTL VOL begins to rise from the currently active supply airflow minimum to the
currently active supply airflow maximum to provide more cool air to the space. If this
value is compatible with correct room pressurization, then it is used as the supply flow
setpoint, SUP FLOW STPT. If not, the actual setpoint may be higher or lower than
TEMP CTL VOL.
Temperature Control Sequence.
BTU Calculations – Constant Volume Mode
During the Constant Volume Mode, the BTU Compensator operates as follows:
During Constant Volume operation, the controller adjusts the supply air temperature
set point as necessary to maintain CTL TEMP at CTL STPT. The room temperature
PID loop calculates the value of TEMP LOOPOUT. This value is used to adjust the
value of the supply air temperature set point. The loop is tuned by adjusting the values
of the feedback gain points, ROOM P GAIN and ROOM I GAIN and the sample interval
point, LOOP TIME.
The output of the room temperature loop, TEMP LOOPOUT, reflects the load
requirements. The value of TEMP LOOPOUT is a supply air temperature expressed as
“degrees above or below the room temperature set point if the supply flow is at 100%”.
If the supply flow is less than 100% of the currently active air supply airflow maximum,
DISCH STPT is adjusted to an amount greater than TEMP LOOPOUT by a
corresponding percentage.