JOHNSON CONTROLS
130
FORM 150.27-NM1
ISSUE DATE: 09/30/2015
SECTION 8 – UNIT OPERATION
If the chilled liquid temperature falls to a value greater
than 0.5 °F (.28 °C) below the Setpoint Low Limit but
not greater than 1.5 °F (.83 °C) below the Setpoint Low
Limit, unloading occurs at a rate of 20 seconds. If the
chilled liquid temperature falls to a value greater than
1.5 °F (.83 °C) below the Setpoint Low Limit, unload-
ing occurs at a rate of 10 seconds. If the chilled liquid
temperature falls below 1 °F above the low chilled liq-
uid temperature cutout, unloading occurs at a rate of 10
seconds if it is greater than 10 seconds.
In water cooling mode on R-410A chillers, the mini-
mum low limit of the control range will be 40.0ºF. For
leaving chilled liquid temperature setpoint and control
range combinations that result in the low limit of the
control range being below 40.0ºF, the low limit will be
reset to 40.0ºF and the difference will be added to the
high limit. This will result in a control range the same
size as programmed but not allow the unit to run below
40.0ºF. This control will not affect glycol chillers.
Hot gas, if present, will be the final step of capacity.
Hot gas is energized when only a single compressor is
running and LWT is less than SP. Hot gas is turned off
as temperature rises when LWT is more than SP plus
CR/2. If temperature remains below the setpoint low
limit on the lowest step of capacity, the microprocessor
will close the liquid line solenoid, after turning off hot
gas, and pump the system down before turning off the
last compressor in a system.
The Leaving Chilled Liquid Setpoint is programmable
from 40 °F to 70 °F (4.4 °C to 21.1 °C) in water chill-
ing mode and from 10 °F to 70 °F (-12.2 °C to 21.1
°C) in glycol chilling mode. In both modes, the cool-
ing range can be from plus or minus 1.5 °F to plus or
minus 2.5 °F (plus or minus .83 °C to 1.39 °C) Leaving
Chilled Liquid Control.
LEAVING CHILLED LIQUID CONTROL
OVERRIDE TO REDUCE CYCLING
To avoid compressor cycling the microprocessor will
adjust the setpoint upward temporarily. The last run
time of the system will be saved. If the last run time was
greater than 5 minutes, no action is to be taken. If the
last run time for the lead system was less than 5 min-
utes, the microprocessor will increase the setpoint high
limit according to the chart below, with a maximum
value allowed of 50 °F (See
If adding the setpoint adjust value to the setpoint high
limit causes the setpoint high limit to be greater than 50
°F, the setpoint high limit will be set to 50 °F, and the
difference will be added to the setpoint low limit.
Once a system runs for more than 5 minutes, the set-
point adjust will be set back to 0. This will occur while
the system is still running.
0
1
2
3
4
5
6
LAST RUN TIME OF LEAD SYSTEM (MINUTES)
SETPOINT
ADJUST
(DEG. F)
6
5
4
3
2
1
0
FIGURE 36 -
SETPOINT ADJUST
LD11415
LEAVING CHILLED LIQUID SYSTEM LEAD/
LAG AND COMPRESSOR SEQUENCING
A Lead/Lag option may be selected to help equalize
average run hours between systems with 2 refriger-
ant systems. This may be programmed under the OP-
TIONS key. Auto Lead/Lag allows automatic Lead/
Lag of the two systems based on average run hours of
the compressors in each system. Manual Lead/Lag se-
lects specifically the sequence which the microproces-
sor starts systems.
On a hot water start, once a system starts, it will turn
on all compressors before the next system starts a com-
pressor. The microprocessor will sequence compres-
sors within each circuit to maximize individual com-
pressor run time on individual compressors within a
system to prevent short cycling.
Each compressor in a system will be assigned an arbi-
trary priority number 1, 2, or 1, 2, 3. The non-running
compressor within a system with the lowest priority
number will always be the next compressor to start.
The running compressor with priority number 1 will
always be the next to shut off. Whenever a compres-
sor is shut off, the priority numbers of all compressors
will be decreased by 1 with wrap-around. This control
scheme assures the same compressor does not repeat-
edly cycle ON and OFF.
Once the second system starts a compressor on a 2 sys-
tem chillers, the microprocessor will attempt to equally
load each system as long as the system is not limit-
ing or pumping down. Once this occurs, loading and
unloading will alternate between systems, loading the
lead system first or unloading the lag system first.
