Lennox G27M Series Unit Information

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Sequence of Operation G27M
1 -- When there is a call for heat, W1 of the thermostat
energizes W of the furnace control with 24VAC.
2 -- SureLight control runs self--check for S10 primary
limit switch and S21 secondary limit switch nor-
mally closed contacts. The control also checks
S128 combustion air prove switch and S102 high
fire pressure switch normally open contacts. Fol-
low steps 4 through 8 per respective thermostat.
Single--stage Mode, Single--stage Thermostat
3 -- SureLight control energizes combustion air blower
B6 on high speed. Combustion air blower runs un-
til S102 high fire pressure switch closes (switch will
close within 2--1/2 minutes or control goes into
Watchguard--Pressure Switch mode). A 15--second
pre--purge follows after S102 closes.
4 -- SureLight control energizes ignitor. A 20--second
warm--up period begins.
5 -- Gas valve is energized on both first and second
stage. Gas valve opens for a 4--second trial for igni-
tion (ignitor remains energized for the first se-
cond).
6 -- Flame is sensed, gas valve remains open for the
heat call.
7 -- After 45--second delay, indoor blower B3 is ener-
gized on high heating speed.
Two--stage Mode, Single--stage Thermostat
1 -- SureLight control energizes combustion air blower
B6 on low speed. Combustion air blower runs until
combustion air blower switch S128 closes (switch
will close within 2--1/2 minutes or control goes into
Watchguard--Pressure Switch mode). A 15--second
pre--purge follows once S128 closes.
2 -- SureLight control energizes ignitor. A 20--second
warm--up period begins.
3 -- Gas valve is energized on first stage. Gas valve
opens for a 4--second trial for ignition (ignitor re-
mains energized for the first second). Signal is sent
from control module to two--stage control board to
begin W2 (second--stage) ON delay.
4 -- Flame is sensed, gas valve remains open for the
heat call.
5 -- After 45--second delay, indoor blower B3 is ener-
gized on low speed.
6 -- Second stage time on delay complete (8, 12, or 15
minutes). Combustion air blower B6 switches to
high speed, indoor blower B3 switches to heating
speed and gas valve opens on high fire (second
stage.)
Two--stage Mode, Two--stage Thermostat
1 -- SureLight control energizes combustion air blower
B6 on low speed. Combustion air blower runs until
combustion air blower switch S128 closes (switch
will close within 2--1/2 minutes or control goes into
Watchguard--Pressure Switch mode). A 15--second
pre--purge follows once S128 closes.
2 -- SureLight control energizes ignitor. A 20--second
warm--up period begins.
3 -- Gas valveis energized on first stage only. Gas valve
opens for a 4--second trial for ignition (ignitor will
remain energized for the first second).
4 -- Flame is sensed, gas valve remains open for the
heat call.
5 -- After 45--second delay, indoor blower B3 is ener-
gized on low speed.
6 -- Call for second stage heat comes from indoor ther-
mostat.

Lennox G27M Series Specifications

General

Brand	Lennox
Model	G27M Series
Category	Furnace
Language	English