Johnson Controls ZL Series Installation Manual page 61

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 M4 for the condenser
fan(s) to run at nominal speed. The UCB
energizes the VFD-equipped blower at the first
stage speed as set in the Smart Equipment™.
• 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 to the
M2 relay which starts compressor #2 on high
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 4-stage board
closes and sends 24 volts to terminal block
TB3 which energizes compressor #1 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 within +/- 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 within
+/- 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 within +/- 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 within +/- 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
Installation Manual ZL Series 7.5- 12.5 Ton 60 Hertz R-410A
Johnson Controls Ducted Systems
61