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PKG-SVX029A-EN
Operating Principles
Control Sequences of Operation
Occupied/Unoccupied Switching
There are three ways to switch occupied/unoccupied:
1. Field-supplied contact closure [hardwired binary input
to field wiring terminal block (2TB3)] - This input
accepts a field supplied switch or contacts closure such
as a time clock. Open Contacts = Unoccupied and
Closed Contacts = Occupied.
2. BACnet® MSTP control system can control the
occupied/unoccupied request of the unit.
3. Through the TD-7 display via the on-board schedule.
Compressor Lead/Lag
Compressor lead/lag is present in the controls for all units.
This is used to balance the run hours between all the
compressors by putting the highest run hours to run last.
Emergency Stop Input
A binary input is provided on the units field wiring terminal
block (2TB3) for installation of a field-provided switch or
contacts to immediately shutdown all unit functions. An
open contact will indicated an emergency shutdown
condition exist and shut down the unit and generate a
manual reset alarm. A closed set of contacts will indicate a
normal condition. After the contacts are closed, the manual
reset alarm must be cleared to resume unit operations.
External Auto/Stop Input
A binary input is provided on the unit’s field wiring terminal
block (2TB3) for installation of a field-provided switch or
contacts to safely shutdown all unit functions. An Open
contact will indicate a Stop condition exist and the unit will
shutdown and generate an automatic reset alarm. A
Closed contact will indicate an Auto condition and allow the
unit to operate.
Local Auto Stop
A binary value is located on the TD-7 which allows the user
to start and stop the unit in a safe manner for maintenance.
On the TD-7 home screen in the upper right hand corner
the unit can be put into Auto to perform normally or Stop to
perform a controlled shutdown and keep the unit off.
Head Pressure Control
Water-cooled condensers: Units are ordered with the
optional flow control valves and configured for variable
water flow will modulate a water valve to maintain a user-
defined condensing pressure setpoint and compressor
refrigerant pressure differential. Condensing pressure and
pressure differential will be referenced utilizing factory
installed suction and discharge pressure sensors located
on each circuit.
Water Purge
This user-definable feature allows the user to select a
purge schedule to automatically circulate water through the
economizer and condensers periodically during non-
operational times. This allows fresh chemicals to circulate
in waterside heat exchangers.
Note: Only available on units with condenser valves.
Supply Air Temperature Control
Unit Sequence of Operation
Discharge Air High Temp Limit Software
Alarm (Software)
When the discharge air temperature is above the discharge
air high temperature cutout setpoint of 100 degrees for 5
minutes, the unit will perform a controlled shutdown. A
Manual Reset alarm will generate and must be cleared
before the unit can resume operating.
Condenser Water Low Temperature
Alarm (Software)
When either the entering or leaving condenser water
temperature is below the condenser water low temperature
cutout setpoint of 35 degrees for 5 minutes, the unit will
perform a controlled shutdown. All condenser water valves
will be commanded open if installed, and a manual reset
diagnostic will be generated and need to be cleared before
the unit can resume operation.
Leaving Air Low Temperature Alarm
When the discharge air temperature is below the discharge
air temperature cutout for 5 minutes, the unit will perform a
controlled shutdown. A Manual Reset alarm will be
generated and need to be cleared before the unit can
resume operation.
Occupied Fan Operation
When all alarms are cleared, the unit is occupied, and the
local auto stop is in auto, all fans that are available will start
up and operate at 30 percent speed for 45 seconds. For
CV, fans will then release to control to the fan fixed speed
setpoint.
VAV
The fans will then release to control to the duct static
pressure setpoint. The fans will modulate to maintain the
duct static pressure at the duct static pressure setpoint.
The fans are commanded off during the unoccupied mode.
Heat/Cool Mode
The unit is in Cooling mode by default. The front end
Building Automation System will switch the Heat/Cool
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