Application

Sequence of Operation
BTU Calculations
22
Siemens Industry, Inc.
Application Note, Application 6759
140-1335
2015-07-07
BTU Calculations – VAV Mode
During VAV operation, the controller adjusts the supply airflow and the supply air
temperature setpoint as necessary to maintain CTL TEMP at CTL STPT. The room
temperature PID loop calculates the value of TEMP LOOPOUT. The Figure
Temperature Control Sequence [➙ 23] shows how this value is used to sequence the
cooling flow and the supply air temperature setpoint. The loop is tuned by adjusting the
values of the feedback gain points that are visible in the database (ROOM P GAIN;
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 setpoint 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.
Example
CTL STPT = 70°F: OCC.UNOCC = OCC
If
TEMP LOOPOUT =
and
SUP AIR VOL =
then
DISCH STPT =
Formula for DISCH STPT:
CTL STPT +
(TEMP LOOPOUT x
100% ÷ SUP AIR VOL)
10°F
OCC SUP MAX
80°F
70° + (10° x 100% ÷ 100%)
10°F
0.5 ´ OCC SUP MAX
90°F
70° + (10° x 100% ÷ 50%)
–5°F
0.25 ´ OCC SUP MAX
50°F
70° + (–5° x 100% ÷ 25%)
0°F
any flow
70°F
70° + (0° x 100% ÷ x%)
CTL STPT = 70°F: OCC.UNOCC = UNOCC
If
TEMP LOOPOUT =
and
SUP AIR VOL =
then
DISCH STPT =
Formula for DISCH STPT:
CTL STPT +
(TEMP LOOPOUT x
100% ÷ SUP AIR VOL)
10°F
UOC SUP MAX
80°F
70° + (10° x 100% ÷ 100%)
10°F
0.5 ´ UOC SUP MAX
90°F
70° + (10° x 100% ÷ 50%)
–5°F
0.25 ´ UOC SUP MAX
50°F
70° + (–5° x 100% ÷ 25%)
0°F
any flow
70°F
70° + (0° x 100% ÷ x%)
While the actual number of BTUs is not explicitly calculated, DISCH STPT varies as
the supply flow varies in order to maintain a constant quantity of heat entering the
room.
This module also limits TEMP LOOPOUT to values that generate supply air
temperature setpoints that are less than DISCH MAX.
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.