Application

Sequence of Operation
VAV versus CV Control
12
Siemens Industry, Inc.
BACnet LCM-OAVS, Application 6754
140-1325
2015-07-07
VAV versus CV Control
In Application 6754, VAV means that the supply airflow can be varied to provide
cooling. CV means the supply airflow is not a source of cooling. However, the supply
and general exhaust can still change in CV mode to keep the volume differential
setpoint constant. This may be necessary if HOOD VOL is varying. Application 6754
can do either Variable Air Volume control (VAV) or Constant Air Volume Control (CV).
It can also change the type of air volume control based on which mode it is operating in
(OCC or UNOCC).
 When OCC.UNOCC equals
OCC, Application 6754 will perform VAV control
provided that VOLUME STATE equals
1 or 3. It will perform constant volume (CV)
control provided that VOLUME STATE equals
0 or 2.
 When OCC.UNOCC equals
UNOCC, Application 6754 will perform VAV control
provided that VOLUME STATE equals
2 or 3. It will perform constant volume
control provided that VOLUME STATE equals
0 or 1.
Depending on how VOLUME STATE is configured, Application 6754 can operate as
either a Variable Air Volume (VAV) LCM or a constant volume (CV) LCM. Also, these
operational modes can vary between the occupied and unoccupied periods, if desired.
The following table shows what the application does when VOLUME STATE is at a
particular value.
VOLUME STATE Values.
VOLUME STATE
(value)
Description
0
Always Constant Volume.
1
(default)
VAV during occupancy, Constant Volume during unoccupied period.
2
Constant Volume during occupancy, VAV during unoccupied period.
3
Always VAV.
NOTE:
If VOLUME STATE is set greater than 3, it will default to 0.
Fume Hood Flow Input
A signal of 1 to 10 Vdc at AI 3 represents the volume of air that is exhausted through
the fume hood(s). By using a Fume Hood Flow Module (FFM), you can connect up to
six fume hoods to AI 3. Multiple fume hood flows must be averaged using a Fume
Hood Flow Module. The resulting airflow is displayed in point HOOD VOL.
MAX HOOD VOL is set during start-up to indicate the flow in cubic feet per minute
(cfm) that corresponds to an input signal of 10 Vdc. A signal of 1 Vdc indicates 0 cfm. If
the signal drops below 1 Vdc, TOTL EXHAUST will fail.