Lennox G21Q Series Unit Information

Page 22
IMPORTANT
When the GC1 is operating normally the LED will not
be lit. If it is lit refer to Ignition Control Troubleshoot
ing Flowchart in the back of manual.
The GC1 ignition control LED may be lit for the following rea
sons: lockout condition (lit), postpurge operation (lit) or con
trol failure (lit). If the control is in lockout (lit) due to unsuccess
ful ignition (five tries), it must be reset by breaking and remak
ing thermostat demand or power to the unit. The GC1 is
equipped with a Safety Dormant Lockout circuit (sleep
mode). This circuit continuously monitors the control for inter
nal circuit faults. In addition the Safety Dormant Lockout cir
cuit senses potentially damaging frequency, voltage or cur
rent. If lockout is due to a selftest failure, or if damaging fre
quency, voltage or current has been sensed, the GC1 will go
into a Safety Dormant Lockout (LED lit). The control will be
shut off. The GC1 control may or may not have been dam
aged. The control can only be reset by breaking and remak
ing power to the unit.
IMPORTANT
When the GC3 and the G891 are operating normal
ly the LED will be lit at all times. If it is not lit or if it
is flashing refer to Ignition Control Troubleshooting
Flowchart in the back of manual.
If any of the the GC3 or the G891 model no. 86H3001 or
34K8301 control is in lockout (LED flashing) due to unsuc
cessful ignition (five tries), breaking and remaking thermostat
demand or power to the unit will reset the control. The later
G891 control (model no. 73K8601) has two flashing modes. If
LED is flashing 0.5 seconds on and 2.5 seconds off, the control
is in lockout due to unsuccessful ignition (five tries). To reset
the control break and remake thermostat demand to the unit. If
LED is flashing 0.1 seconds on and 0.1 seconds off (flickering)
the control reads a low flame sense. Inspect sensor and refer
to Troubleshooting Flowchart in back of manual. If the LED is
not lit, break and remake power to unit. If LED is still not lit refer
to Ignition Control Troubleshooting Flowchart in back of manu
al. The GC3 and G891 do not have Safety Dormant Lockout
circuit.
Watchguard
All G21 / GSR21 units are equipped with a watchguard circuit.
The GC1 uses a separate external watchguard WG1 control
board (A18) located above the control box. The GC3 and
G891 ignition control incorporate this function.
The watchguard feature serves as an automatic reset de
vice for ignition controls locked out because the furnace
has failed to ignite.
The external watchguard used with the GC1 ignition control
will break and remake thermostat demand after one hour of
continuous thermostat demand, (unit locked out or operation
al). This will reset the ignition control to attempt ignition.
The internal watchguard used with the GC3 and the G891 is
activated only when the unit has failed to light. The internal
watchguard is activated after the fifth unsuccessful ignition trial.
Internal watchguard will reset the ignition control one hour after
the unit has locked out.
DANGER
Shock Hazard. Spark related com
ponents contain high voltage. Dis
connect power before servicing
unit. Ignition control is not field re
pairable. Can cause injury or
death.
1 − Lennox GC1 Ignition Control (Figure 18)
The Lennoxbuilt GC1 ignition control (A3) has a black
plastic cover, and is illustrated in figure 18. The unit wiring
harness (P72) plugs directly into the jack (J72) at the cor
ner of the control. A diagnostic lockout indicator light, a red
LED, is visible through the GC1 cover.
Spark wire connection is made to a spark plug type connec
tor on the control. Sensor wire connection is made to a
quick connect terminal.
GC1 IGNITION CONTROL
RED
LED
SENSE TERMINAL
SPARK PLUG WIRE CONNECTION
123
45 6
1 24 VAC FROM TSTAT W"
2 COMMON
3 24 VAC TO GAS VALVE
4 24 VAC INPUT FROM
TRANSFORMER T1
5 120 VAC TO PURGE
BLOWER
6 120 VAC INPUT
J72 CONNECTOR
FIGURE 18
2 − Lennox GC3 Ignition Control (Figure 19)
The Lennoxbuilt GC3 ignition control (A3) has a white or a
green plastic cover, and is illustrated in figure 19. The unit
wiring harness (P72) plugs directly into the jack (J72) at the
bottom of the control. A diagnostic indicator light, a green
LED, is visible through the GC3 cover.

Lennox G21Q Series Specifications

General

Brand	Lennox
Model	G21Q Series
Category	Industrial Equipment
Language	English