Lennox LGA Series Unit Information page 50

First or Single Stage Heat:
1. Heating demand initiates at W1 in the thermostat.
2. 24VAC is routed through TB34 to the main control
module A55. After A55 proves N.C. primary limit S21,
the combustion air blower relay K13 is energized.
3. N.O. K13-1 contacts close allowing line voltage to en-
ergize combustion air blower B6.
4. After the combustion air blower B6 has reached full
speed, the combustion air proving switch S18 contacts
close. The A55 routes 24VAC through N.C. burner
flame rollout switch S47 and the closed contacts of
combustion air proving switch S18 to energize the igni-
tion module A3.
5. After a 30 second delay A3 energizes the ignitor and
W1 terminal (low fire) of two stage gas valve GV1.
Second Stage Heat:
6. With first stage heat operating, an additional heating
demand initiates W2 in the thermostat.
SEQUENCE OF OPERATION LGA036/072 GAS HEAT
Page 50
7. A second stage heating demand is received by A55
control module.
8. A55 energizes W2 terminal (high fire) of gas valve
GV1.
End of Second Stage Heat:
9. Heating demand is satisfied. Terminal W2 (high fire) is
de-energized.
10. Terminal W2 of GV1 is de-energized by A55 control
module.
End of First Stage Heat:
11. Heating demand is satisfied. Terminal W1 (low fire) is
de-energized.
12. Ignition A3 is de-energized by control module A55 in
turn de-energizing terminal W1 of GV1. Combustion
air blower relay K13 is also de-energized.
Optional Low Ambient Kit:
(C.G.A. -50° C Low Ambient Kit)