Lennox G21Q3-40 User Manual

Page 72
Sequence of Operation GSR21Q
C−APPLICATION OF GSR21Q WITH GC3 IGNITION CONTROL
This flowchart is used to show the step by step sequence that takes
place when thermostat demand is sent to the GSR21Q. The se
quence describes the actions of devices in the unit which control blow
ers, fans, gas valve and other components in the system. The se
quence is outlined by numbered steps which correspond to circled
numbers on the adjacent diagram.
NOTE−This is a basic operation sequence for the GSR21Q. The ther
mostat shown is a basic electromechanical thermostat .
Operation Sequence
1− Line voltage feeds through door interlock switch S51. The blower ac
cess 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. The heating de
mand energizes K9 which closes N.O. K91. The heating demand
then passes through the secondary gas limit S21, primary gas limit
S10, heat relay K91 and 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. (Early GC3, in later versions and G891 spark
for a full 8 seconds even if ignition is sensed).
9− Combustion air blower B6 is deenergized .5 seconds after ignition is
sensed or after 8.0 seconds if ignition is not sensed. (Early GC3, in
later versions and G891 the combustion air blower B6 is deener
gized after 8 seconds).
10−After the adjustable time of 30 − 60 seconds of successful ignition
A28 energizes K36.
11−K361 N.O. closes energizing the blower B3 on heating speed
and ACC terminal of TB2 is energized.
End of Heating Demand:
12−Heating demand is satisfied.
13−Ignition control A3 is deenergized.
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.
120240 seconds.
17−After the adjustable delay, K36 is deenergized.
18−K361 closes deenergizing blower motor B3.
Blower Only:
19−Blower demand originates in the thermostat terminal G.
20−Blower contactor K3 is energized.
21−K31 N.O closes energizing blower (black tap).
22−K32 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 IN
TERNAL CONNECTIONS TO THE UNIT.
23−Blower demand originates in the thermostat terminal G.
24−Blower contactor K3 is energized.
25−K31 N.O closes energizing blower on cooling speed (black tap).
26−K32 N.O. closes energizing accessories terminal on TB2 terminal
strip.
SingleStage or TwoStage Cooling With CCB1 (EBR1 Required)
See Table 20 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.

Lennox G21Q3-40 Specifications

General

Brand	Lennox
Model	G21Q3-40
Category	Industrial Equipment
Language	English