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Page 27
1 − When there is a call for heat, W1 of the thermostat energizes W of the furnace control with 24VAC.
2 − Integrated control runs a self−check. S10 primary limit, S47 rollout switch and S21 secondary limit contacts are found to be closed. Call for
heat can continue.
3 − Integrated control (A92) energizes combustion air inducer B6. Combustion air inducer runs until S18 combustion air pressure switch closes .
Once S18 closes, a 15−second pre−purge follows.
4 − Integrated control (A92) energizes ignitor. A 20−second warm−up period begins.
5 − Gas valve opens for a 4−second trial for ignition
6 − Flame is sensed, gas valve remains open for the heat call.
7 − After 45−second delay, integrated control (A92) energizes indoor blower B3.
8 − When heat demand is satisfied, W1 of the indoor thermostat de−energizes W of the ignition control which de−energizes the gas valve
Combustion air inducer B6 continues a 5−second post−purge period, and indoor blower B3 completes a selected OFF time delay.
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6
7
1
2
3
4
3
8
FIGURE 32
ML180UH Schematic Wiring Diagram and Sequence of Operation