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Sequence of Operation G21Q with GC3 Ignition Control
F−EXPLANATION OF LENNOX’ MODEL NUMBER DESIGNATIONS:
The following is an explanation of Lennox’ model number designations:
G21Q: Residential unit with a up flow cabinet and a multispeed blower motor.
G21V: Residential unit with a up flow cabinet and an electronically controlled blower
motor.
GSR21Q: Residential unit with down flow/horizontal cabinet and a multispeed
blower motor.
GSR21V: Residential unit with a down flow/horizontal cabinet and an electronically
controlled blower motor.
G−APPLICATION OF G21Q WITH GC3 IGNITION CONTROL
This flowchart is used to show the step by step sequence that takes place when thermostat demand
is sent to a G21Q. The sequence describes the actions of devices in the unit which control blowers,
fans, gas valve and other components in the system. The sequence is outlined by numbered steps
which correspond to circled numbers on the adjacent diagram.
NOTE−This is a basic operation sequence for the G21Q. The thermostat shown is a basic electro
mechanical thermostat .
Operation Sequence:
1− Line voltage feeds through door interlock switch S51. The blower access panel must be in place to
energize the unit.
2− Line voltage energizes transformer T1. Transformer T1 provides 24VAC power to all unit controls,
terminal strip and thermostat.
Heating:
3− Heating demand initiates at W1 in the thermostat. Heating demand passes through primary gas
limit S10 through the differential pressure switch S72 enabling ignition control A3.
4− Combustion air blower B6 is energized for 30 + 5 seconds by ignition control A3.
5− Gas valve opens and attempts for ignition occur.
6− Internal Watchguard is enabled only if ignition fails to light unit (5 tries). Watchguard will break and
remake thermostat demand in 60 + 5 minutes.
7− Fan timing control A28 is energized by gas valve GV1.
8− Spark stops .5 seconds after ignition is sensed or after 8.0 seconds if ignition is not sensed.
9− Combustion air blower B6 is deenergized .5 seconds after ignition is sensed or after 8.0 seconds
if ignition is not sensed.
10− After 45 seconds of successful ignition A28 energizes K36.
11− K361 N.O. closes energizing the blower B3 on heating speed and ACC terminal of TB2 is ener
gized.
End of Heating Demand:
12− Heating demand is satisfied.
13− Ignition control A3 is deenergized.
14− Gas valve GV1 closes. Pulse cycle stops.
15− Combustion air blower B6 is energized for 30 + 5 seconds post purge by ignition control A3.
16− A28 continues to power blower motor B3 for the adjustable time of 120240 seconds.
17− After the adjustable delay, K36 is de energized.
18− K361 closes deenergizing blower motor B3.
Blower Only:
19− Blower demand originates in the thermostat terminal G.
20− Blower contactor K3 is energized.
21− K31 N.O closes energizing blower (black tap).
22− K32 N.O. closes energizing ACC terminal on TB2 terminal strip.
Cooling Demand:
NOTE: Y TERMINAL ON LOW VOLTAGE TERMINAL STRIP IS USED FOR
CONNECTING THERMOSTAT TO OUTDOOR UNIT (JUNCTION BLOCK).
THE Y TERMINAL HAS NO INTERNAL CONNECTIONS TO THE UNIT.
23− Blower demand originates in the thermostat terminal G.
24− Blower contactor K3 is energized.
25− K31 N.O closes energizing blower on cooling speed (black tap).
26− K32 N.O. closes energizing accessories terminal on TB2 terminal strip.
SingleStage or TwoStage Cooling With CCB1 (EBR1 Required)
See Table20 for operating characteristics.
Continuous Low Blower Kit−Optional
27− S68 is a DPST switch.
28− One pole (S68−1) is connected to the blower (low speed tap).
29− The other pole (S68−2) is connected to the ACC terminal of the TB2 terminal block.
30− When the switch is energized S68−1,2 both close energizing the blower on
low speed and the ACC terminal. This assures a blower supply when optional
accessories such as an electronic air cleaner operate.
