JOHNSON CONTROLS
175
SECTION 8 – UNIT OPERATION
FORM 150.67-NM2
ISSUE DATE: 01/31/2019
8
LOAD LIMITING
Load Limiting is a feature that prevents the unit from
loading beyond the desired value. 2 and 4 compres-
sor units can be load limited to 50%. This would allow
only 1 compressor per system to run. 3 and 6 compres-
sor units can be load limited to 33% or 66%. The 66%
limit would allow up to 2 compressors per system to
run, and the 33% limit would allow only 1 compressor
per system to run. Five-compressor units may be load
limited to 40% (1 compressor per system runs) or 80%
(up to 2 compressors per system) are permitted to run.
No other values of limiting are available.
There are two ways to load limit the unit. The first is
through remote communication via an ISN. Load limit
stages are sent through YORK Talk on pages 9 and 10
of feature 54. Page 9 is stage 1 load limit and page 10
is stage 2 load limit.
A second method of load limiting the unit is through
closing dry contacts connected to the Load Limit
(CTB1 – Terminals 13 -21). Load limiting involves
closing the Load Limit input with a dry contact. Load
limiting is either 80%, 66% or 50%, depending on the
number of compressors on the unit. Table 29 on page
175 shows the load limiting permitted for the various
number of compressors. Only Stage 1 is available uti-
lizing a dry contact.
TABLE 29 - COMPRESSOR OPERATION – LOAD
LIMITING
COMPRESSORS
IN UNIT
STAGE 1 STAGE 2
2 50% -
3 66% 33%
4 50% -
5 80% 40%
6 66% 33%
Simultaneous operation of Remote Load
Limiting and EMS-PWM Temperature
Reset (described on following pages)
cannot occur.
COMPRESSOR RUN STATUS
Compressor run status is indicated by closure of con-
tacts at CTB2 – terminals 25 to 26 for system 1 and
CTB2 – terminals 27 to 28 for system 2.
ALARM STATUS
System or unit shutdown is indicated by normally-open
alarm contacts opening whenever the unit shuts down
on a unit fault, locks out on a system fault, or experi-
ences a loss of power to the chiller electronics . System
1 alarm contacts are located at CTB2 – terminals 29
to 30. System 2 alarm contacts are located at CTB2 –
terminals 31 to 32. The alarm contacts will close when
conditions allow the unit to operate, or the fault is reset
during a loss of power, the contacts will remain open
until power is reapplied and no fault conditions exist.
BAS/EMS TEMPERATURE RESET USING A
VOLTAGE OR CURRENT SIGNAL
The Remote Reset Option allows the Control Center
of the unit to reset the chilled liquid setpoint using a
0 - 10VDC input, or a 4-20mA input connected to
CTB1 terminals A- and A+. Whenever a reset is called
for, the change may be noted by pressing the Cooling
Setpoints key twice. The new value will be displayed
as “REM SETP = XXX °F.”
If a 0 - 10VDC signal is supplied, it is applied to termi-
nals A+ and A-, and jumper JP1 on the I/O board must
be inserted between pins 2 and 3. To calculate the reset
chilled liquid setpoint for values between 0VDC and
10VDC use the following formula:
Setpoint = Local Chilled Liquid Setpoint + °Reset
°Reset = (DC voltage signal) x (*Max Reset Value)
10
Example:
Local Chilled Liquid Setpoint = 45 °F (7.22 °C)
*Max Reset Value = 20 °F (11.11 °C)
Input Signal = 6VDC
(English)
°Reset = 6VDC x 20 °F = 12 °F Reset
10
New Setpoint = 45 °F + 12 °F = 57 °F
(Metric)
°Reset = 6VDC x 11. 11 °C = 6.67 °C Reset
10
New Setpoint = 7.22 °C + 6.67 °C = 13.89 °C
* Max Reset Value is the “Max EMS-PWM Remote Temp. Reset”
setpoint value described in the program ming section under Cooling
Setpoints. Programmable values are from 2 °F to 40 °F (1.11 °C to
11.11 °C).
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