Lennox G21Q Series Unit Information

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THERMOSTAT DEMAND
PURGE BLOWER
GAS VALVE
IGNITION SPARK
0
30
ON
OFF
1
GC1, GC3, and G891 NORMAL IGNITION SEQUENCE
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IGNITION TRIAL
END OF HEATING DEMAND
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POSTPURGE 30 SEC
ONDS
FIGURE 21
38
PREPURGE 30 SEC.
GC1 GC3
GC3GC1
(Timings Nominal)
1 − Thermostat demand for heat.
2 − Purge blower is energized.
3 − At 30 seconds gas valve and ignition spark are energized for
eight seconds.
4 − When ignition occurs (sensed by flame rectification), the
spark and purge blower remain energized for the remainder
of the 8.0 seconds (GC1, G891, and later GC3 controls with part
numbers 97H02 and 52J18). Early GC3 controls, with part
number 72H68 deenergizes spark and purge .5 seconds after
flame is sensed or after 8.0 seconds if flame is not sensed.
5 − At end of heating demand, gas valve is deenergized and purge
blower is started.
6 − Post purge continues for 30 seconds after heating cycle, then
is deenergized.
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THERMOSTAT DEMAND
PURGE BLOWER
GAS VALVE
IGNITION SPARK
0
30
ON
OFF
1
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ÉÉ
ÉÉ
IGNITION TRIAL
RESET AT
THERMOSTAT
BY BREAKING
DEMAND
38
38
GC3GC1
2
3
4
5
68 76 106 144 152 182 190
63 71
GC1, GC3, AND G891 IGNITION ATTEMPT SEQUENCE FOR RETRIALS
1 − Thermostat demand for heat.
2 − Purge blower is energized.
3 − At 30 (30 + 2 GC1, 30 + 5 GC3 and G891) seconds gas valve
and ignition spark are energized for 8 seconds.
4 − If no ignition is sensed, purge blower continues to run.
5 − After additional 30 seconds (GC1 and G891) (25 seconds
GC3), ignition retrial takes place for another 8 seconds.
6 − If no ignition is sensed, purge blower continues to run for
another 30 seconds (GC1 and G891) (25 seconds GC3).
7 − This sequence is repeated for five trials; without ignition the
control locks out until reset at the thermostat.
8 − With ignition occurring at any trial, the ignition spark and
purge blower remain energized for the duration of that trial
(GC1, G891, and later GC3 controls with part numbers
97H02 and 52J18) or are deenergized after .5 seconds if
flame is sensed (Early GC3 with part number 72H68).
96 104
114
129 137 162 170
GC1 / G891 TIMINGS (SEC.)
GC3 TIMINGS (SEC.)
(Specific Timings)
FIGURE 22
G − Gas Valve and Expansion Tank
(Figure 23)
1 − Gas Valve
Gas valves used on G21/GSR21 series units have vari
ous opening times. All gas valves are internally redun
dant to assure safety shutoff. If replacement is neces
sary, the valve must be replaced with the same type of
valve. For example, replace a White Rodgers as shown
in figure 23 with a White Rodgers valve.
2 − Expansion Tank
An expansion tank downstream of the gas valve ab
sorbs back pressure created during combustion to pre
vent damage to gas valve diaphragm.
3 − Gas Valve Conversion (Figure 24)
A gas changeover kit is available to convert A.G.A. / C.G.A.
natural gas units to LP gas. Refer to the instructions provided
in each specific kit for proper installation procedures.
A.G.A. / C.G.A. GSR21 (50,80) units are shipped with the
components required for field conversion to LP gas.

Lennox G21Q Series Specifications

General

Brand	Lennox
Model	G21Q Series
Category	Industrial Equipment
Language	English