284806-YIM-A-0107
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Unitary Products Group
enthalpy of the outdoor air is below the setpoint of the
enthalpy controller (previously determined), "Y1" ener-
gizes the economizer. The dampers will modulate to
maintain a constant supply air temperature as moni-
tored by the discharge air sensor. If the outdoor air
enthalpy is above the setpoint, "Y1" energizes com-
pressor #1, condenser fan motor #1, and condenser
fan motor #2, 15 and 20T only (if the ambient tempera-
ture is above 60°F).
When the thermostat calls for "second-stage" cooling,
the low voltage control circuit from "R" to "Y2" is
completed. The UCB energizes the first available
compressor If the enthalpy of the outdoor air is below
the setpoint of the enthalpy controller (i.e. first stage
has energized the economizer), "Y2" will energize
compressor #1. If the outdoor air is above the setpoint,
"Y2" will energize compressor #2.
Once the thermostat has been satisfied, it will de-ener-
gize Y1 and Y2. 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 fol-
lowing the elapse of the fan off delay for cooling, and
the economizer damper goes to the closed position. If
the unit is in continues fan operation the economizer
damper goes to the min. position.
ECONOMIZER WITH DUAL ENTHALPY SENSORS -
The operation with the dual enthalpy sensors is identi-
cal to the single sensor except that a second enthalpy
sensor is mounted in the return air. This return air sen-
sor allows the economizer to choose between outdoor
air and return air, whichever has the lowest enthalpy
value, to provide maximum operating efficiency.
ECONOMIZER (SINGLE OR DUAL) WITH POWER
EXHAUST -
This system operates as specified above with one
addition. The power exhaust motor is energized 45
seconds after the actuator position exceeds the
exhaust fan setpoint on the economizer control. When
the power exhaust is operating, the second stage 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 thumbwheel 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 associated com-
pressor, initiate the ASCD (Anti-short cycle delay), and,
if the other compressor is idle, stop the condenser
fans. If the call for cooling is still present at the conclu-
sion of the ASCD, the UCB will re-energize the halted
compressor.
Should a high-pressure switch open three times within
two hours of operation, the UCB will lock-out the asso-
ciated compressor and flash a code (see Table 29). If
the other compressor is 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-pres-
sure switch fails to close after the 30-second monitor-
ing phase, the UCB will de-energize the associated
compressor, initiate the ASCD, and, if the other com-
pressor is 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 com-
pressor.
Should a low-pressure switch open three times within
one hour of operation, the UCB will lock-out the associ-
ated compressor and flash a code (Table 29). If the
other compressor is inactive, the condenser fans will
be de-energized.
