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826081-XIM-C-0214
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Johnson Controls Unitary Products
of mechanical cooling will not operate. As always, the “R” to “G”
connection provides minimum position but does not provide
power exhaust operation.
Motorized Outdoor Air Dampers
This system operation is the same as the units with no outdoor
air options with one exception. When the “R” to “G” circuit is
complete, the motorized damper drives open to a position set
by the thumbnail on the damper motor. When the “R” to “G”
circuit is opened, the damper spring returns fully closed.
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 compressor, initiate the ASCD
(Anti-short cycle delay), 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 high-pressure switch open three times within two
hours of operation, the UCB will lock-out the compressor and
flash a code (see Table 24).
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
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 compressor and flash a
code (see Table 24).
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 cooling 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 24).
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 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 compressor due to a freezestat trip. However, 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 -3°C (26°F) and resets at 3°F
(38°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 2660 kPa (380 psig)
and resets at 2100 kPa (300 psig)).
3.
A low-pressure switch to protect against loss of refrigerant
charge, (opens at 152 kPa (22 psig) and resets at 310 kPa
(45 psig)).
The above pressure switches are hard-soldered to the unit. The
refrigeration system is monitored and controlled. 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 and the temperature limit switch and the gas
valve of gas furnace units.
Compressor Protection
The compressor also has 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 damage. 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 compressor is
energized.