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Sequence of Operation HPXA16−024/060
First Stage Cool (low capacity)
Transformer from indoor unit supplies 24VAC power to
the thermostat and outdoor unit controls.
1− Internal wiring energizes terminal O by cooling mode
selection, energizing the reversing valve. Cooling de-
mand initiates at Y1 in the thermostat.
2− 24VAC passes through high pressure switch S4 and
discharge thermostat switch S5 energizing compres-
sor contactor K1.
HPXA16−024 − Compressor begins start up. Relay
K31 remains closed during start up and capacitor
C7 remains in the circuit. As compressor speeds up
K31 is energized, de−energizing capacitor C7.
3− K1−1 N.O. closes energizing compressor B1 and out-
door fan motor B4. Transformer T46 is also energized.
4− Solenoid L34 is NOT energized. The slider ring re-
mains open limiting compressor to low capacity.
Second Stage Cool (high capacity)
5− Second stage thermostat demand energizes solenoid
relay K195. K195−1 closes sending voltage to rectifier
plug D4. D4 converts the AC voltage to DC voltage and
energizes L34 unloader solenoid. L34 then closes the
slider ring, allowing the compressor to operate at high
capacity.
Heating
A – Low Capacity
1−
Room thermostat in heating mode. Room thermostat
outputs Y1 signal to the defrost board in the heat pump
and to the indoor air handler.
2 − The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3− K1−1 closes, energizing the compressor and outdoor
fan motor through the normally closed fan relay con-
tacts on the defrost board.
4− The compressor will run on low capacity along with the
single speed outdoor fan motor.
B – High Capacity (Ambient temperature above de-
frost board Y2 lock−in temperature)
1− Room thermostat in heating mode. Room thermostat
outputs Y1 and Y2 (if applicable to that room thermo-
stat) signal to the defrost board in the heat pump and to
the indoor unit.
2− The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3 − The defrost board sends 24 volts through Y2 OUT to
the L34 compressor solenoid plug. The 2− wire com-
pressor solenoid plug converts the 24volt AC outputs
to a 24volt DC signal input to the L34 internal high ca-
pacity solenoid valve in the compressor.
4 − K1−1 closes, energizing the compressor and outdoor
fan motor through the normally closed fan relay con-
tacts on the defrost board. The compressor runs high
capacity.
B – High Capacity (Ambient temperature below de-
frost board Y2 lock−in temperature)
1 − Room thermostat in heating mode. Room thermostat
outputs Y1 signal to the defrost board in the heat pump
and to the indoor unit.
2 − The defrost board checks for open low or high−pres-
sure switches and proper coil, ambient and discharge
sensor readings. If checks show no issues, the defrost
board sends 24 volts through Y1 OUT signal to the K1
compressor contactor coil.
3 − The defrost board Y2 locks in sends 24 volts through
Y2 OUT to the L34 compressor solenoid plug. The plug
converts the 24volt AC outputs to a 24volt DC signal in-
put to the L34 internal high capacity solenoid valve in
the compressor.
4 − K1−1 closes, energizing the compressor and outdoor
fan motor through the normally closed fan relay con-
tacts on the defrost board. The compressor runs on
high capacity.
When a defrost cycle is initiated, the control ener-
gizes the reversing valve solenoid and turns off the
condenser fan. The control will also put 24VAC on
the W1" (auxiliary heat) line. The unit will stay in this
mode until either the coil sensor temperature is
above the selected termination temperature, the de-
frost time of 14 minutes has been completed, or the
room thermostat demand cycle has been satisfied.
(If the temperature select shunt is not installed, the
default termination temperature will be 90°F.) If the
room thermostat demand cycle terminates the
cycle, the defrost cycle will be held until the next
room thermostat demand cycle. If the coil sensor
temperature is still below the selected termination
temperature, the control will continue the defrost
cycle until the cycle is terminated in one of the meth-
ods mentioned above. If a defrost is terminated by
time and the coil temperature did not remain above
35°F (2°C) for 4 minutes the control will go to the
34−minute Time/Temperature mode.
Page 23
Defrost Mode
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