284816-BIM-B-0707
Unitary Products Group
47
outside air dampers will be closed. When the supply
fan is powered and there is no Y1 call, or if free-cooling
is unavailable, the control opens the economizer
dampers to the minimum position setting.
Free-cooling is available if the outdoor air temperature
meets one of the three criteria discussed below, based
upon the unit's configuration.
•
Dry Bulb:
The control refers to input from the Out-
side 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, Out-
door Air Humidity sensor is connected to the con-
trol. When the measured outdoor enthalpy is below
the
Outside Air Enthalpy
setpoint, and the outdoor
temperature is less than the
First-Stage SAT Con-
trol
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 tem-
perature 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 con-
trol 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 satis-
fied. 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 control-
ler'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™
WITH POWER EXHAUST
The power exhaust motor is energized via the exhaust
relay based on the position of the economizer actua-
tor'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 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 27). If
the other compressor is inactive, the condenser fans
will be de-energized.