Lennox SSB*H4 Manual page 18

VII − WIRING DIAGRAMS AND SEQUENCE OF OPERATION
Sequence of Operation SSB*H4
NOTE − First and second stage cool operate independent of each other and can modulate back and
forth according to thermostat demand.
First Stage Cool (low capacity)
1. Cooling demand initiates at Y1 in the thermostat.
2. Voltage from terminal Y passes through S4 high pressure switch, energizes K1 compressor
contactor, passes through S87 low pressure switch and returns to common side of the
24VAC power.
3. K1 closes energizing B1 compressor and B4 outdoor fan.
4. Solenoid L34 is NOT energized so the slider ring remains open, limiting compressor to low
capacity.
Second Stage Cool (high capacity)
Compressor is operating in first stage cool
5− Second stage thermostat demand sends 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.
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