Lennox LGT Series Unit Information page 59

Power:
1 - Line voltage through the S48 unit disconnect, TB2 terminal block, or CB10 circuit breaker energizes the T1 transformer. T1 provides 24VAC power to A55
Unit Controller which provides 24VAC to the unit cooling, heating and blower controls.
2 - Line voltage is also routed to compressor crankcase heaters, compressor contactors, the blower motor, condenser fan relays and exhaust fan relays.
Blower Operation:
3 - The A55 Unit Controller module receives a demand from thermostat terminal G.
4 - B3 recieves the pre-set blower setting through MODUS.
Economizer Operation:
5 - A55 receives a demand and energizes exhaust fan relay K65 with 24VAC at 50% outside air damper open (adjustable).
6 - N.O. K65-1 and N.O. K65-2 both close, energizing exhaust fan motor B10.
1st Stage Cooling (compressor B1)
7 - A55 receives a Y1 thermostat demand.
8 - After A55 proves N.C. low pressure switch S87, RT46 reading above freeze point and N.C. high pressure switch S4, compressor contactor K1 is energized.
9 - N.O. contacts K1-1 close energizing compressor B1. Crankcase heater HR1 is de-energized.
10 - At the same time, A55 energizes condenser fan relays K10 and K68.
11 - N.O. contacts K10-1 close energizing condenser fan B4 and N.O. contacts K68-1 close energizing condenser fan B5.
2nd Stage Cooling (compressor B2 is energized)
12 - A55 receives a Y2 thermostat demand.
13 - After A55 proves N.C. low pressure switch S88, RT47 reading above freeze point, and N.C. high pressure switch S7, compressor contactor K2 is energized.
14 - N.O. contacts K2-1 close energizing compressor B2. Crankcase heater HR2 is de-energized.
3nd Stage Cooling (compressor B1 in full load and compressor 2 is energized)
15 - A55 receives a Y3 thermostat demand (Y1 + Y2 thermostat inputs).
16 - A55 sends 24VAC to B1 compressor solenoid (L14), B1 compressor runs at full load.
LGT092H-150H SEQUENCE OF OPERATION
Page 59