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Marvair Scholar 2.0 I&S Manual 04/2022 Rev.2
1.03 STANDARD CONTROLS
A. Programmable Logic Controller (PLC) Microprocessor
Scholar 2.0 heat pumps and air conditioners use a factory installed PLC
microprocessor to control the operation, the safety switches and function
options. LED’s show operational status and provide assistance with diagnosis if
troubleshooting is ever required. Various control functions are field selectable. The
PLC is also capable of communicating to PLC’s in other Scholar units to allow run
time leveling and does not require additional equipment installed in the Scholar 2.0
unit. The PLC microprocessor provides improved reliability because of the reduction
of components, the components utilized are more durable and the control box
wiring has been simplified. Pertinent statistical data about the life of the refrigeration
system can be accessed through the PLC.
The PLC microprocessor provides for the following control and operation functions:
• Anti-Short Cycle Timer -
Prevents the compressor from destructive short
cycling due to momentary power interruptions. One of three time intervals can
be field selected.
• Defrost Timer (heat pumps only) -
Adjustable defrost control that is based upon
both time and temperature. The time interval presets are 30, 60 and 90 minutes.
• BAS Control Relay -
Provides 24 VAC coil to control operation from Building
Automation System (BAS). Note - an additional BAS control relay can be added
when 120 or 240 VAC coils are required.
B. High and Low Refrigerant Pressure and Loss of Charge Switches
These switches render the compressor and outdoor fan motor inoperative whenever
the limits of the pressure switches are exceeded. In the event of high pressure, the
Scholar 2.0 heat pump or air conditioner will turn off the compressor and activate the
ASCT.
Low pressure switch is bypassed for 3 minutes during each start-up. After the
system has started, if the low pressure and high pressure switch is open while the
system is running, the system will fault and the compressor will shut down, which
in turn will activate the Anti-Short-Cycle-Timer. Once the ASCT has elapse and the
switch(s) resets, if there is still a call for cooling the unit will start back up.
The high and low pressure and loss of charge switches are resettable at the wall
thermostat or by turning power off and then on to the Scholar 2.0 heat pump or air
conditioner.
Note:
Scholar units built before 8/23/2021 will no longer use wires and jumper for
S-CKT and EHDD. The selection of both features will be done from PLC screen.
C. Heat Related Controls
The
outdoor thermostat (heat pump only)
, if installed, factory set at 40°F,
determines the temperature at which the supplemental electric heat (or wet heat)
turns on to heat the classroom. The temperature set point can be adjusted in the
field for local conditions. When the outdoor thermostat activates wet heat, the
compressor simultaneously turns off.
The
electric heat control
allows the user to select whether the electric heat
operates simultaneously with the compressor. Scholar 2.0 units are wired at the
