Operation Sequence - Low Voltage; Unit Diagram Low Voltage Operation Sequence - Lennox HP22-261 Service Manual

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3
9
5
B-Operation Sequence - Low Voltage
1- Transformer in indoor unit supplies 24VAC power to
the thermostat and outdoor unit controls.
2- Cooling demand energizes thermostat terminals M
and R. Voltage from terminal M passes through high
pressure switch and energizes compressor contac
tor.
3- Thermostat demand (from thermostat terminal M) is
also supplied to the defrost control. Defrost control
cannot operate in cooling mode because defrost
thermostat cannot close.
4- Thermostat demand (from thermostat terminal R) en
ergizes reversing valve.
5- Heating demand energizes thermostat terminal M.
Voltage from terminal M passes through high pres
sure switch and energizes compressor contactor.
UNIT DIAGRAM
LOW VOLTAGE OPERATION SEQUENCE
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2
7
4
6- During heating operation, when outdoor coil drops
below 35 + 4_ F, the defrost thermostat closes.
When defrost thermostat closes, defrost timer begins
timing. If defrost thermostat remains closed at the
end of 30, 60 or 90 minutes, defrost relay energizes
and defrost begins.
7- When defrost relay energizes, reversing valve and in
door electric heat relay are energized.
8- Defrost continues until 14 + 1 minutes have elapsed
or until the defrost thermostat opens. When defrost
thermostat opens to terminate defrost, the defrost
timer loses power and resets. Defrost timing is
stopped until the next call for defrost (when defrost
thermostat closes).
9- After each thermostat demand, time delay locks-out
the circuit to conpressor contactor coil and defrost
control for 5 + 2 minutes. At the end of the timed peri
od, the time delay allows the compressor contactor
and defrost control to be energized upon demand as
in step 2.
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