Wiring And Sequence Of Operation - Lennox F8AUH series Service Manual

VII− Wiring and Sequence of Operation
1 − When there is a call for heat, W1 of the thermostat en-
ergizes W of the furnace control with 24VAC.
2 − S10 primary limit switch and S47 rollout switch are
closed. Call for heat can continue.
3 − Control board energizes combustion air blower B6.
Combustion air blower 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−se-
cond pre−purge follows.
4 − Control board energizes ignitor. An 11 to 21 second
warm−up period begins.
5 − Gas valve opens for a 4−second trial for ignition (igni-
tor will remain energized for the first second).
6 − Flame is sensed, gas valve remains open for the heat
call.
7 − After 45−second delay, control board energizes in-
door blower B3.
8 − When heat demand is satisfied, W1 of the indoor ther-
mostat de−energizes W of the control board which de−
energizes the gas valve. Combustion air blower B6
continues a 5−second post−purge period, and indoor
blower B3 completes a selected OFF time delay.
Page 19