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SEQUENCE OF OPERATION
FIRST STAGE HEAT:
1 − Heating demand initiates at W1 in thermostat.
2 − 24VAC is routed through TB35 to the main con-
trol module A55. After A55 proves N.C. primary
limit S10 and N.C. secondary limit S21 the com-
bustion air blower relay K13 is energized.
3 − N.O. K13-1 contacts close allowing line voltage
(or transformer T3 in 460V and 575V only) to ener-
gize combustion air blower B6.
4 − After the combustion air inducer B6 has reached full
speed, the combustion air proving switch (S18) con-
tacts close. The A55 routes 24VAC through N.C.
burner 1 flame rollout switch S47 and the closed con-
tacts of the combustion air proving switch (S18) to
energize the ignition module A3. After a 30 second
delay A3 energizes gas valve GV1 on low fire.
5 − As steps 2, 3 and 4 occur, 24VAC is also routed to the
gas valve control module A58. After A58 proves N.C.
primary gas heat limit S99 and N.C. secondary limit
S100 the combustion air inducer relay K19 is ener-
gized.
6 − N.O. K19-1 contacts close allowing line voltage
(or transformer T13 in 460V and 575V only) to
energize combustion air inducer B15.
7 − After the combustion air inducer B15 has reached full
speed, the combustion air proving switch (S45) con-
tacts close. The A58 routes 24VAC through N.C.
burner 2 flame rollout switch S69 and the closed con-
tacts of the combustion air proving switch (S45) to
energize the ignition module A12. After a 30 second
delay A12 energizes gas valve GV3 on low fire. In-
door blower energizes after time delay Time delay is
field adjustable with a factory set default of 40 sec-
onds.
SECOND STAGE HEAT:
8 − With first stage heat operating, an additional
heating demand initiates W2 in the thermostat.
9 − A second stage heating demand is received by both
A55 and A58 modules.
10 − Each module will energize the corresponding gas
valves GV1 and GV3 on high fire.
OPTIONAL LOW AMBIENT KIT
(C.G.A. -50°C LOW AMBIENT KIT):
11 − 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.
END OF SECOND STAGE HEAT:
12 − Heating demand is satisfied. Terminal W2 is
de-energized.
13 − High fire is de-energized on gas valves GV1
and GV3 by the A55 and A58 Module.
END OF FIRST STAGE HEAT:
14 − Heating demand is satisfied. Terminal W1 is
de-energized.
15 − Ignition module A3 is de-energized by A55 in turn
de-energizing low fire on GV1. Combustion in-
ducer relay K13 is also de-energized. At the same
instant, ignition module A12 is de-energized by
A58 module in turn de-energizing low fire on GV3.
K19 combustion air b inducerrelay is also de-ener-
gized.
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