Leaving Chilled Liquid Control Override To Reduce Cycling; Leaving Chilled Liquid System Lead/Lag And Compressor Sequencing; Return Chilled Liquid Control; Figure 41 - Setpoint Adjust - York YCAL0019 Installation Operation & Maintenance

SECTION 8 – UNIT OPERATION
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<SP. Hot gas is turned off as tem-
perature rises when LWT>SP + CR/2. If temperature
remains below the setpoint low limit on the lowest step
of capacity, the micro will close the liquid line sole-
noid, after turning off hot gas, and pump the system
down before turning off the last compressor in a sys-
tem.
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 cooling
range can be from +/-1.5 °F to +/-2.5 °F (+/-.83 °C to
1.39 °C).leaving chilled liquid control
LEAVING CHILLED LIQUID CONTROL
OVERRIDE TO REDUCE CYCLING
To avoid compressor cycling the micro 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 minutes, the
microprocessor will increase the setpoint high limit ac-
cording to the chart at right, with a maximum value
allowed of 50 °F (See Figure 41 on page 158).
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 greater than 5 minutes, the set-
point adjust will be set back to 0. This will occur while
the system is still running.
6
5
4
3
2
1
0
0 1 2
LAST RUN TIME OF LEAD SYSTEM (MINUTES)

FIGURE 41 - SETPOINT ADJUST

158
3 4 5 6
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 micro 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
system chillers, the micro will attempt to equally load
each system as long as the system is not limiting or
pumping down. Once this occurs, loading and unload-
ing will alternate between systems, loading the lead
system first or unloading the lag system first.

RETURN CHILLED LIQUID CONTROL

(Can be used on Dual System 4, 5 & 6 Comp Units
Only)
Return chilled liquid control is based on staging the
compressors to match the cooling load. The chiller
will be fully loaded when the return water temperature
is equal to the Cooling Setpoint plus the Range. The
chiller will be totally unloaded (all compressors off)
when the return water temperature is equal to the Cool-
ing Setpoint (See sample in Table 19 on page 159).
At return water temperatures between the Cooling
Setpoint and Cooling Setpoint plus Range, compres-
sor loading and unloading will be determined by the
formulas in Table 20 on page 160.
FORM 150.67-NM2
ISSUE DATE: 01/31/2019
JOHNSON CONTROLS