66389-BIM-C-1208
34
Johnson Controls Unitary Products
LOW-PRESSURE LIMIT SWITCH
The low-pressure limit switch is not monitored during the ini-
tial 30 seconds of a cooling system's operation. For the fol-
lowing 30 seconds, the UCB will monitor the low-pressure
switch to ensure it closes. If the low-pressure switch fails to
close after the 30-second monitoring phase, the UCB will de-
energize the associated compressor, initiate the ASCD, and,
if the other compressor is idle, stop the condenser fans.
Once the low-pressure switch has been proven (closed dur-
ing the 30-second monitor period described above), the UCB
will monitor the low-pressure limit switch for any openings. If
the low-pressure switch opens for greater than 5 seconds,
the UCB will de-energize the associated compressor, initiate
the ASCD, and, if the other compressor is idle, stop the con-
denser fans.
If the call for cooling is still present at the conclusion of the
ASCD, the UCB will re-energize the halted compressor.
Should a low-pressure switch open three times within one
hour of operation, the UCB will lock-out the associated com-
pressor and flash a code (Table 29). If the other compressor
is inactive, the condenser fans will be de-energized.
FREEZESTAT
During cooling operation, if a freezestat opens, the UCB will
de-energize the associated compressor, initiate the ASCD,
and, if the other compressor is idle, stop the condenser fans.
If the call for cooling is still present at the conclusion of the
ASCD, the UCB will re-energize the halted compressor.
Should a freezestat open three times within two hours of
operation, the UCB will lock-out the associated compressor
and flash a code (Table 29). If the other compressor is inac-
tive, the condenser fans will be de-energized.
LOW AMBIENT COOLING
To determine when to operate in low ambient mode, the UCB
has a pair of terminals connected to a temperature-activated
switch set at 45ºF. When the low ambient switch is closed
and the thermostat is calling for cooling, the UCB will operate
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 opera-
tion. Compressor minimum run time may extend the minutes
of compressor operation. The defrost cycle will begin immedi-
ately following the elapse of the minimum run time.
When operating in low ambient mode, the UCB will not lock-
out the compressors due to a freezestat trip. However, a
freezestat trip will de-energize the associated compressor. If
the call for cooling is still present at the end of the ASCD and
the freezestat has closed, the unit will resume operation.
SAFETY CONTROLS
The unit control board monitors the following inputs for each
cooling system:
1.
A suction line freezestat to protect against low evapora-
tor temperatures due to a low airflow or a low return air
temperature, (opens at 26 ± 5 °F and resets at 38 ± 5°F).
2.
A high-pressure switch to protect against excessive dis-
charge pressures due to a blocked condenser coil or a
condenser motor failure, (opens at 380 ± 10 psig and
resets at 300 ± 10 psig).
3.
A low-pressure switch to protect against loss of refrigerant
charge, (opens at 22 ± 5 psig and resets at 45 ± 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 will be
affected by any safety/preventive action. The other refrigerant
system will continue in operation unless it is affected by the
fault as well.
The unit control board monitors the temperature limit switch
of electric heat units and the temperature limit switch and the
gas valve of gas furnace units.
COMPRESSOR PROTECTION
The compressors also have inherent (internal) protection. If
there is an abnormal temperature rise in a compressor, the
protector will open to shut down the compressor. The UCB
incorporates features to minimize compressor wear and dam-
age. An anti-short cycle delay (ASCD) is utilized to prevent
operation of a compressor too soon after its previous run.
Additionally, a minimum run time is imposed any time a com-
pressor is energized.
The ASCD is initiated on unit start-up and on any compressor
reset or lock-out.
FLASH CODES
The UCB will initiate a flash code associated with errors
within the system. Refer to UNIT CONTROL BOARD FLASH
CODES Table 29.
RESET
Remove the call for cooling, by raising thermostat setting
higher than the conditioned space temperature. This resets
any pressure or freezestat flash codes.
ELECTRIC HEATING SEQUENCE OF OPERATIONS
The following sequence describes the operation of the elec-
tric heat section.
Two-stage heating:
a.
Upon a call for first stage heat by the thermostat, the
heater relay (RA) will be energized. After completing
the specified fan on delay for heating, the UCB will
