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NXW REVERSIBLE CHILLER INSTALLATION MANUAL
Control Features cont.
when the leaving water temperature is 1.5°F above the
freeze setpoint.
Dual Circuited Heat Exchanger Protection
A low temperature condition on either refrigerant circuit
will prevent the start of both compressors. If the low
temperature condition exists for 5 minutes when both
compressors are off, a lockout is triggered for both
compressors. However, if –for instance-both compressors
are operating and circuit 1 experiences a refrigerant
temperature below the freeze detection setpoint such
that compressor 1 is halted, compressor 2 will not be
halted as a result.
Advanced Freeze Detection Operation: “Pre Freeze”
Alarm
Predictive freeze condition detection:
If the refrigerant temperature is within 7.2°F of the freeze
detection setpoint, the predictive freeze detection
algorithm is enabled, and if the logic determines that
a freeze condition is likely to happen based on current
conditions, the compressor of the involved refrigerant
circuit is immediately stopped. Three (3) such events
in 60 minutes will trigger a compressor lockout that
requires a manual reset. In the absence of such a
condition, the compressor is allowed to operate so that
the refrigerant temperature may eventually be at the
threshold of the freeze detection setpoint.
NOTE: This
Lockout produces a “Pre Freeze” detection error on
the MUI display.
Capacity Limiting
If the leaving water temperature drops to 1.8°F above the
freeze detection setpoint, the lead compressor is halted.
When the leaving water temperature rises to 3.6°F above
the freeze detection setpoint, it will be allowed to resume
operation. This limiting is allowed to repeat indefinitely with
no lockout or indication on the display.
If the leaving water temperature drops to the freeze
detection setpoint, the lag compressor is halted. When the
leaving water temperature rises to 1.8°F above the freeze
detection setpoint, it will be allowed to resume operation.
This limiting is allowed to repeat indefinitely with no lockout
or indication on the display.
Compressor Current Switch (AI-3 EXP and AI-4 EXP)
The compressor current switch is designed to insure that
the compressor is on when the compressor output is
energized. This switch is normally open and closes when
current is flowing to the compressor. If the compressor fails
to start the switch will open. The switch must be open for
a continuous 5 seconds for a fault to occur. After 3 faults in
60 minutes the control will put the unit into an alarm state.
Flow Proving Switch (DI-1 and DI-10)
The load and source flow-proving switches are a field
installed option. These switches shall be normally open
flow switches that will close when the water flow through
the heat exchangers reach an acceptable level. The flow-
proving switches must be closed 15 seconds prior to
enabling either compressor output (DO-2 and DO-4). If
the load flow-proving switch opens at any time both
compressor outputs (DO-2 and DO-4) must be disabled
immediately.
High Pressure (DI-11 and DI-12)
The high-pressure switches shall be a normally closed (NC)
switch that monitors the systems compressor discharge
refrigerant pressures. There shall be an individual high
pressure switch for each circuit. If the input senses the
high-pressure switch is open during the period that
the compressor output is enabled, it must shut down
the compressor immediately and count the fault. The
compressor minimum on time does not apply if the high-
pressure switch trips. The compressor will not restart until
the short cycle time delay has been satisfied. If the high-
pressure fault occurs in one circuit the other compressor
will continue to operate based on the heating or cooling
demand.
Low Pressure (DI-3 and DI-6)
The low-pressure switches shall be a normally closed (NC)
switch that monitors the systems compressor suction line
refrigerant pressure. The input shall be checked 15 seconds
before compressor start up to insure the pressure switch
is closed and then ignored for the first 2 minutes after the
compressor output (DO-1 or DO-2) is enabled. If the switch
is open continuously for (30) seconds the compressor
output for that circuit will be disabled. The compressor
will not restart until the short cycle time delay has been
satisfied. If a low-pressure fault occurs in one circuit the
other compressor will continue to operate based on the
heating or cooling demand.
Compressor 1 Alarm Output (DO-5)
The compressor 1 alarm output will be enabled when stage
1 is in the lockout mode and will be disabled when the
lockout is reset.
Compressor 2 Alarm Output (DO-6)
The compressor 2 alarm output will be enabled when stage
2 is in the lockout mode and will be disabled when the
lockout is reset.
Test Mode
The unit controls system can be put into test mode to
eliminate startup delays to aid in trouble shooting. To put
the unit into test mode hold the “ESC” and “Down Arrow”
keys until LED 8 begins to flash. The control will remain in
test mode until power is cycled or after 30 minutes.
