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177820-YIM-A-0506
Unitary Products Group 29
Should a high-pressure switch open three times within
two hours of operation, the UCB will lock-out the com-
pressor and flash a code (see Table 23).
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-pres-
sure switch fails to close after the 30-second monitor-
ing phase, the UCB will de-energize the compressor,
initiate the ASCD, and stop the condenser fan.
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
compressor, initiate the ASCD, and stop the condenser
fan.
If the call for cooling is still present at the conclusion of
the ASCD, the UCB will re-energize the compressor.
Should a low-pressure switch open three times within
one hour of operation, the UCB will lock-out the com-
pressor and flash a code (see Table 23).
FREEZESTAT
During cooling operation, if a freezestat opens, the
UCB will de-energize the compressor, initiate the
ASCD, and stop the condenser fan. If the call for cool-
ing is still present at the conclusion of the ASCD, the
UCB will re-energize the compressor.
Should a freezestat open three times within two hours
of operation, the UCB will lock-out the compressor and
flash a code (see Table 23).
LOW AMBIENT COOLING
To determine when to operate in low ambient mode,
the UCB has a pair of terminals connected to a temper-
ature-activated switch set at 45ºF. When the low ambi-
ent 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 mini-
mum run time.
When operating in low ambient mode, the UCB will not
lockout the compressor due to a freezestat trip. How-
ever, a freezestat trip will de-energize the 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
the cooling system:
1. A suction line freezestat to protect against low
evaporator 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 exces-
sive discharge pressures due to a blocked con-
denser 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 7 ± 3 psig and resets
at 22 ± 5 psig).
The above pressure switches are hard-soldered to the
unit. The refrigeration system is monitored and con-
trolled. On any fault, the system will be affected by any
safety/preventive action.
The unit control board monitors the temperature limit
switch of electric heat units.
COMPRESSOR PROTECTION
The compressor also has inherent (internal) protection.
If there is an abnormal temperature rise in a compres-
sor, the protector will open to shut down the compres-
sor. The UCB incorporates features to minimize
compressor wear and damage. An anti-short cycle
delay (ASCD) is utilized to prevent operation of a com-
pressor too soon after its previous run. Additionally, a
minimum run time is imposed any time a compressor is
energized.