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Marvair MGA3090-3180 Wall Mount AC I&O Manual 05/2023 Rev.14
1.5 General Operation
Optional Hot Gas By-Pass
Normally used in specialty applications (i.e Magnetic Resonance Imaging (MRI) buildings) to prevent
magnetic voltage disturbance caused by cycling. This technology is applied in this product to extend
the operation envelope for the compressor to 20º F (-6.6ºC). Combined with a condenser low ambient
Fan Cycle feature, compressor operation can be extended to 0º F (-17.8ºC). During Hot Gas operation
mode, system performance will be reduced. If product operation is in mild outdoor ambient conditions,
the installed shut-off ball valve may be closed, thus disabling the Hot Gas By-Pass feature.
Refrigerant Cycle (Cooling Mode)
The air conditioners use R-410A refrigerant in a conventional vapor-compression refrigeration cycle to
transfer heat from air in an enclosed space to the outside. A motorized impeller assembly blows indoor
air across the evaporator. Cold liquid refrigerant passing through the evaporator is boiled into gas by
heat removed from the air. The warmed refrigerant gas enters the compressor where its temperature
and pressure are increased. The hot refrigerant gas condenses to liquid as heat is transferred to outdoor
air drawn across the condenser by the condenser fan. Liquid refrigerant is metered with a thermal
expansion valve (TXV) into the evaporator to repeat the cycle.
MGA3090A/3120A/3150A (Dual Compressor) Units:
These units are factory wired for maximum
cooling utilizing both compressors. If 2 stage compressor operation is desired, the factory installed
jumper between terminals 1 and 2 of the low voltage terminal strip must be removed. The 1st stage
cooling input is terminal 1 of the low voltage terminal strip and the 2nd stage cooling input is terminal
2 of the low voltage terminal strip. The thermostat must be programmed for 2 stage cooling operation
when 2 stage compressor operation is desired.
Heating Mode
A wall-mounted thermostat controls the heating cycle of models which incorporate resistance heating
elements. On a call for heat, the thermostat closes the heat relay to energize the indoor fan and the
resistance elements.
1.6 Programmable Logic Controller (MGA3120)
MGA3120A-3180A models include a factory installed PLC
to control the operation of the HVAC system (MGA3090
models are standard with the Marvair printed circuit board,
see Chapter 2). LEDs on the PLC show operational status and
provide assistance with diagnosis if troubleshooting is ever
required. Various control functions are field selectable and
programmable. The PLC is also capable of communicating
to other PLCs to allow run time leveling and does not require
additional equipment to be installed. The PLC 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.
Figure 1.
Programmable Logic Controller
The PLC is factory installed and tested, requires no adjustments or changes when the air conditioning
system is installed.
Location
The PLC is located in the unit control panel. Remove the HVAC unit front access panel and open the
panel door to the control cabinet.
