Three-Phase Heating Sequence - Lennox HP21 Unit Information

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K−THREE-PHASE HEATING SEQUENCE
1− Speed control thermostat S55 determines whether the compressor is
to operate on high or low speed based on outdoor temperature. If out-
door temperature drops below set temperature, S55 closes to shunt
across Y2. When Y2 is shunted, a 2nd stage demand is sent to the two-
speed control and the compressor runs on high speed. If outdoor
temperature rises above set temperature, S55 opens and the com-
pressor runs on low speed.
2− 1st stage demand energizes TB15-Y1. TB15-Y1 energizes TSC,
speed control thermostat S55 and defrost control terminal HOLD. De-
frost control initializes timing sequence.
NOTE-2nd stage heating demand may be energized directly without
passing through 1st stage.
3− TSC delays 3 seconds before responding to the new command. Then
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2
S1, S2 DENOTE
TERMINAL
DESIGNATIONS
FIGURE 36
compressor begins operating at low or high speed as determined by
speed control thermostat S55.
4− 2nd stage heating demand energizes TB15-Y1 and TB15-W1.
TB15-Y1 energizes TSC, speed control thermostat S55 and defrost
control terminal HOLD. The compressor continues to operate at low or
high speed as determined by speed control thermostat S55. TB15- W1
energizes the indoor auxiliary heat relay for 2nd stage heat.
5− Speed control thermostat can open or close during 1st stage or 2nd
stage heating operation. When S55 opens, the compressor runs on
low speed. When S55 closes, the compressor runs on high speed. If
speed control thermostat S55 opens or closes during compressor op-
eration, TSC initiates speed change delay and stops all unit operation
for 60+5 seconds (indoor unit continues to operate and if 2nd stage
thermostat demand is present auxiliary heat continues operate). If all
safety circuits check-out, TSC energizes compressor.
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