18
OPERATION
Occupancy Determination —
The Omnizone™
controller allows several methods of occupancy determination,
based upon the setting of the occupancy configuration. Local
occupancy is determined by either a local schedule (Local
Sched) contained in the controller, the use of the ROCC dis-
crete input point (Remote DI) or by setting the unit configura-
tion to always occupied (Always Occ).
When the Omnizone unit is connected to a network, occu-
pancy can be controlled the occupancy configuration. To oper-
ate to a BACnet schedule, set the configuration to "BACnet
Sched." To use a remote On/Off signal from a building automa-
tion system, set the configuration to "BAS On/Off."
Fan Control —
All variable air volume (VAV) units have a
variable frequency drive (VFD) to provide variable fan motor
speed and thus variable airflow. Fan control turns the fan on
and off based on unit operating mode, and controls fan speed to
maintain a particular duct static pressure at a duct static pres-
sure (DSP) sensor. The objective is to maintain a reasonably
constant supply-air exit velocity at VAV system outlet grilles,
regardless of damper opening positions. The duct static pres-
sure sensor is field-installed about 2/3 of the way toward the
"far end" of the ductwork. A duct high static (DHS) limit
switch provides protection by shutting the fan down if the duct
static pressure exceeds a maximum setting.
For the 50XJ unit, a VFD interface display is mounted in
the front of the unit. A number of user-adjustable features can
be entered/changed using the keypad on the display. These fea-
tures are described in detail in the VFD Control and Display
section.
Sequence of Operation —
The following control
sequence of operation for the 50XJ unit describes the various
sequences that occur depending upon the way an operation is
triggered and which software control points are involved.
SUPPLY FAN — The supply fan is enabled whenever the unit
enters occupied mode. The fan is also enabled in the unoccu-
pied mode with a cooling or heating demand. Fan operation
will be prevented if there is an active duct high static pressure
alarm, the optional phase rotation monitor indicates a problem,
the duct static pressure sensor has failed or has an alarm, or
there is a fire/smoke shutdown alarm.
When there are no alarms preventing fan operation and the
fan is enabled, the (remote terminal control) TRMCT should
turn on. Thirty seconds later the output to the VFD will in-
crease. The VFD output level is determined by a PID calcula-
tion, based on the duct static pressure (DSP) input and the sup-
ply static pressure set point. Once the supply fan is running and
the static pressure increases above the supply fan status set
point the supply fan status point (SFS) will indicate ON.
The space temperature sensor point indicates the current
value of the sensor used to determine cooling/heating demand.
The sensor selection is based on the sensors installed and the
configuration of these sensors in the custom configuration. The
following sensors, when valid, are used for the space tempera-
ture sensor with the highest precedence at the top of the list:
• Space temperature sensor (SPT)
• Return air temperature sensor (MA_RA) - only used if
the unit is NOT configured for reset by EWT.
• Entering water temperature (EWT) - only if the unit is
configured for reset by EWT.
• Default with no sensors is the occupied cooling set point
of 10 F (prevents unoccupied operation).
If the space has unoccupied demand, the supply fan is en-
abled. If conditions allow, the fan will start as described above.
If the fan is running due to unoccupied heating or cooling de-
mand, the controlling temperature input must rise or drop to
within half way between the occupied and unoccupied set
points in order for the fan to turn back off.
Once the unoccupied conditions are satisfied, there is a
fixed 5-minute delay for normal fan shutdown. During the 5-
minute delay, the cooling and heating routines become dis-
abled. This delay allows a compressor that may have just start-
ed to run for its 5-minute minimum on-time with the supply fan
on. For example, if the staging routine had just started com-
pressor no. 3 at the time the call was satisfied, the cooling rou-
tine would become disabled and compressor no. 1 and 2 would
shut off right away. Compressor no. 3 would continue to run
for its minimum on time of 5 minutes. The fan continues run-
ning until all compressors meet the minimum on time and run
with a load, preventing them from shutting down due to a
safety.
Supply Fan operation with Optional Bypass — If the option-
al VFD bypass is installed and the bypass switch has been
turned to bypass (line), and the VFD compartment access panel
is in place, the software point VFD Access Panel Secure has
been set to YES, there are no faults to prevent fan operation
and there is a call for fan operation, then the bypass start stop
point BPS_S will be engaged to start the fan. The terminal
open point TRMOP will go on with the TRMCT point 30 sec-
onds before the fan starts.
COMPRESSOR COOLING — If the fan is on and there is no
demand for heat, internal software point (COOLOK) will
switch to on. The unit MODE will not change to "Cooling" unit
there is an active call for compressor operation.
If the unit is configured for variable flow the reverse/head
pressure control valve will open (otherwise it will already be
open), and if there is condenser water flow (or no flow switch
installed), then mechanical cooling will be enabled.
Units are equipped with 2, 3, or 4 compressors piped in sep-
arate refrigerant circuits, and staged on/off in a fixed sequential
manner (compressor no. 1 through compressor no. 4). The
compressor control routine uses a PID calculation to determine
the percentage of cooling required, from 1 to 100%. Demand
for the PID calculation is determined from the supply air tem-
perature and the supply air set point.
Compressor cooling will be disabled for any of the follow-
ing reasons:
• There is no water flow (CDWF is off, with switch
installed).
• Economizer freezestat (FREEZ) has been in alarm for
more than 15 minutes.
Fig. 10 — Condenser Air Vent
a50-7124tf
