Lennox G21Q Series Unit Information

Page 74
Sequence of Operation G21V
D−OPERATION SEQUENCE G21V 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 G21V. The sequence describes the actions of devices in the unit which control blow
ers, 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−If a singlestage thermostat is to be used, the heating blower jumper should be in position #
1. This forces the blower to run on the high speed heat tap and stops frequent cycling which would
occur. When the blower operates on the low speed heat/cool tap, reduced airflow causes heat
exchanger temperatures to rise above the modulation control (S78) setpoint. The upper modula
tion limit will trip interrupting the W1 signal and energizing the blower. As the blower continues to
run and the temperatures decrease, the lower modulation limit will trip resetting the W1 signal if
there is still a call for heat.
If in a nonzoning application and no modulation of heat exchanger is desired, then the heat de
mand from the thermostat should be wired to terminal NM instead of W1.
NOTE−This is a basic operation sequence for the G21V. 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. (NOTE− Blower motor is always energized, S51 does not interrupt power to
blower motor.)
2− Line voltage energizes transformer T1. Transformer T1 provides 24VAC power to all unit controls,
terminal strip. Thermostat is energized after 24 VAC passes through normally closed primary limit
S10.
HeatingSingle Stage−Heating Blower jumper in position #2
(APPLICATIONS WITH A SINGLESTAGE THERMOSTAT)
3− Heating demand initiates at W1 in the thermostat.
4− Voltage passes through modulating switch S78 to energize ignition control A3.
5− Combustion air blower B6 is energized for 30 +
5 seconds by ignition control A3.
6− Gas valve GV1 is energized by terminal 3 of ignition control A3.
7− Attempts for ignition occur. Internal Watchguard is enabled only if ignition fails to light unit (5 tries).
Watchguard will break and remake thermostat demand in 60 + 5 minutes.
8− Combustion air blower B6 and ignition control A3 are 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 spark
for a full 8 seconds even if ignition is sensed).
9− VSP2 internal circuitry starts blower on low speed heat/cool tap after a 45 second delay. After
approximately 3−10 minutes heat exchanger temperatures will have risen to the set point of modu
lation control S78.
10− S78 will open, interrupting signal to VSP2 and in turn ignition control A3 is deenergized closing
gas valve.
11− Combustion air blower B6 is energized for 30 + 5 seconds by ignition control A3.
12− VSP2 internal circuitry forces blower to operate on low speed heat/cool tap, reducing heat ex
changer temperature.
13− When heat exchanger temperature falls below the modulation control set point, modulation limit
S78 closes and signal to VSP2 is remade. If there is still a call for heat, unit will attempt ignition and
operate until S78 cycles again.
HeatingSingle Stage  Heating Blower Jumper in Position #1
(APPLICATIONS WITH A SINGLESTAGE THERMOSTAT)
14− The thermostat calls for heat (W1). Ignition is established. See steps 3−8.
15− Internal circuitry of the VSP2 forces the blower to run on high speed heat tap.
16− Heat exchanger temperatures are kept below modulation control S78 opening set point. Modula
tion control S78 is now acting as a safety limit.
HeatingTwo Stage (APPLICATIONS WITH A TWOSTAGE THERMOSTAT / NM CAN NOT BE
USED WITH TWOSTAGE) − Heating Blower jumper in position # 2
Stage One:
17− The thermostat calls for heat (W1). Ignition is established. See steps 3−9.
18− The unit operates in the first stage of heating. See steps 1013.
Stage Two:
19− The thermostat calls for heat (W2). Ignition is established. See steps 3−9.
20− Pin 3 of VSP2 is energized. Internal circuitry of the VSP2 forces the blower to run on high speed
heat tap.
21− Heat exchanger temperatures are kept below modulation control S78 opening set point.
HeatingSingle Stage (NO MODULATION) − Heating Blower jumper in position # 1, W1 to NM
22− The thermostat calls for heat (W1 to NM). See steps 3−8.
23− Internal circuitry of the VSP2 forces the blower to run on high speed heat tap.
HeatingSingle Stage (NO MODULATION) − Heating Blower jumper in position # 2, W1 to NM
24− The thermostat calls for heat (W1 to NM). See steps 3−8.
25− Internal circuitry of the VSP2 forces the blower to run on low speed (heat/cool) tap.
HeatingZone Control System Used−Heating Blower jumper in position # 2
Stage One:
26− The thermostat calls for heat (W1). Ignition is established. See steps 3−9.
27− When the Harmony Zone Control System is used, the motor does not run at the low speed heat/
cool or the high speed heating tap. Zone controller internal circuitry provides the DS terminal (Pin
4) to control blower speed based upon zone demands. Speed is determined by a PWM signal.
Blower speed is based on the PWM signal determined by the number of zones calling.
End of Heating Demand:
28− Heating demand is satisfied. Ignition control A3 is de−energized. Gas valve GV1 closes. Pulse
cycle stops.
29− Combustion air blower B6 is energized for 30 + 5 seconds by ignition control A3.
30− VSP2 internal circuitry continues to power blower motor B3 on low speed heat/cool tap for the ad
justable time 90−330 seconds. After the adjustable delay, B3 is de energized.
Blower Only:
31− Blower demand G originates in the thermostat. Terminal 2 of VSP2 is energized.
32− VSP2 internal circuitry forces blower to operate on low speed heat/cool tap.
CoolingSingle Stage−No CCB1, No Harmony, DS Jumpered to G
(APPLICATIONS WITH A SINGLESTAGE THERMOSTAT)
33− Refer to table 19 for operation characteristics.
CoolingTwo StageNo CCB1, No Harmony, DS Jumpered to Y2
(APPLICATIONS WITH A TWOSTAGE THERMOSTAT)
Stage One:
34− Refer to table 19 for operation characteristics.
Stage Two:
35− Refer to table 19 for operation characteristics.
CoolingCCB1 Used with SingleSpeed or TwoSpeed Compressor, No Harmony,
No Jumpers
36− Refer to table 19 for operation characteristics.
CoolingHarmony Zone Control Used With SingleSpeed or TwoSpeed Compressor,
No CCB1, No Jumpers
37− Refer to table 19 for operation characteristics.

Lennox G21Q Series Specifications

General

Brand	Lennox
Model	G21Q Series
Category	Industrial Equipment
Language	English