6021980-UIM-A-1120
60
Johnson Controls Ducted Systems
Operation
Cooling operation
• A Y1 call for the first stage of cooling is passed to the unit
control board (UCB). The UCB then determines whether
the requested operation is available and if so, which
components to energize. With a Y1 call for first stage
cooling, the UCB determines if a first stage cooling output
is valid as long as all safeties and time-delays allow a C1
output for cooling. The C1 relay on the UCB closes and
sends 24 volts to the M1 relay. This starts compressor #1
on low and also energizes terminal block TB3-1 for cond.
fan #1 and cond. fan #2 on low speed. The UCB
energizes the VFD-equipped blower at the first stage
speed as set in the Smart Equipment™ control.
• If a Y2 call is present, it is passed to the UCB. The UCB
then determines whether the requested operation is
available and if so, which components to energize. With a
Y2 call for second stage cooling, the UCB determines if a
second stage 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 24 volts to the M2
relay. This starts compressor #2 on low speed and also
energizes for cond. fan #3 and cond. fan #4 on low speed.
The UCB energizes the VFD-equipped blower at the
second stage speed as set in the Smart Equipment™
control.
• If a Y3 call is present, it is passed to the UCB. The UCB
then determines whether the requested operation is
available and if so, which components to energize. With a
Y3 call for third stage cooling, the UCB determines if a
third stage cooling output is valid as long as all safeties
and time-delays allow a C3 output for cooling. The C3
relay on the UCB closes and sends 24 volts to the
terminal block TB3-2 which energizes compressors #1 &
#2 on high speed and also cond. fan #1, cond. fan #2,
cond. fan #3, and cond. fan #4 on high speed. The UCB
energizes the VFD-equipped blower at the third stage
speed as set in the Smart Equipment™ control.
Free cooling operation with economizer
• With a demand for first stage cooling either from a
thermostat or space sensor and the outside air is suitable
for free cooling, the dampers are modulated to maintain
supply air temperature to /- 1 degree of the free
cooling SAT upper setpoint. If the output to the
economizer actuator is at 10 VDC and the supply air
temperature cannot be controlled to within 5 degrees of
the free cooling SAT upper setpoint, one compressor is
energized. The economizer is then modulated to maintain
supply air temperature to /- 1 degree of the free
cooling SAT upper setpoint.
• If a demand for second stage cooling occurs and the
economizer is already modulating to maintain the supply
air temperature to the cooling SAT upper setpoint, the
setpoint is changed to the free cooling SAT lower setpoint.
Any compressors that are energized remain energized
and the economizer dampers are modulated to maintain
supply air temperature to /- 1 degree of the cooling
SAT lower setpoint. If the output to the economizer
actuator is at 10 VDC and the supply air temperature
cannot be controlled to within 5 degrees of the cooling
SAT lower setpoint, compressor stage 2 is energized. The
economizer is then modulated to maintain supply air
temperature to /- 1 degree of the cooling SAT
lower setpoint.
• If the output to the economizer actuator is at minimum
position and the supply air temperature drops more than 5
degrees below the current economizer SAT setpoint, the
highest stage compressor is de-energized as long as the
minimum run time has elapsed.
Economizer with single enthalpy sensor
When the room thermostat calls for 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 to 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
supply air temperature as monitored by the discharge air sensor.
If the outdoor air enthalpy is above the setpoint, Y1 energizes
the compressor and condenser fan motor only.
When the thermostat has been satisfied, it de-energizes Y1. If
the compressor has satisfied its minimum run time, the
compressor and condenser fan are de-energized. Otherwise, the
unit operates the cooling system until the minimum run times for
the compressor are completed. After the compressor de-
energizes, 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 continues fan operation, the
economizer damper goes to the min. 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 setpoint on
the economizer control. 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
by the thumbwheel on the damper motor. When the R to G
circuit is opened, the damper spring returns fully closed.
