5011492-XIM-B-0815
Johnson Controls Unitary Products
37
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 (dry bulb 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
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 flash a code on the Simplicity Lite Control (see
Table 28) or sent error message to the LCD Simplicity SE
Control.
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 on the Simplicity Lite Control (Table 28) or sent error
message to the LCD Simplicity SE Control.
Freezestat (Simplicity Lite Control)
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 halted
compressor.
Should a freezestat open three times within two hours of
operation, the UCB will lock-out the associated compressor and
flash a code on the Simplicity Lite Control (Table 28).
Evaporator Low Limit (Simplicity SE Control)
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 (Simplicity Lite Control)
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 compressors due to a freezestat trip. However, a freezestat
trip will de-energize the associated 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.
Low Ambient Cooling (Simplicity SE Control)
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
