Page 14 of 24
506468-01
Issue 1946
Operation
Outdoor unit and indoor blower cycle on demand from
the room thermostat. When the thermostat blower switch
is moved to the ON position, the indoor blower operates
continuously.
Filter Drier
The unit is equipped with a large capacity bi-flow filter
which keeps the system clean and dry. If replacement is
necessary, replace with one of similar design and capacity.
The replacement filter drier must be suitable for use with
R410A refrigerant.
Crankcase Heater
If unit is equipped with a crankcase heater, it should
be energized 24 hours before unit start-up to prevent
compressor damage as a result of slugging.
Emergency Heat Function (Room Thermostat)
See Figure 6 for thermostat designations.
An emergency heat function is designed into some room
thermostats. This feature is applicable when isolation of
outdoor unit is required or when auxiliary electric heat is
stage by outdoor thermostats. When the room thermostat
is placed in the emergency heat position, the outdoor unit
control circuit is isolated from power and the field-supplied
relays bypass the outdoor thermostats. An amber indicating
light simultaneously comes on to remind the homeowner
that the unit is operating in the emergency heat mode.
Emergency heat is usually used during an outdoor
shutdown, but it should also be used following a power
outage if power has been off for over an hour and the
outdoor temperature is below 50°F. System should be left
in the emergency heat mode at least 6 hours to allow the
crankcase heater sufficient time to prevent compressor
slugging.
Pressure Switch Connections
The unit’s automatic reset pressure switches (LO PS - S87
and HI PS - S4) are factory-wired into the control board on
the LO-PS and HI-PS terminals, respectively.
Low Pressure Switch (LO-PS)
When the low pressure switch trips, the control board will
cycle off the compressor, and the strike counter in the
board will count one strike. The low pressure switch is
ignored under the following conditions:
• during the defrost cycle and 90 seconds after the
termination of defrost
• when the average ambient sensor temperature is
below 15° F (-9°C)
• for 90 seconds following the start up of the compressor
• during “test” mode
High Pressure Switch (HI-PS)
When the high pressure switch trips, the control board
will cycle off the compressor, and the strike counter in the
board will count one strike.
Pressure Switch Settings
• High Pressure (auto reset)
- trip at 590 psig; reset
at 418.
• Low Pressure (auto reset)
- trip at 25 psig; reset at
40.
5-Strike Lockout Feature
The internal control logic of the board counts the pressure
switch trips only while the Y1 (Input) line is active. If a
pressure switch opens and closes four times during a Y1
(Input), the control logic will reset the pressure switch trip
counter to zero at the end of the Y1 (Input). If the pressure
switch opens for a fifth time during the current Y1 (Input),
the control will enter a lockout condition.
The 5-strike pressure switch lockout condition can be reset
by cycling OFF the 24-volt power to the control board or by
shorting the TEST pins between 1 and 2 seconds. All timer
functions (run times) will also be reset.
If a pressure switch opens while the Y1 Out line is engaged,
a 5-minute short cycle will occur after the switch closes.
Demand Defrost System
The demand defrost system measures differential
temperatures to detect when the system is performing
poorly because of ice build-up on the outdoor coil. The
system “self-calibrates” when the defrost system starts
and after each system defrost cycle. The demand defrost
components on the control board are listed below.
NOTE:
The demand defrost system accurately measures
the performance of the system as frost accumulates on the
outdoor coil. This typically will translate into longer running
time between defrost cycles as more frost accumulates on
the outdoor coil before the board initiates defrost cycles.
Defrost System Sensors
Sensors connect to the control board through a field–
replaceable harness assembly that plugs into the board.
Through the sensors, the board detects outdoor ambient,
coil, and discharge temperature fault conditions. As the
detected temperature changes, the resistance across the
sensor changes. Sensor resistance values can be checked
by ohming across pins.
NOTE:
When checking the ohms across a sensor, be
aware that a sensor showing a resistance value that is not
within the range shown, may be performing as designed.
However, if a shorted or open circuit is detected, then the
sensor may be faulty and the sensor harness will needs to
be replaced.
