5566020-JIM-C-1019
Johnson Controls Ducted Systems
57
supply air temperature as monitored by the discharge air
sensor. If the outdoor air enthalpy is above the setpoint, Y1
energizes compressor #1.
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 energizes
compressor #1. If the outdoor air is above the setpoint, Y2l
energizes compressor #2.
When the thermostat is satisfied, it de-energizes 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 are completed. Upon the final
compressor de-energizing, 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 continuous
fan operation, the economizer damper goes to the minimum
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. 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.
Note:
The following components are needed to access the
control points in the Smart Equipment™ control.
1.
Local LCD on the unit control board.
OR
2.
Mobile Access Portal (MAP) Gateway (Portable).
• Source 1 P/N S1-JC-MAP1810-OP
• MAP Gateway Quick Start Guide P/N 24-10737-16
• MAP Gateway Instruction P/N 24-10737-8
High-pressure limit switch
During cooling operation, if a high-pressure limit switch opens,
the UCB de-energizes the associated compressor, initiates the
ASCD (Anti-short cycle delay), and, if the other compressor is
idle, stops the condenser fans. If the call for cooling is still
present at the conclusion of the ASCD, the UCB re-energizes
the halted compressor.
If a high-pressure switch opens three times within two hours of
operation, the UCB locks out the associated compressor. If the
other compressor is inactive, the condenser fans are 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 monitors the low-pressure switch to
ensure it closes. If the low-pressure switch fails to close after
the 30-second monitoring phase, the UCB de-energizes the
associated compressor, initiates the ASCD, and, if the other
compressor is idle, stops the condenser fans.
When the low-pressure switch is proven (closed during the 30-
second monitor period described above), the UCB monitors the
low-pressure limit switch for any openings. If the low-pressure
switch opens for greater than 5 seconds, the UCB de-energizes
the associated compressor, initiates the ASCD, and, if the other
compressor is idle, stops the condenser fans.
If the call for cooling is still present at the conclusion of the
ASCD, the UCB re-energizes the halted compressor.
If a low-pressure switch opens three times within one hour of
operation, the UCB locks out the associated compressor. If the
other compressor is inactive, the condenser fans are de-
energized.
Evaporator low limit
During cooling operation, if the
Evaporator Low Limit Sensor
(EC1, 2)
(Located on the Suction Line at the Evaporator Coil.)
detects a temperature below 26
°
F (default), the UCB de-
energizes 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 re-energizes the halted compressor. If the UCB detects
the evaporator low limit sensor
(EC1, 2)
falling below 26
°
F
(default) three times within two hours of operation, the UCB
locks out the associated compressor. If the other compressor is
inactive, the condenser fans are de-energized.
Low ambient cooling
To determine when to operate in low ambient mode, the UCB
has an
Outdoor Air Temperature Sensor (OAT)
with a low
