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LGH092/150S & H SEQUENCE OF OPERATION
Power:
1- Line voltage through the S48 unit disconnect, TB2 terminal block, or CB10 circuit breaker energizes the T1 trans
former. 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 contac
tor, condenser fan relays and exhaust fan relays.
Blower Operation:
3- The A55 Unit Controller module receives a demand from thermostat terminal G. A55 energizes blower contactor
K3 with 24VAC.
4- N.O. K3-1 closes, energizing blower B3.
Economizer Operation:
5- A55 receives a demand and energizes exhaust fan relay K65 with 24VAC at 50% outside air damper open (ad
justable).
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, N.C. freezestat S49 and N.C. high pressure switch S4, compres
sor contactor K1 is energized.
9- N.O. contacts K1-1 close energizing compressor B1.
10- At the same time, A55 energizes condenser fan relays K10 (when N.O. low ambient switches S11 and S84 close)
and K68.
11- N.O. contacts K10-1 close energizing condenser fan B4 and N.C. contacts K10-2 open, de-energizing compres
sor crankcase heaters HR1 and HR2. 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, N.C. freezestat S50 and N.C. high pressure switch S7, compres
sor contactor K2 is energized.
14- N.O. contacts K2-1 close energizing compressor B2.
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