JOHNSON CONTROLS
133
SECTION 8 – UNIT OPERATION
FORM 150.27-NM1
ISSUE DATE: 09/30/2015
8
ANTI-COINCIDENCE TIMER
This timer is not present on single-system units. Two
timing controls are present in software to assure com-
pressors within a circuit or between systems, do not
start simultaneously. The anti-coincidence timer assures
there is at least a one minute delay between system starts
on 2-circuit systems. This timer is NOT programmable.
The load timers further assure that there is a minimum
time between compressor starts within a system.
EVAPORATOR PUMP CONTROL AND YORK
HYDRO KIT PUMP CONTROL
The evaporator pump dry contacts (CTB2 – terminals
23 and 24) are energized when any of the following
conditions are true:
1. Low Leaving Chilled Liquid Fault.
2. Any compressor is running.
3. Daily Schedule is ON, Unit Switch is ON and Re-
mote Stop is closed.
The pump will not run if the micro panel has been pow-
ered up for less than 30 seconds or if the pump has run
in the last 30 seconds to prevent pump motor overheat-
ing.
Whenever the option “YORK HYDRO KIT PUMPS
= 1” is selected under the OPTIONS key, the pump
control will be as described above. DO NOT SELECT
the option “YORK HYDRO KIT PUMPS = 2” under
the OPTIONS key.
EVAPORATOR HEATER CONTROL
The evaporator heater is controlled by ambient air tem-
perature. When the ambient temperature drops below
40 °F (4.4 °C) the heater is turned ON. When the tem-
perature rises above 45 °F (7.2 °C) the heater is turned
off. An under voltage condition will keep the heater off
until full voltage is restored to the system.
PUMPDOWN CONTROL
Each system has a pump-down feature upon shut-off.
Manual pumpdown from the keypad is not possible. On
a non-Safety, non-Unit Switch shutdown, all compres-
sors but one in the system will be shut off. The LLSV
will also be turned off. The final compressor will be
allowed to run until the suction pressure falls below
the cutout, or for 180 seconds, whichever comes first.
LOAD LIMITING
Load Limiting is a feature that prevents the unit from
loading beyond the desired value. Four-compressor
units can be load limited to 50%. This would allow
only 1 compressor per system to run. Six-compressor
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 compres-
sor per system 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 and 21). Stage 1 load limiting
involves closing the Load Limit input (13 and 21) with
a dry contact. Load limiting is either 66% or 50%, de-
pending on the number of compressors on the unit. A
second step of load limiting on six-compressor chillers
is available by closing the CTB1 terminals 13 and 20
with dry contact. This allows only a single compressor
to run on each system, unloading the chiller to 33%.
shows the load limiting permit-
ted for the various number of compressors. Only Stage
1 is available utilizing a dry contact.
TABLE 19 -
COMPRESSOR OPERATION – LOAD
LIMITING
COMPRESSORS
IN UNIT
STAGE 1
STAGE 2
4
50%
-
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.
