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JOHNSON CONTROLS
30
FORM 100.50-EG5 (108)
proportionally controlled open and the Exhaust Fan is
controlled ON. If the Building Pressure falls, the Exhaust
Damper is proportionally controlled closed and the Ex-
haust Fan is controlled OFF. The position of the Exhaust
Damper in which the Exhaust Fan is controlled ON and
OFF as well as the Building Pressure setpoint are user-
selectable from the Rooftop Unit User Interface.
Powered, Variable-Volume Exhaust-VFD
Controlled
This optional variable-volume, powered-exhaust system
consists of an Exhaust Fan driven by a Variable Fre-
quency Drive (VFD), which is controlled by the Rooftop
Unit controller. The Rooftop Unit controller monitors the
pressure within the building. As the pressure rises, the
VFD is controlled to increase Exhaust Fan speed. As
the pressure falls, the VFD is controlled to decrease
Exhaust Fan speed. The Building Pressure Setpoint is
user-selectable from the Rooftop Unit User Interface.
On/Off control is maintained the same as Exhaust-Dis-
charge Damper control stated above.
LOW-AMBIENT/HEAD-PRESSURE CONTROL
OPERATION
The Rooftop Unit controller continuously monitors the
outside air temperature to determine if mechanical cool-
ing should be allowed. As a safety, if the Outside Air
temperature falls to or below the Low Ambient Lockout
temperature, mechanical cooling is prevented from
operating.
For units with economizers, the Low Ambient Lockout
temperature is typically low enough that mechanical
cooling will rarely be required. However, for some ap-
plications, mechanical cooling is required when the
Outside Air temperature is lower than the Low Ambient
Lockout temperature.
For these applications, the unit must be equipped with
optional Low Ambient controls. For optional Low Ambi-
ent operation, the Rooftop Unit controller monitors the
refrigeration-system discharge pressure and controls the
speed of the fi rst stage condenser fan. If the discharge
pressure falls, the speeds of the condenser fan is re-
duced to maintain acceptable condensing pressures in
the refrigeration system. With the optional Low Ambient
controls, mechanical cooling is allowed down to Outside
Air temperatures of 0°F.
SMOKE PURGE SEQUENCE
A contact closure input (PURGE) is provided to place the
unit in smoke purge mode. When the contact is closed
the unit will operate as follows:
• Turn off all heating and cooling operation
• Set the outdoor air damper output to 100%
• Close the return to 0%
• Turn the supply fan on
• On VAV unit set the supply fan output to 100%.
• Turn the power exhaust fan on
• On VFD driven exhaust fans set the exhaust fan
output to 100%
Note that 24 volts terminal (R) on the Simplicity control
board must be used as the 24 Volt AC source for switch
the contact to the Unit Controller Smoke Purge (PURGE)
input. Use of any power source external to the controller
will result in damage to the Unit Controller.
VAV SPECIFIC SEQUENCES
SUPPLY FAN OPERATION
For VAV units, the supply fan is controlled ON and OFF
based on the occupancy state. When the unit goes
into the Occupied mode of operation, the Rooftop Unit
controller will monitor the static pressure within the
supply-duct system and control the speed of the supply
fan to maintain a specifi ed Duct Static Pressure setpoint.
A Variable Frequency Drive (VFD) is used on all VAV
units to vary the speed of the supply fan. Note, the use
of a VFD in lieu of inlet guide vanes provides for higher
energy effi ciency for the unit by eliminating the losses
(air-pressure drop) typical of inlet guide vane systems.
CV SPECIFIC SEQUENCES
COOLING OPERATION
Thermostat Control
If a 7-wire thermostat (2 Cool/2 Heat) controls the unit,
all zone-temperature setpoint-control is maintained at
the thermostat. With this operation, the unit remains idle
until it receives a stage call from the thermostat. If “G”
Controls (continued)