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Alarms
The DDC controller includes a digital output for
remote indication of an alarm condition. Possible
alarms include:
• Dirty Filter Alarm: If the outside air or return
air filter differential pressure rises above the
differential pressure switch set point, the DDC
controller will activate an alarm.
• S upply and Exhaust Air Proving Alarm: DDC
controller monitors proving switch on each blower
and displays an alarm in case of blower failure.
• T emperature Sensor Alarm: DDC controller
will send an alarm in the case of a failed air
temperature sensor.
• S upply Air Low Limit: If the supply air temperature
drops below the supply air low limit (35°F), the DDC
controller will de-energize the unit and activate the
alarm output after a preset time delay (300s).
• O ther Alarms: Wheel Rotation, High Wheel
Pressure, High/Low Refrigerant Pressure.
Energy Recovery Wheel Sequences
Economizer (optional): If the unit is equipped with an
energy recovery wheel, the economizer will modulate/
stop the energy wheel to achieve free cooling. The
economizer will be locked out when:
- The outside air is less than the economizer
lockout (40°F).
- The unit is operating in dehumidification mode.
- The unit is operating in heating mode.
• Stop Wheel: When economizer mode is enabled
and there is a signal for cooling, the wheel will stop
rotating to allow free cooling.
• M odulate Wheel: When economizer mode is
enabled and there is a signal for cooling, the wheel
VFD modulates wheel speed to maintain the supply
temperature set point.
Frost Control (optional): The DDC controller will
activate the frost control method when the outdoor
air temperature is less than the defrost set point (5°F)
and the wheel pressure switch is closed, due to a
high wheel pressure drop.
• E lectric Preheater: When frosting is occurring, the
preheater is energized to defrost the wheel. Once
the pressure drop decreases below the pressure
switch set point, the preheater is de-energized.
• M odulate Wheel: When frosting is occurring, the
VFD slows the wheel to allow defrosting to occur.
Once the pressure drop decreases below the
pressure switch set point, the wheel returns to full
speed.
4
DDC Controller for Tempered Air Products
Sequence of Operation
Energy Recovery Wheel Sequences, continued
• T imed Exhaust: When frosting is occurring, the
supply fan is cycled off along with the tempering
for 5 minutes. The exhaust fan will continue to
run, allowing the warm exhaust air to defrost the
wheel. After the 5 minute cycle, the supply fan and
tempering are re-energized to continue normal
operation. The unit will continue normal operation
for 30 minutes before allowing another defrost
cycle to occur.
®
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