Installation, Operation and Maintenance Manual : 036-1011(0806)
Page 19
If the compressors have satisfied their minimum run
times, the compressors and condenser fans are
de-energized. Otherwise, the unit operates each cooling
system until the minimum run times for the compressors
have been completed. Upon the final compressor
de-energizing, the blower is stopped following the elapse of
the fan off delay for cooling.
To be available, a compressor must not be locked out due to a
high or low pressure switch or freezestat trip and the anti short
cycle delay (ASCD) must have elapsed.
COOLING OPERATION ERRORS
Each cooling system is monitored for operation outside of the
intended parameters. Errors are handled as described
below. All system errors override minimum run times for
compressors.
HIGH PRESSURE LIMIT SWITCH
During cooling operation, if a high pressure limit switch
opens, the UCB will de-energize the associated compressor,
initiate the ASCD (anti short cycle delay), and, if the other
compressors are 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 compressors.
Should a high pressure switch open three times within two
hours of operation, the UCB will lock out the associated
compressors and flash a code (see Table 11). If the other
compressors are inactive, the condenser fans will be
de-energized.
LOW PRESSURE LIMIT SWITCH
The low pressure limit switch is not monitored during the
initial 30 seconds of a cooling system’s operation. For the
following 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 compressors are idle, stop the condenser
fans.
Once the low pressure switch has been proven (closed
during 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 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 low pressure
switch open three times within one hour of operation, the
UCB will lock out the associated compressor and flash a
code (Table 11). 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 reenergize 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 11). If the other compressor is
inactive, the condenser fans will be de-energized.
LOW AMBIENT COOLING (OPTIONAL)
To determine when to operate in low ambient mode, the UCB
has a pair of terminals connected to a temperature activated
switch set at 50°F. 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 operation. Compressor
minimum run time may extend the minutes of compressor
operation.
The defrost cycle will begin immediately following the elapse
of the minimum run time. When operating in low ambient
mode, the UCB will not lockout 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
evaporator temperatures due to a low air flow or a low
Installation, return air temperature, (opens at 26 ± 5°F
and resets at 38 ± 5°F).
2. A high pressure switch to protect against excessive
discharge pressures due to a blocked condenser coil or
a condenser motor failure, (opens at 415 ± 15 psig and
resets at 280 ± 15 psig).
3.
A low pressure switch to protect against loss of
refrigerant charge, (opens at 35 ± 5 psig and resets at 60
± 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.
COMPRESSOR PROTECTION
The compressors also have inherent (internal) protection. If
there is an abnormal temperature rise in a compressor, the
