349218-JIM-F-0512
Johnson Controls Unitary Products
47
•
Dry Bulb:
The control refers to input from the Outside Air
Temperature sensor and will allow free-cooling when the
outdoor temperature is less than both the
First-Stage SAT
Control
setpoint plus 5 °F, and the
Economizer OAT
Enable
setpoint.
•
Single Enthalpy (optional):
A field-installed, Outdoor Air
Humidity sensor is connected to the control. When the
measured outdoor enthalpy is below the
Outside Air
Enthalpy
setpoint, and the outdoor temperature is less
than the
First-Stage SAT Control
setpoint plus 5 °F, free-
cooling is available.
•
Dual Enthalpy (optional):
Both the field-installed
Outdoor Air Humidity and the Return Air Humidity sensors
are connected to the control. When the measured outdoor
air enthalpy is less than the measured return air enthalpy,
and the outdoor temperature is less than the
First-Stage
SAT Control
setpoint plus 5 °F, free-cooling is available.
If free-cooling is available with a Y1 call, then the control
modulates the economizer dampers to maintain the
First-Stage
SAT Control
setpoint, plus or minus one degree. If free-cooling
is unavailable, then 1st-stage mechanical cooling is initiated.
If at anytime the outdoor air temperature rises above the
First-
Stage SAT Control
setpoint plus 5 °F, while free-cooling is
available, then a Y1 call will also initiate 1st-stage mechanical
cooling.
For a Y2 call, free-cooling is available based upon the criteria
described above, except a
Second-Stage SAT Control
setpoint
is used in the determination.
Once the call for cooling has been satisfied, it will de-energize
any compressors and condenser fans, after the minimum
compressor run times have been satisfied. Otherwise, the unit
operates each cooling system until the minimum run times for
the compressors have been completed.
Upon de-energizing the final compressor, the blower will
continue to run with the economizer damper in its minimum
position if in the Occupied mode; otherwise, the blower will stop
following the elapse of the fan-off delay for cooling, and the
economizer outdoor damper will close.
Economizer With Optional VAV Blower With Power
Exhaust
The power exhaust motor is energized via the controller's EXH~
terminal and the ER relay, based on the position of the
economizer damper parameter settings in the VAV control.
Minimum run time is 10 seconds; minimum off time is 60
seconds. The outlet pressure of the power exhaust fan forces
the barometric relief dampers open; gravity closes the dampers
when the exhaust fan is off.
Economizer With Optional Intelli-comfort II™ With Power
Exhaust
The power exhaust motor is energized via the exhaust relay based
on the position of the economizer actuator's auxiliary switch
adjustment screw. The adjustment screw represents the outdoor
damper position at which to activate power exhaust, and can be
set between 25 to 85 degrees open. The outlet pressure of the
power exhaust fan forces the barometric relief dampers open;
gravity closes the dampers when the exhaust fan is off.
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 compressor, 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 conclusion 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 associated
compressor and flash a code (see Table 25). 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-pressure switch fails to close after
the 30-second monitoring phase, the UCB will de-energize the
associated compressor, initiate the ASCD, and, if the other
compressor is idle, stop the condenser fans. If the LPS is still
open after the ASCD, the compressor will not be energized for
30 seconds. The second and third times that the UCB sees an
open LPS will count towards the three occurrences that will
cause a UCB lock-out.
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 compressor.
Should a low-pressure switch open three times within one hour
of operation, the UCB will lock-out the associated compressor
and flash a code (Table 25). If the other compressor is inactive,
the condenser fans will be de-energized.
