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directs hot gas through part of the indoor coil and decreases the
sensible capacity of the unit.
Standard A/C Mode: Input: Y1 On, Y2 On, Humidistat Open
Or Closed
If there is a call for Y1 and Y2, the unit will be in normal cooling
mode regardless of the humidistat input. The Y2 signal will
energize the hot gas relay (RHG) which will keep the liquid line
solenoid (SLL) open and the hot gas solenoid (SHG) closed
and the unit will operate in the normal cooling mode.
NOTE: Y2 must be used or the function of the unit will be
permanently in the Re-Heat mode and sensible
cooling capacity will be affected.
Economizer With Dry Bulb Sensor
When the room thermostat calls for cooling, the low voltage
control circuit from "R" to "G" and "Y1" is completed. The UCB
energizes the blower motor (if the fan switch on the room
thermostat is set in the "AUTO" position) and drives the dry bulb
temperature dampers from fully closed to their minimum
position. If the enthalpy of the outdoor air is below the setpoint of
the dry bulb controller (previously determined), "Y1" energizes
the economizer. The dampers will modulate to maintain a
constant supply air temperature as monitored by the discharge
air sensor. If the outdoor air enthalpy is above the setpoint, "Y1"
energizes the compressor and condenser fan motor only.
Once the thermostat has been satisfied, it will de-energize "Y1". If
the compressor has satisfied its minimum run time, the
compressor and condenser fan are de-energized. Otherwise, the
unit operates the cooling system until the minimum run times for
the compressor has been completed. After the compressor de-
energizes, the blower is stopped following 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 identical to the
single sensor except that a second enthalpy sensor is mounted
in the return air. This return air sensor 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 set point 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
Johnson Controls Unitary Products
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 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 associated
compressor and shows an error message.
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 shows
an error message.
Evaporator Low Limit
During cooling operation, if the evaporator sensor detects a
temperature below 26 Deg. F (default), 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 halted
compressor.
Should the evaporator sensor detect a temperature below 26°F
three times within two hours of operation, the UCB will lock-out
the associated compressor and flash an error message.
Low Ambient Cooling
To determine when to operate in low ambient mode, the UCB
has an Outdoor Air Temperature Sensor (OAT) with a low
ambient setpoint at 45ºF (default). When the OAT Sensor
senses a temperature below the low ambient setpoint and the
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