Lennox LGT Series Unit Information page 53

First Stage Heat:
1 - Heating demand initiates at W1 in the thermostat.
2 - 24VAC is routed through the A55 unit controller to
A3 Ignition Control. The Ignition control then routes
the 24VAC to the N.C. primary limit S10. The A3
Ignition control energizes the combustion air blower
B6.
3 - After the combustion air blower B6 has reached
full speed, the combustion air proving switch S18
contacts close. The A3 routes 24VAC through N.C.
burner flame roll-out switch S47 and the closed
contacts of combustion air proving switch S18 to
energize the ignition module A3.
4 - After a 30 second delay A3 energizes the ignitor
and LO terminal (low fire) of gas valve GV1.
Second Stage Heat:
5 - With first stage heat operating, an additional heating
demand initiates W2 in the thermostat.
6 - A second stage heating demand is received by A55
Unit Controller.
7 - A55 provides the 24VAC to the A3 Ignition control.
This is routed to the HI Terminal (high fire) of gas
valve GV1.
GAS HEAT SEQUENCE OF OPERATION LGT092H-150H
End of Second Stage Heat:
8 - Heating demand is satisfied. Terminal W2 (high fire)
is de-energized.
9 - Terminal HI of GV1 is de-energized by A3 control
module.
End of First Stage Heat:
10 - Heating demand is satisfied. Terminal W1 (low fire)
is de-energized.
11 - Ignition A3 is de-energized by control module A55 in
turn de-energizing terminal LO of GV1. Combustion
air blower relay K13 located in the A3 ignition control
is also de-energized.
Optional Low Ambient Kit:
(C.S.A. -50° C Low Ambient Kit)
12 - Line voltage (or transformer T20 in 460V and 575V
only) is routed through the low ambient kit fuses
F20 and N.C. low ambient kit thermostats S60 and
S61,to energize low ambient kit heater HR6.
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