Lennox LHT Series Service Manual page 44

Sequence of Operation LHT156/180/240
1 - Line voltage from TB13 energizes transformer T1 and T18. Transformer T1 and T18 provides 24VACpower to the main controller A55. The transformers also
provides 24VAC power to the unit cooling, heating and blower controls and thermostat.
ECONOMIZER OPERATION
2 - The A55 Unit Controller receives a demand and energizes exhaust fan relay K65 and K231 with 24VAC at 50% (travel) outside air damper open (adjustable).
3 - N.O. K65-1, K65-2, K231-01 and K231-02 close, energizing exhaust fan motors B10 and B11.
1ST STAGE COOLING
4 - First stage cooling demand energizes Y1 and G in the thermostat. G energizes blower, if blower is not already running.
5 - 24VAC is routed to the A55 Unit Controller. After A55 proves N.C. low pressure switch S87 and high pressure switch S4, compressor contactor K1 and L34
are energized.
6 - A55 energizes outdoor fan B21 directly and fans B4 and B5 through K10.
7 - N.O. K1 closes energizing compressor B1, and N.C. K1-52 opens denergizing HR1.
2ND STAGE COOLING
8 - Second stage cooling demand energizes Y2.
9 - After A55 proves N.C. low pressure switch S88 and N.C. high pressure switch S7, contacotor K2 is energized.
10 - N.O. K2 closes energizing compressor B2 and K2-52 opens de-energizing crankcase heater HR2.
BLOWER OPERATION With By Pass
1 - Control inverter A96 energizes contactor K202 and main control A55 energizes relay 203.
2 - K203-1 N.C. contacts open to de-energize K3 relay coil. K3 contacts open to interrupt power to B3 blower motor through K3 N.O. relay contacts.
3 - K202 contacts close to allow power to B3 blower motor from A96.
BLOWER OPERATION No By-Pass
1 - Control inverter A96 energizes B3 on a G demand.
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