284806-YIM-B-0707
Unitary Products Group
43
enthalpy of the outdoor air is below the setpoint of the
enthalpy controller (previously determined), "Y1" ener-
gizes the economizer. The dampers will modulate to
maintain a constant supply air temperature as moni-
tored by the discharge air sensor. If the outdoor air
enthalpy is above the setpoint, "Y1" energizes com-
pressor #1, condenser fan motor #1, and condenser
fan motor #2, 15 and 20T only (if the ambient tempera-
ture is above 60°F).
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" will energize
compressor #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.
ECONOMIZER WITH OPTIONAL VAV OR INTELLI-COM-
FORT™ 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.
