Johnson Controls ZL Series Installation Manual page 63

If the UCB detects the evaporator low limit sensor
(EC1) falling below 26°F (default) three times within
two hours of operation, the UCB locks out the
compressor.
Low ambient cooling
To determine when to operate in low ambient
mode, the UCB has an outdoor air temperature
sensor (OAT) with a low ambient setpoint at
45ºF (default). When the OAT sensor senses a
temperature below the low ambient setpoint
and the thermostat is calling for cooling, the UCB
operates in the low ambient mode.
Low ambient mode operates the compressors in
this manner: 10 minutes on, 5 minutes off. The
indoor blower is operated throughout the cycle.
The 5-minute off period is necessary to defrost the
indoor coil.
Low ambient mode always begins with compressor
operation. Compressor minimum run time may
extend the minutes of compressor operation. The
off cycle begins immediately following the elapse of
the minimum run time.
When operating in low ambient mode, an
evaporator low limit sensor (EC1) temperature
below 26ºF de-energizes the compressor. If the
call for cooling is still present at the end of the
ASCD and the evaporator temperature sensor
(EC1) temperature is above 26°F, the unit resumes
operation.
Safety controls
The unit control board monitors the following inputs
for each cooling system:
1. An evaporator low limit sensor (EC1) to
protect against low evaporator temperatures
due to a low airflow or a low return air
temperature, set at 26°F. The evaporator low
limit sensor is located on the suction line at
the evaporator coil.
2. A high-pressure switch to protect against
excessive discharge pressures due to a
blocked condenser coil or a condenser motor
failure. The switch opens at 625 ± 25 psig.
3. A low-pressure switch to protect against loss
of refrigerant charge. the switch opens at
opens at 50 ± 5 psig.
The above pressure switches are hard-soldered
to the unit. The refrigeration systems are
independently monitored and controlled. On any
fault, only the associated system is affected by any
safety or preventive action.
The UCB monitors the temperature limit switch of
electric heat units and the temperature limit switch
and the gas valve of gas furnace units.
Compressor protection
In addition to the external pressure switches,
the compressors also have inherent (internal)
protection. If there is an abnormal temperature rise
in a compressor, the protector opens to shut down
the compressor. The UCB incorporates features
to minimize compressor wear and damage. An
anti-short cycle delay (ASCD) is used to prevent
operation of a compressor too soon after its
previous run. Additionally, a minimum run time is
imposed any time a compressor is energized.
The ASCD is initiated on unit start-up and on any
compressor reset or lock-out.
Space humidity sensing
There are five zone relative humidity sensing
options listed by priority (authority).
Priority #1: BAS override. BAS network overrides
zone humidity
Priority #2: Network space humidity sensor,
communicating thermostat or sensor
Priority #3: Local, hardwired 0-10V sensor input
(return air or wall mount)
Priority #4: Local, hardwired dry contact input
Unreliable: No value provided.
Note: Refer to Technical Bulletin: Smart
Equipment™ Controls Sequence of Operation
Overview for details on all options
• BAS and network space humidity sensor:
The RTU's control system can accept a
network communicating thermostat, NS-8000
series, as the primary temperature, humidity,
temperature setpoint and more for the RTU.
The NS-8000 series does not provide local
humidity setpoint adjustment. The relative
humidity setpoint is adjustable using the
MAP tool. The NS-8000 sensor is a 4 wire
device using 22 gauge shielded wire. The
device connects to the SA Bus located on the
SSE board. Refer to the wiring diagram for
details. Network Space Sensor is priority #2.
A building automation system can directly
override and write to the input value, (NetSH)
which is priority #1.
Installation Manual ZL Series 7.5- 12.5 Ton 60 Hertz R-410A
Johnson Controls Ducted Systems
63