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Marvair Scholar 2.0 I&S Manual 04/2022 Rev.2
N. Defrost Timer (DFT) (heat pump function only)
Located on the “Preset Timer” screen. The defrost control is based upon both
time and temperature. The DFT initiates a defrost cycle whenever the outdoor coil
temperature is 28°F or below and the selected time interval from the previous defrost
cycle has been exceeded. The Defrost time is selectable from the Preset Timer
screen form 30 to 90 minutes. The Marvair
®
factory set point is 30 minutes.
O. Low Pressure Lockout
The Low Pressure Switch is designed to guard against the operation of the system
in the event of a loss of refrigerant. If the Low Pressure switch is open the system
will shut down with a Low Pressure fault, indicated by a flashing amber LED on the
PLC. The interval for the flash is once per second. Once the switch is closed being
that there is still a call for cooling the unit will resume operation. In cold weather the
pressure in the refrigerant system is low prior to operation. When the Scholar 2.0
unit starts in the heat pump mode during cold weather, low pressure could cause the
system to lock out. To guard against nuisance lockouts, the Scholar 2.0 unit will not
shut off if the Low Pressure Switch (LPS) comes on during the first three minutes
bypass time of operation. In other words, the compressor will start and operate for
three minutes even with the LPS switch open. Once the bypass timer has elapse
and the LPS is closed the system will continue to operate, if the LPS is open twice
on the same call for cooling the system will go into a lock out mode.
P. High Pressure Switch
The system has a High Pressure Switch (HPS) that indicates a high system
pressure. If there is an HPS fault or the HPS is open during operation the system
will shut down with a High Pressure fault, indicated by a flashing Red LED on the
PLC. Once the switch is closed being that there is still a call for cooling the unit will
resume operation. If the HPS is open twice on the same call for cooling the system
will go into a lock out mode.
Q. Troubleshooting
The PLC is a microprocessor-based system that has the ability to provide a great
amount of information to aid in troubleshooting a malfunctioning system. With the
Input and Output screen user can easily diagnose most problem by just seeing what
is on and or what is off.
R. Normal Operation
The High Pressure Switch (HPS) and Low Pressure Switch (LPS) should be close
all the time. An exception will be during a low temperature start-up during which the
LPS should come on within a few moments of start-up. The chart below shows the
standard inputs and what functions should be on as a result.
