Marvair MAA/MGA Wall Mount AC Installation & Operation Manual
05/2023 Rev.21
29
on the fan purge timer setpoint (default 90 seconds) selected in setting 3 of the configuration
menu of the control board.
Note:
A request for Cooling and Heating at the same time is not allowed and control board will
not allow the electric heat to operate.
4.2 Heat Pump Heating Operation –
This section will describe the sequence of operation which
takes place during Direct Expansion (DX) heating from a control standpoint. The reversing
valve (O Input) must NOT be energized for Heating operation on Heat Pump systems.
Note:
The system type must be configured for “HP” operation in setting 1 of the configuration
menu of the control board for proper HP system operation. This system type is selected at the
factory and there’s no need to change this setting.
In normal operating mode, a request for “Heat Pump Heating” Y1 signal via Digital input or
Modbus), energizes the Compressor 1 and 2 Relay Output (CC1+CC2) on the PCB. The controller
provides a continuous control signal associated to the Indoor Fan Motor, that is proportional to
the “Y2” speed setting on the board or via Modbus. The system is defaulted to operate at Full
capacity in heat pump heating mode. It also outputs a request for the Outdoor Fan to operate at
the max speed. In heat pump Heating operation, the compressor operates at Full capacity and
the indoor and outdoor fans remain on continuously. The outdoor fan does not modulate in heat
pump heat mode.
4.3 Defrost Operation (Heat Pump Systems Only) –
The board has an integral accumulation run
timer that runs on every request for heat pump heating. The system has a Defrost Sensor (DS)
that monitors the outdoor coil circuit for freezing conditions. If the defrost sensor registers a
temperature of 32°F while in the heat pump heating mode, it will initiate a defrost cycle based
on the time selected in the control board configuration menu setting 8. The defrost time intervals
selections are 30, 60 and 90 minutes and the factory default setting is 30 minutes. If the defrost
sensor is still calling for defrost at the end of this delay, it will de-energize the outdoor fan,
energize the reversing valve through the RV terminal, and keep the compressor energized. The
defrost cycle will terminate on time or temperature. It will have a maximum defrost run time
of 10 minutes, however, if the defrost sensor registers a temperature of 70°F or higher before
the 10-minute maximum run time, the defrost cycle will terminate. If the defrost cycle has not
completed (DS sensor above 70F) due the heating call being satisfied, the defrost cycle will
continue upon the next call for heat pump heating to complete the defrost cycle. If freezing
conditions are not present at the end of the defrost interval timer, the system will not perform
a defrost cycle.
Note:
If the DS sensor is disconnected or faulty, the compressor will not operate in heat pump
heating mode and the control board display will annunciate the fault as “DS-F”.
Note:
If “Yes” has been selected for Electric Heat During Defrost (EHDD) and “No” selected
for the SCKT, the electric heat (EH) output will be energized during a defrost cycle to supply
heat while the coil defrosts.
If the ODS is used, the electric heat will operate only if the outdoor
temperature is below the ODS setpoint.
4.4 Electric Heat / Outdoor Sensor / SCKT / EHDD (Heat Pumps) – Electric Heat (HP 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
