Lennox ML193UHE Series Unit Information page 49

VII-WIRING DIAGRAM AND SEQUENCE OF OPERATION
1 - When there is a call for heat, W1 of the thermostat energizes W of the furnace control with 24VAC.
2 - S10 primary limit switch and S47 rollout switch are closed. Call for heat can continue.
3 - The integrated control (A92) energizes combustion air inducer B6. Combustion air inducer runs until S18 combustion air prove switch closes
(switch must close within 2-1/2 minutes or control goes into 5 minute Watchguard Pressure Switch delay). Once S18 closes, a 15-second pre-
purge follows.
4 - The 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 30-second delay, the integrated control (A92) energizes indoor blower B3.
8 - When heat demand is satisfied, W1 of the indoor thermostat de-energizes W of the integrated 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
FIGURE 61
Page 49
.