24VNA6 / 25VNA4: Service Manual
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
9
CONTROL FEATURES AND OPERATION
Equipment Configuration (Model Plug)
The PCM detects the model plug value at power-up and configures
operation of the equipment to the model identification specified by the
model plug for the remainder of the power cycle. Model plug
diagnostics will occur while the equipment is operating, but changes to a
model plug will not affect equipment operation until power is cycled.
The PCM remembers the most recent valid model plug detected at
power-up. If a model plug is lost or missing during equipment
operation, the PCM will continue to operate the equipment based on the
most recent valid model plug detected at power-up. Model plug memory
is preserved across power cycles, so if a model plug becomes missing,
proper equipment operation will continue even if power to the
equipment is lost and then restored.
An installed model plug at power-up takes precedence over a model plug
value stored in memory. If a new valid model plug value is detected at
power-up, the model plug memory will be updated to that new value.
Performance Mode
The PCM supports equipment operation in two performance modes:
Efficiency and Comfort. Efficiency mode provides maximum possible
operating efficiency. Comfort mode modifies operation to provide
optimal comfort to the home.
Compressor Operation
Compressor Protect Delay
The compressor is inhibited from running within 5 minutes of power
being applied to the equipment or within 5 minutes of previous
operation. The Compressor Protect Delay can be overridden as described
in Overrides in the Troubleshooting section.
Starting
The compressor will not start if the differential pressure between suction
and discharge is greater than 100 psi. When a capacity demand is
received, the outdoor fan begins operating first. Once the fan begins
operating, the compressor is started. The compressor will ramp to 2,700
rpm and dwell at that speed until the required amount of discharge super
heat is developed.
Running
Once start-up super heat is reached, the compressor will ramp up or
down to the appropriate continuous operating speed. Compressor
operating speed depends on the capacity being requested by the Infinity
Control, the Operating Mode (Cooling vs. Heating), the outdoor air
temperature, and the Performance Mode setting.
Compressor Reliability Protection
During operation, the PCM monitors the operating point of the
compressor relative to the reliability operating envelope. If operation
attempts to exceed a reliability limit, system operation is modified to
bring operation back within the envelope. Examples of modified
operation include increasing fan speed and reducing compressor speed.
During extreme operating conditions, operation may not be able to be
maintained within the reliability envelope. If this occurs, the PCM will
shut down the system and set a diagnostic code indicating the type of
limit being exceeded. Extreme operating conditions are usually partially
or completely created by non-ideal operating conditions that can be
corrected by equipment service. Examples of such conditions include
blocked or partially blocked coil on the indoor or outdoor equipment,
and under-charged or over-charged systems.
Sump Heat
When the compressor is not running, the compressor sump may need to
be heated to prevent liquid refrigerant from collecting in the sump. This
condition is most likely with reduced outdoor temperatures.
Some systems use an external Crankcase Heater installed around the
bottom portion of the compressor. Other systems may use a stator
heating function where the VFD flows a DC current through the motor
windings. This stator current heats the motor. That heat is transferred to
the compressor shell and heats the compressor sump.
Fan Operation
The fan generally runs when the compressor is running. When the
equipment begins to operate, the fan will start first, then the compressor
will start. Fan speed is adjusted based on the Operating Mode (Cooling
vs. Heating), the outdoor air temperature and compressor speed.
Intermittent Low Fan Speed
In some operating conditions such as Low Ambient Cooling, airflow
requirements are below the minimum fan speed. In these cases, the fan
will operate intermittently. The fan on/off cycle during this operation is
approximately 40 seconds.
Fan Operating Exceptions
There are some operating conditions where the fan will operate
independently of the compressor. During the defrost cycle, the fan will
not operate while the compressor is running. It is possible for the
equipment to shut down during a defrost cycle. On the next operating
cycle, the equipment will resume in defrost mode. In this case, the fan
will run briefly at power-up and then shut down while the previously
interrupted defrost cycle completes.
The fan may intermittently shut down during Low Ambient Cooling
operation, especially at colder temperatures and if the wind is blowing
during operation.
If the compressor is shut down due to a discharge pressure limit or VFD
or compressor high discharge temperature limit, the fan will continue to
run for a period of time after the compressor shuts down in order to
alleviate the excessive temperature or pressure condition.
Cooling Control
Cooling mode will operate at any outdoor ambient temperature of 55°F
(12.78°C) or higher. There is no upper limit of outdoor ambient
temperature operation. Full cooling capacity is available up to at least
125°F (51.67°C) for properly installed and maintained equipment. If
Low Ambient Cooling is enabled on the Infinity Control, the cooling
mode will operate down to 0°F (-17.78°C).
Heating Control
(
25VNA4
Only)
Heating mode will operate up to an outdoor ambient temperature of 75°F
(23.9°C). Heating mode will start down to 0°F (-17.78°C). If the
equipment is already operating when ambient temperature drops below
0°F (-17.78°C), the equipment will continue to operate until some limit
is reached. Heating capability should be available down to -10°F
(-23.3°C) or lower for properly installed and maintained equipment.
Super Heat Control
Super heat control is provided by the EXV-H. The PCM continuously
adjusts the EXV-H to keep suction super heat operating at the target
control value.
Defrost Mode
(25VNA4 Only)
Defrost mode is part of Heating Control. Because the outdoor coil
operates at a temperature below ambient, it has the potential for
condensing moisture from the air. When the coil temperature is at
freezing conditions, that condensation will occur as frost. As frost builds
on the coil, the frost acts as an insulator and reduces the heating capacity
of the coil. Periodic defrost is required to remove frost and restore
heating capacity.
