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SEQUENCE OF OPERATION LHT078-150
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.T
he A55 Unit Controller module receives a demand from thermostat terminal G.
4.B3 receives 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.
First Stage Cooling Demand (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 condenser fans B4 and B5 and are energized.
Second Stage Cooling Demand (compressor B2)
11.A55 receives a Y2 thermostat demand.
12.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.
13.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)
14.A55 receives a Y3 thermostat demand (Y1 + Y2 thermostat inputs).
15.A55 sends 24VAC to B1 compressor solenoid (L14), B1 compressor runs at full load.
First Stage Heat -
1 Unit controller A55 receives W1 demand. If the OD air temperature is above the balance point setpoint ( 35 F default), HP heating is initiated.
2 After A55 proves N.C. low pressure switch S87, high pressure switch S4, compressors contactors K1, K2 are energized.
3 K1-1 and K2-1 close energizing compressor B1 and B2. K1 and K2 auxiliary switch open de-energizing crankcase heaters
4 Outdoor ECM fans B4, B5 and (B21 in 122, 150 units) receive pre-set fan settings at high speed from A55 unit controller
.
Second Stage Heat -
5 Second stage heat demand energizes W2 in the thermostat. A55 unit controller will activate electric heat in addition to heat pump operation.
6 See sequence of operation for electric heat.
Defrost Mode:
7 Defrost is enabled when outdoor coil temperature is below 35F. The Unit Controller will cycle in and out of defrost depending on the tempera
ture differen
ce between the outdoor coil and outdoor air temperature. Defrost is also initiated when the accumulated run time with the outdoor
coil temperature below 35F reaches six hours. Electric heat is energized during a defrost cycle to maintain discharge air temperature.
NOTE - Only one refrigerant circuit will go into defrost at a time.
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