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Once placed into the Occupied Mode, the speed of the indoor
blower motor is controlled by duct static pressure. The Duct
Static set point (default = 1.5") is the pressure that the VFD
drive will maintain when operating the unit in VAV mode. If the
duct static pressure reaches or exceeds the high-limit set-point
(default = 4.5"), then the supply fan motor will be shutdown.
The Supply Air Temperature (SAT) is controlled by staging
compressors on and off to satisfy the "Operating Cooling
Supply Air Temp Set point". There are 3 set points that
determine the resulting "Operating Cooling Supply Air Temp
Set point".
1.
VAV Cooling Supply Air Temp Upper Set point
(default 60° F)
2.
VAV Cooling Supply Air Temp Lower Set point
(default 55° F)
3.
VAV Supply Air Temp Reset Set point (default 72° F)
When the Return Air Temp (RAT) is above the "VAV Supply Air
Temp Reset Set point" the SAT will be maintained at +/- 5
degrees of the "VAV Cooling Supply Air Temp Lower Set point".
When the Return Air Temp (RAT) is below the "VAV Supply Air
Temp Reset Set point" the SAT will be maintained at +/- 5
degrees of the "VAV Cooling Supply Air Temp Upper Set point".
When the Outdoor air condition is sufficient for free cooling, the
economizer will modulate to control the SAT to +/- 1 degrees of
the operational set point.
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.
The UCB energizes the economizer (if installed and free cooling
is available) or the first available compressor
condenser fans. For first stage cooling, compressor #1 is
energized. If compressor #1 is unavailable, compressor #2 is
energized. 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 cooling, the
low-voltage control circuit from "R" to "Y2" is completed. The
control board energizes the first available compressor. If free
cooling is being used for the first stage of cooling, compressor
#1 is energized. If compressor #1 is active for first stage cooling
or the first compressor is locked-out, compressor #2 is
energized. In free-cooling mode, if the call for the second stage
of cooling continues for 20 minutes, compressor #2 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 seconds in order to
avoid a power rush.
Once the thermostat has been satisfied, it will de-energize 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
following the elapse of the fan off delay for cooling.
Johnson Controls Unitary Products
* To be available, a compressor must not be locked-out due to a
high or low-pressure switch or the Evaporator Low Limit
Sensor (EC1, 2) detects a temperature below 26 Deg. F and
the Anti-Short Cycle Delay (ASCD) must have elapsed.
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 set point
of the enthalpy controller (previously determined), "Y1"
energizes the economizer. 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 set
point, "Y1" energizes compressor #1.
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 set point of the enthalpy controller (i.e.
first stage has energized the economizer), "Y2" will energize
compressor #1. If the outdoor air is above the set point, "Y2" will
energize compressor #2.
Once the thermostat has been satisfied, it will de-energize "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
following the elapse of the fan off delay for cooling, and the
economizer damper goes to the closed position. If the unit is in
continuous fan operation, the economizer damper goes to the
*
and the
minimum position.
Economizer With Dual Enthalpy Sensors
The operation with the dual enthalpy sensors is identical to the
single sensor except that a second enthalpy sensor is mounted
in the return air. This return air sensor allows the economizer to
choose between outdoor air and return air, whichever has the
lowest enthalpy value, to provide maximum operating
efficiency.
Economizer With Power Exhaust
A unit equipped with an economizer (single or dual enthalpy)
and a power exhaust operates as specified above with one
addition. The power exhaust motor is energized 45 seconds
after the actuator position exceeds the exhaust fan set point on
the economizer control. As always, the "R" to "G" connection
provides minimum position but does not provide power exhaust
operation.
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
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