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Marvair GAA A/C w/Gas Heat I&O Manual
06/2023 Rev.5
The sensor is secured to the common suction line of the system to monitor the suction line
temperature.
This condition describes the abnormal fall in Suction Line Temperature below 9ºF. This fault
will only be active when the Low-Pressure Sensor detects suction line temperatures below 9ºF
during a request for Cooling / Heat pump heating. The Low-Pressure Sensor is bypassed on the
initial call for cooling for 3 minutes to allow low ambient start-up of the system. Once these 3
minutes have elapsed plus a 2-minute validation timer (5 minutes total), if the sensor remains
below 9ºF, the system cuts the compressor off. The system will remain off for the defaulted
3-minute anti-short cycle time and then the system will restart the cooling / heat pump heating
operation and the 3-minute bypass timer begins. Once these 3 minutes have elapsed plus a
2-minute validation timer (5 minutes total), if the sensor remains below 9ºF the system locks
out. Lockouts can be monitored using the control board display for the current fault. The fault
for a low-pressure lockout will displayed as “LP-1” or “LP-2” followed by “F”. The faults can
also be monitored via Modbus by reading the respective value based on the Modbus map that
is provided. After this lockout condition is reached, the cooling/heat pump heating call must be
cycled (on/off of respective request) or the system must be power cycled to clear the fault. For
a 2-compressor system, each lockout is isolated to the respective circuit and will not interfere
with the operation of the other circuit providing that the circuits operate independently. The
system will continue to lockout until the problem is rectified.
6.3 Anti-Short Cycle –
This is a built-in protection mechanism that increases the reliability of the
compressor by protecting it from excessive short cycling. When the compressor goes off, due
to any fault, emergency or if the cooling / heat pump heating setpoint is satisfied, a built-in
3-minute timer locks the compressor for that respective circuit out. This can be monitored and
adjusted via Modbus by referencing the appropriate register. However, it can only be monitored
at the board level by waiting for the 3 minutes to elapse.
7.0 Additional Features
7.1 Modulating Head Pressure Control (ECM Outdoor Motors) –
A10K Nominal NTC Thermistor
is connected to the “FCC1” and “FCC2” inputs for circuit 1 and circuit 2 respectively. The
sensor is secured to the liquid line of the system to monitor the liquid line temperature. When
this sensor is connected, the thermistor value is used as the process variable when controlling the
head pressure. The controller modulates the Outdoor Fan Motor to maintain a 90ºF temperature
setpoint. The controller will always use the greater value of FCC1 and FCC2 input as the reference
point. In the event that the Thermistor is disconnected, the controller reverts to the outdoor fan
operating at the max speed setting selected on the control board configuration menu setting 7.
Note:
Systems with PSC outdoor motors utilize a fan cycle switch in the refrigeration circuit
for on/off fan cycle head pressure control.
PSC Motor Head Pressure Control
The onboard outdoor fan relay (OFR) is used to control the Fan Cycle Switch based on CC
output to the Compressor. Once the compressor out (CC1) is energized the OFR output will
also be energized and the fan cycle switch will cycle on and off at 290psi and 400psi. This will
turn the Outdoor Fan Motor on and off to maintain the head-pressure of the system, the OFR
output is only used on systems with PSC out door motor and Fan Cycle Switch (FCC) only.
7.2 Emergency Ventilation (Motorized Damper or Economizer Equipped Systems) –
When there
is a request for “Emergency Ventilation” (EV-signal via Digital input or Modbus), the system
will run the Indoor Motor at the “Y2” speed setting on the board or via Modbus and provide a
