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Section 7 - Operation
Unit switch
Figure 116 - Unit switch
A double pole single throw ON/OFF rocker switch on
the front of the control panel is used to turn the entire
chiller on and off. When the switch is placed in the
OFF position, the entire unit shuts down immediately.
One pole of the UNIT switch contacts is wired to the
Sys 1/3 and the other to Sys 2/4 VSD Run Signal input
and the Chiller Control Board "UNIT switch X" digital
input (X equals System 1 or 2). Separate System Fuses
are also wired in series with each set of UNIT switch
contacts. If either fuse is pulled or blown, only the sys-
tem with the good fuse (Input is high) will run. When
both inputs are high, the entire chiller will be enabled
to run. When both inputs are low, the chiller will be
disabled as a UNIT switch OFF Shutdown.
The UNIT switch should never be used to
shut down the chiller except in an emer-
gency. When the switch is thrown, the
compressors will immediately shut down.
Since the compressors are not permitted
to come to a controlled stop, the rotors
may back-spin, which may result in some
unusual compressor noise. The back-spin
will not hurt the compressors, but should
be avoided.
It is suggested that the System Switches
on the keypad be used whenever possible
to turn a system off and allow the com-
pressor to complete a controlled shut-
down.
202
LD10605
UNIT
SWITCH
Basic operating sequence
Start sequence and loading
To initiate the start sequence of the chiller, the follow-
ing conditions must be satisfied before the precharge of
the DC Bus will take place:
• SWITCH must be ON.
• At least one System Switch is ON
• Run permissive inputs (Remote Cycling Contacts)
must be closed.
• No unit faults exist.
• No unit start inhibits exist.
• At least one system not faulted or inhibited.
• The Daily Schedule is calling for the chiller to
run.
• The Flow Switch is closed.
• Leaving Chilled Liquid Setpoint is above the
Setpoint plus CR (Setpoint High Limit).
Once the precharge takes place, if the anti-recycle tim-
er is timed out the chiller control system on the Chiller
Control Board will select the number of compressors
to start and begin operation of the compressors. The
compressor(s) speed will be ramped to the minimum
start frequency and increase speed as needed in an ef-
fort to regulate the leaving chilled liquid temperature to
meet the desired Setpoint.
When a compressor starts, the Feed and Drain Valves
on the system will immediately begin to control super-
heat and the liquid level in the flash tank and the Chill-
er Control Board microprocessor will begin to regulate
the speed on the VSD to bring the chilled liquid tem-
perature to within the Control Range (CR). The micro-
processor will regulate the speed of the compressor(s)
primarily based on temperature offset as the loading
timer permits.
Form 201.23-NM2
Issue date: 12/19/2023
Johnson Controls
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