66396-YIM-C-1004
28
Unitary Products Group
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 minimum 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 WITH POWER EXHAUST -
A unit equipped with an economizer (single or dual
enthalpy) and a power exhaust 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 sec-
ond 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 21). 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 21). 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, ini-
tiate 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 compres-
sor and flash a code (Table 21). If the other compressor
is inactive, 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 temper-
ature-activated switch set at 30°F. 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.
