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349215-JIM-B-1008
gized after any specified delay associated with the
operation.
When energized, the indoor blower has a minimum run
time of 30 seconds. Additionally, the indoor blower has
a delay of 10 seconds between operations.
OPTIONAL VAV BLOWER OPERATION
For units with VFD, the speed of the indoor blower
motor is controlled by duct static pressure. The duct
static set point is the pressure that the drive will main-
tain when operating the unit in VAV mode. If the duct
static pressure reaches or exceeds the high-limit set-
point, then the supply fan motor will be shutdown.
If the unit is operated with the manual bypass
switch in the LINE (BYPASS) position and
there are VAV boxes present in the duct sys-
tem, then boxes must be driven to the full-open
position using a customer-supplied power
source to prevent over-pressurizing and possi-
ble damage to the ductwork.
NO OUTDOOR AIR OPTIONS
When the thermostat calls for the first stage of cooling,
the low-voltage control circuit from "R" to "Y1" and "G"
is completed. For first stage cooling, compressor #1,
condenser fan motor #1, and condenser fan motor #2
(if the ambient temperature is above 60ºF), is ener-
gized. After completing the specified fan on delay for
cooling, the UCB will energize the blower motor.
When the thermostat calls for the second stage of cool-
ing, the low-voltage control circuit from "R" to "Y2" is
completed. Compressor #2, condenser fan motor #3,
and condenser fan motor #4 (if the ambient tempera-
ture is above 60ºF), is energized, provided it has not
been locked-out.
If there is an initial call for both stages of cooling, the
UCB will delay energizing compressor #2 by 30 sec-
onds in order to avoid a power in rush.
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
46
compressor de-energizing, the blower is stopped fol-
lowing the elapse of the fan off delay for cooling.
To be available, a compressor must not be locked-out
due to a high or low-pressure switch or freezestat trip
and the anti-short cycle delay (ASCD) must have
elapsed.
ECONOMIZER WITH SINGLE ENTHALPY SENSOR
When the room thermostat calls for "first-stage" cool-
ing, 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 damp-
ers from fully closed to their minimum position. If the
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 (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.
Johnson Controls Unitary Products
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