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ECONOMIZER WITH SINGLE ENTHALPY SENSOR
When the room thermostat calls for "first-stage" 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 economizer dampers from fully
closed to their minimum position. If the enthalpy of the
outdoor air is below the setpoint of the enthalpy control-
ler (previously determined), "Y1" energizes the econo-
mizer. 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 compressor #1, condenser fan motor #1,
and condenser fan motor #2 (if the ambient temperature
is above 60°F).
When the thermostat calls for "second-stage" cooling,
the low voltage control circuit from "R" to "Y2" is com-
pleted. The UCB energizes the first available compres-
sor If the enthalpy of the outdoor air is below the
setpoint of the enthalpy controller (i.e. first stage has
energized the economizer), "Y2" will energize com-
pressor #1. If the outdoor air is above the setpoint, "Y2"
will energize compressor #2.
Once the thermostat has been satisfied, it will de-ener-
gize 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 have been completed. Upon the final
compressor de-energizing, the blower is stopped fol-
lowing 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 identi-
cal to the single sensor except that a second enthalpy
sensor is mounted in the return air. This return air sen-
sor allows the economizer to choose between outdoor
air and return air, whichever has the lowest enthalpy
value, to provide maximum operating efficiency.
ECONOMIZER (SINGLE OR DUAL ENTHALPY WITH
POWER EXHAUST
This system operates as specified above with one
addition. The power exhaust motor is energized 45
seconds after the actuator position exceeds the
exhaust fan setpoint 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.
Johnson Controls Unitary Products
ECONOMIZER WITH OPTIONAL VAV OR
INTELLI-COMFORT™ CONTROL
The position of the outside air and return air dampers
are controlled through a 2-10 VDC signal from the VAV
or Intelli-Comfort™ control board. The economizer is
enabled only in Occupied or Recovery mode. When the
control is not powered or is in Unoccupied mode, the
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.
284812-YIM-D-1008
57
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