5566020-JIM-C-1019
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Johnson Controls Ducted Systems
cooling output is valid as long as all safeties and time-delays
allow a C2 output for cooling. The C2 relay on the UCB closes
and sends 24v to the M2 relay starting the second stage
compressor also relay starting the associated condenser fans.
During any call for fan or cooling the FAN output on the UCB
energizes the M3 relay starting the supply fan.
Continuous blower
By setting the room thermostat fan switch to ON, the supply air
blower operates continuously.
Intermittent blower
With the room thermostat fan switch set to AUTO and the
system switch set to either the AUTO or HEAT settings, the
blower is energized whenever a cooling or heating operation is
requested. The blower is energized 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 minimum off.
Optional VAV startup and control
For units with VFD and VAV control, the unit must first be put
into the Occupied Mode to start operation. The default setting
for all VAV units is 'Unoccupied', therefore the installer must
add a jumper wire between terminals R - OCC on the VAV add-
on board to put the unit into 'Occupied' Mode. Additionally, the
unit can be switched between Unoccupied/Occupied mode
through network communications.
When 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 in.) is the pressure that the VFD
drive maintains when operating the unit in VAV mode. If the
duct static pressure reaches or exceeds the high-limit setpoint
(default = 4.5 in.), then the supply fan motor is shutdown.
The Supply Air Temperature (SAT) is controlled by staging
compressors on and off to satisfy the Operating Cooling Supply
Air Temp setpoint. There are 3 setpoints that determine the
resulting Operating Cooling Supply Air Temp setpoint.
1.
VAV Cooling Supply Air Temp Upper setpoint
(default 60° F)
2.
VAV Cooling Supply Air Temp Lower setpoint
(default 55° F)
3.
VAV Supply Air Temp Reset setpoint (default 72° F)
When the Return Air Temp (RAT) is above the VAV Supply Air
Temp Reset setpoint the SAT is maintained at +/- 5 degrees of
the VAV Cooling Supply Air Temp Lower setpoint.
When the Return Air Temp (RAT) is below the VAV Supply Air
Temp Reset setpoint the SAT is maintained at +/- 5 degrees of
the VAV Cooling Supply Air Temp Upper setpoint.
When the Outdoor air condition is sufficient for free cooling, the
economizer modulates to control the SAT to +/- 1 degrees of
the operational setpoint.
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
*
and the 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
energizes 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
delays energizing compressor #2 by 30 seconds in order to
avoid a power rush.
When the thermostat is satisfied, it de-energizes 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.
* 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
°
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 setpoint
of the enthalpy controller (previously determined), Y1 energizes
the economizer. The dampers modulate to maintain a constant
If the unit is operated with the manual bypass switch in
the LINE (BYPASS) position and there are VAV boxes
present in the duct system, then boxes must be driven to
the full-open position using a customer-supplied power
source to prevent over-pressurizing and possible
damage to the ductwork.
