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Marvair MAA/MGA Wall Mount AC Installation & Operation Manual
05/2023 Rev.21
Figure 3. Dry Bulb Sensor
On a call for cooling from the wall-mounted thermostat, if outdoor conditions are suitable, the
sensor will open the damper and admit outside air (i.e., economizer free cooling). If the outdoor
ambient is too hot (dry bulb sensor only), the sensor will place the actuator in the closed or
minimum open position and activate mechanical cooling. The compressor is locked-out during
the economizer cooling mode. The Supply Air sensor monitors the supply air of the system in
economizer mode to modulate the economizer damper to maintain a 55F supply air temperature.
During the testing of the air conditioner at the factory, the control board has been configured for
the sensor type used in the system. There should be no need to change the sensor configuration. If
a dry bulb sensor is being used, setting 13 of the control board configuration menu will be set for
DB=Dry Bulb. A disconnected or faulty dry bulb sensor is annunciated on the control board
display as “db-F”. Economizer operation is only allowed in Cooling Mode.
Note:
A call for Y2 or FC Inputs or a faulty or disconnected dry bulb sensor will override the
economizer operation and force the compressor to operate.
3.3 Supply Air Sensor –
The supply air sensor is a 10K NTC sensor probe mounted on a bracket
in the supply air opening of the system. The probe senses the air temperature entering the
structure, and provides a signal to the economizer controller for modulating the position of the
damper. The default supply air temperature is 55F in economizer cooling and is not adjustable. A
disconnected or faulty supply air sensor is annunciated on the control board display as “AS-F”.
Note:
A faulty or disconnected supply air sensor will override the economizer operation and
force the compressor to operate.
3.4 Minimum Position –
The minimum position controls the amount of outside air introduced in
the building when not in economizer operation or to meet a minimum outside air requirement to
keep the damper open. This feature is only applicable to systems with economizer or ventilation
damper options. The minimum position can be adjusted in the control board configuration menu
with setting 15. The setpoint adjustments are from 20 (2vdc) to 100 (10vdc) in 1vdc increments.
This is an independent adjustment and can be set to drive the ventilation damper open to any
position from closed to fully opened. A selection of 20 (2vdc) is the same as being set to OFF
or fully closed.
4.0 Heating Operation
4.1 Electric Heat (AC Systems with Electric Heat) –
When there is a request for “Electric Heat”
(W2-signal via Digital input or Modbus), the system will run the Indoor Motor for 10 seconds
prior to energizing the Heater Output. The Indoor Motor will operate at the “W2” speed setting
on the board or via Modbus. Once the Heater output is energized, the Heater comes on at full
capacity (no staging). The Heater will remain on until the Heating setpoint is satisfied and the
request is dropped. Once the heating setpoint is satisfied, the Indoor motor continues to run based
