Lennox G27M Series Installation and operation manual

1998 Lennox Industries Inc.
Litho U.S.A.
Page 1
Corp. 9703-L2
G27M
Service Literature Revised 05-2000
G27M SERIES UNITS
G27M series units are mid-efficiency gas furnaces
manufactured with tubular aluminized steel heat ex
changers. G27M units are available in heating capaci
ties of 60,000 to 120,000 Btuh (17.6 to 35.2 kW) and cool
ing applications from 1.5 through 5 tons (3.5 through
17.6 kW). Refer to Engineering Handbook for proper siz
ing.
Units are factory equipped for use with natural gas only.
LP kits are not available. All G27M units feature the Len
nox SureLighthot surface ignition system. The
G27MX units meet the California Nitrogen Oxides (NOx)
Standards and California Seasonal Efficiency require
ments. All units use a two-stage gas valve along with a
two-stage induced draft blower assembly. The gas
valve is redundant to assure safety shut-off as required
by A.G.A. and C.G.A.
Units may be installed in upflow, downflow or horizon
tal position. The heat exchanger is designed for upright
or horizontal use only. When the unit is installed in the
downflow position, the heat exchanger must be field
removed and reinstalled so it is upright when the cabi
net is inverted. No field conversion is required when
the unit is installed in the horizontal position.
The heat exchanger, burners and manifold assembly
can easily be removed for inspection and service by sim
ply disconnecting gas, unplugging wiring harnesses
and removing four screws holding the heat exchanger in
place. The heat exchanger slides out of the cabinet.
Information contained in this manual is intended for use
by qualified service technicians only. All specifications
are subject to change. Procedures outlined in this manu
al are presented as a recommendation only and do not
supersede or replace local or state codes.
G27M
TABLE OF CONTENTS
General 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Specifications 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Blower Performance Data 2. . . . . . . . . . . . . . . . . . . . .
I-Unit Components 4. . . . . . . . . . . . . . . . . . . . . . . . . . .
II-Placement and Installation 11. . . . . . . . . . . . . . . . . .
III-Start-Up 11. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV-Heating System Service Checks 12. . . . . . . . . . . .
V-Typical Operating Characteristics 14. . . . . . . . . . . .
VI-Maintenance 15. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VII-Wiring Diagram and Sequence of Operation 17
VIII-Troubleshooting 23. . . . . . . . . . . . . . . . . . . . . . . . .

3DJH 
SPECIFICATIONS
Model No. G27M2(X)-60 G27M3(X)-75 G27M4(X)-100 G27M5(X)-120
Input Btuh (kW) low fire 40,800 (12.0) 51,000 (14.9) 68,000 (19.9) 81,600 (23.9)
Output Btuh (kW) low fire 32,600 (9.6) 40,800 (12.0) 54,400 (16.0) 65,300 (19.1)
Input Btuh (kW) high fire 60,000 (17.6) 75,000 (22.0) 100,000 (29.3) 120,000 (35.2)
Output Btuh (kW) high fire 48,000 (14.1) 60,000 (17.6) 80,000 (23.5) 96,000 (28.2)
- A.F.U.E. 80%
Flue size connection diameter  in. (mm) round 3 (76) 4 (102)
Temperature rise range
E
F(
E
C)
Low fire 30 -- 60 (17 -- 33) 40 -- 70 (22 -- 39) 30 -- 60 (17 -- 33)
Temperature rise range  E F( E C) High fire 40 -- 70 (22 -- 39)
High static certified by A.G.A./C.G.A.  in wg. (Pa) .50 (124)
Gas Piping Size I.P.S. Natural gas only 1/2 (13)
Blower wheel nominal in. 9x7 10 x 7 12x8 12 x 9
Blower wheel nominal
diameter x width mm 229 x 178 254 x 178 305 x 203 305 x 229
Blower motor output  hp (W) 1/4 (187) 1/3 (249) 1/2 (373) 3/4 (560)
Electrical characteristics 120 volts  60 hertz  1 phase (12 amps or less -- all models)
Nominal coolin
g
Tons 2 or 2.5 2, 2.5 or 3 3.5 or 4 3.5, 4 or 5
Nominal cooling
that can be added kW 7.0 or 8.8 7.0, 8.8 or 10.6 12.3 or 14.1 12.3, 14.1 or 17.6
Up-flow/Horizontal Filter Kit (furnished)
c filter size  in. (mm) (1) 16 x 20 x 1 (406 x 508 x 25) (1) 20 x 20 x 1(508 x 508 x 25)
Shipping weight  lbs. (kg) 1 package 135 (61) 175 (79)
H Optional Accessories (Must Be Ordered Extra) H
o
Do n flo
Catalog No. LB-69843A (32J01)  3 lbs. (1 kg)
o Down-flow
Filter Kit No. & Size of Filters  in.
(mm) (2) 16 x 20 x 1 (406 x 508 x 25)
Down-flow Combustible Floor Base LB-79239A (67J91)  10 lbs. (4 kg) LB-79239B (67J92)  10 lbs. (4 kg)
Hanging Bracket Kit LB-69957 (46J66)  15 lbs. (7 kg)
High Altitude Pressure Switch Kits (high fire)
80K40 80K43 18L25 18L25
- Annual Fuel Utilization Efficiency based on U.S. DOE test procedures and according to FTC labeling regulations. Isolated combustion system rating for non-weatherized
furnaces.
c Polyurethane frame type filter.
o Filters are not furnished with kit and must be ordered extra.
BLOWER PERFORMANCE DATA
G27M2(X)-60 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
External Static
Pressure High Medium-High Medium-Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s
0 0 1270 600 980 460 770 365 570 270
.05 12 1245 590 975 460 770 365 565 265
.10 25 1220 575 975 460 770 365 565 265
.15 37 1195 565 965 455 765 360 560 265
.20 50 1170 550 960 455 760 360 560 265
.25 62 1140 540 950 450 760 360 555 260
.30 75 1110 525 940 445 760 360 550 260
.40 100 1060 500 910 430 750 355 545 255
.50 125 990 465 880 415 740 350 540 255
.60 150 900 425 810 380 690 325 530 250
.70 175 800 380 740 350 630 295 520 245
NOTE  All air data is measured external to unit with 1 inch (25 mm) air filter in place. NOTE -- Filter not furnished with unit.

Page 3 1997 Lennox Industries Inc.
Litho U.S.A.
BLOWER PERFORMANCE DATA
G27M3(X)-75 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
Pressure High Medium-High Medium-Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s
0 0 1425 670 1240 585 1000 470 800 380
.05 12 1415 670 1230 580 995 470 800 380
.10 25 1400 660 1220 575 990 465 795 375
.15 37 1385 655 1200 565 985 465 795 375
.20 50 1370 645 1180 555 980 460 790 375
.25 62 1350 635 1160 545 970 460 780 370
.30 75 1330 630 1140 540 955 450 770 365
.40 100 1280 605 1095 515 925 435 750 355
.50 125 1210 570 1040 490 900 425 720 340
.60 150 1135 535 985 465 860 405 680 320
.70 175 1070 505 920 435 800 380 630 300
NOTE — All air data is measured external to unit with 1 inch (25 mm) air filter in place. NOTE – Filter not furnished with unit.
G27M4(X)-100 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
Pressure High Medium-High Medium Medium-Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s
0 0 1830 865 1600 755 1325 625 1070 505 880 415
.05 12 1815 855 1585 750 1320 625 1070 505 880 415
.10 25 1800 850 1570 740 1315 620 1070 505 880 415
.15 37 1875 885 1550 730 1310 620 1065 505 875 415
.20 50 1750 825 1530 720 1300 615 1060 500 875 415
.25 62 1725 815 1515 715 1290 610 1050 495 870 410
.30 75 1700 800 1500 710 1275 600 1040 490 870 410
.40 100 1650 780 1460 690 1245 590 1020 480 860 405
.50 125 1600 755 1420 670 1210 570 1000 470 840 395
.60 150 1550 730 1380 650 1170 550 980 460 820 385
.70 175 1480 700 1330 630 1130 535 960 455 790 375
NOTE — All air data is measured external to unit with 1 inch (25 mm) air filter in place. NOTE – Filter not furnished with unit.
G27M5(X)-120 BLOWER PERFORMANCE
External Static Air Volume at Various Blower Speeds
Pressure High Medium-High Medium Medium-Low Low
in. w.g. Pa cfm L/s cfm L/s cfm L/s cfm L/s cfm L/s
0 0 2450 1155 2160 1020 1970 930 1700 800 1500 710
.05 12 2440 1150 2155 1015 1965 925 1695 800 1500 710
.10 25 2430 1145 2150 1015 1960 925 1690 800 1495 705
.15 37 2415 1140 2135 1010 1950 920 1685 795 1495 705
.20 50 2400 1135 2120 1000 1940 915 1680 795 1490 705
.25 62 2380 1125 2105 995 1930 910 1675 790 1480 700
.30 75 2360 1115 2090 985 1915 905 1670 790 1470 695
.40 100 2310 1090 2050 965 1870 880 1650 780 1440 680
.50 125 2260 1065 2000 945 1810 855 1610 760 1410 665
.60 150 2180 1030 1950 920 1750 825 1560 735 1370 645
.70 175 2100 990 1890 890 1700 800 1520 715 1330 630
NOTE — All air data is measured external to unit with 1 inch (25 mm) air filter in place. NOTE – Filter not furnished with unit.

Page 4
FIGURE 1
G27M
PARTS IDENTIFICATION
VENT ADAPTER
CABINET TOP CAP
G27M
CABINET
CABINET
BOTTOM
HEAT EXCHANGER
ASSEMBLY
BURNER BOX
ASSEMBLY
BLOWER ASSEMBLY
COMBUSTION AIR
BLOWER
FLUE BOX
SECONDARY
LIMIT CONTROLS
FRONT LOUVERED
PANEL
SureLight
IGNITION
CONTROL
BOARD
TRANSFORMER
PRESSURE
SWITCHES
NOx
TURBULATOR
(For G27MX units only)
TWO–STAGE
CONTROL
BOARD
CABINET
BOTTOM
CAP
DOOR INTERLOCK SWITCH
I–UNIT COMPONENTS (Figure 1)
G27M unit components are shown in figure 1. The
blower controls, gas valve and burners can be accessed
by removing the front access panel. A separate blower
access door is located behind the front access panel.
Electrical control components are mounted on the
blower access door. Units are factory–equipped with
bottom return air panels
A–Blower Door Components
The SureLight integrated ignition and blower control is lo-
cated on the outside surface of the blower access door
along with the unit transformer (T1), the two–stage con-
trol and a door interlock switch (S51). Jackplugs allow the
blower door to be removed for blower service.
1– Control Transformer (T1)
A transformer located on the blower door provides power
to the low voltage section of the unit. Transformers on all
models are rated 40VA with a 120V primary and a 24V
secondary. The transformer is protected by a 3.0 amp
circuit breaker.
2–Door Interlock Switch (S51)
A door interlock switch rated 16A at 125VAC is located
on the blower access door. The switch is wired in series
with line voltage. When the blower door is removed the
unit will shut down.
3– SureLight Ignition System (A3)
All G27M units are equipped with the Lennox Sure –
Light ignition system. The system consists of an igni-
tion control board (figure 2) and ignitor (figure 3). The
board and ignitor work in combination to ensure fur-
nace ignition and ignitor durability. The SureLight in-
tegrated board controls all major furnace operations.
The board also features two LED lights for trouble-
shooting and two accessory terminals. See table 1 for
troubleshooting diagnostic codes. Table 2 and 3 show
jack plug terminal designations. Units equipped with
the SureLight board can be used with either electronic
or electro–mechanical thermostats without modifica-
tion. The SureLight ignitor is made of durable silicon–
nitrite. Ignitor longevity is also enhanced by voltage
ramping by the control board. The board finds the low-
est ignitor temperature which will successfully light
the burner, thus increasing the life of the ignitor.
DANGER
Shock hazard.
Disconnect power before servicing. Control is not
field repairable. If control is inoperable, simply
replace entire control.
Can cause injury or death. Unsafe operation will
result if repair is attempted.

Page 5
NOTE - The G27M furnace contains electronic compo
nents that are polarity sensitive. Make sure that the fur
nace is wired correctly and is properly grounded.
CAUTION
Electrostatic discharge can affect electronic com
ponents. Take precautions during furnace instal
lation and service to protect the furnace's elec
tronic controls. Precautions will help to avoid
control exposure to electrostatic discharge by
putting the furnace, the control and the techni
cian at the same electrostatic potential. Neutral
ize electrostatic charge by touching hand and all
tools on an unpainted unit surface, such as the
gas valve or blower deck, before performing any
service procedure.
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
a-Electronic Ignition
On a call for heat the SureLight control monitors the
combustion air blower pressure switch. The control
will not begin the heating cycle if the pressure switch is
closed (by-passed). Once the pressure switch is deter
mined to be open, the combustion air blower is ener
gized. When the differential in the pressure switch is
great enough, the pressure switch closes and a 15-se
cond pre-purge begins. If the pressure switch is not
proven within 2-1/2 minutes, the control goes into
Watchguard-Pressure Switch mode for a 5-minute re-
set period.
After the 15-second pre-purge period, the SureLight
ignitor warms up for 20 seconds after which the gas
valve opens for a 4-second trial for ignition. The ignitor
stays energized for the first second of the 4-second
trial. If ignition is not proved during the 4-second peri
od, the control will try four more times. After a total of
five trials for ignition (including the initial trial), the con
trol goes into Watchguard-Flame Failure mode. After a
60-minute reset period, the control will begin the igni
tion sequence again.
The SureLight control board has an added feature that
prolongs the life of the ignitor. After a successful igni
tion, the SureLight control utilizes less power to ener
gize the ignitor on successive calls for heat. The control
continues to ramp down the voltage to the ignitor until
it finds the lowest amount of power that will provide a
successful ignition. This amount of power is used for
255 cycles. On the 256th call for heat, the control will
again ramp down until the lowest power is determined
and the cycle begins again.
TABLE 1
DIAGNOSTIC CODES
MAKE SURE TO ID LED'S CORRECTLY: REFER TO INSTALLATION INSTRUCTIONS FOR CONTROL BOARD LAYOUT.
LED #1 LED #2 DESCRIPTION
SIMULTANEOUS
SLOW FLASH
SIMULTANEOUS
SLOW FLASH
Power - Normal operation
Also signaled during cooling and continues fan.
SIMULTANEOUS FAST
FLASH
SIMULTANEOUS FAST
FLASH Normal operation - signaled when heating demand initiated at thermostat.
SLOW FLASH ON
Primary or Secondary limit open. Units with board 63K8901 or 24L85: Limit must
close within 5 trials for ignition or board goes into one hour limit Watchguard. Units
with board 56L83: Limit must close within 3 minutes or board goes into one hour
limit Watchguard.
OFF SLOW FLASH
Pressure switch open or has opened 5 times during a single call for heat; OR:
Blocked inlet/exhaust vent; OR: Condensate line blocked; OR: Pressure switch
closed prior to activation of combustion air blower.
ALTERNATING SLOW
FLASH
ALTERNATING SLOW
FLASH Watchguard - burners fail to ignite.
SLOW FLASH OFF Flame sensed without gas valve energized.
ON SLOW FLASH Rollout switch open. OR: 9 pin connector improperly attached.
ON
ON
OFF
ON
OFF
ON
Circuit board failure or control wired incorrectly.
FAST FLASH SLOW FLASH Main power polarity reversed. Switch line and neutral.
SLOW FLASH FAST FLASH Low flame signal. Measures below .7 microAmps. Replace flame sense rod.
ALTERNATING FAST
FLASH
ALTERNATING FAST
FLASH
Improper main ground or line voltage below 75 volts; OR: Broken ignitor; OR: Open
ignitor circuit.
NOTE - Slow flash equals 1 Hz (one flash per second). Fast flash equals 3 Hz (three flashes per second). Drop out flame sense current < 0.15 microAmps

Page 6
SureLight CONTROL BOARD
FIGURE 2
FIGURE 3
SureLight IGNITOR
TABLE 2
SureLight BOARD J156 (J2) TERMINAL
DESIGNATIONS
PIN # FUNCTION
1 Ignitor
2
Not Used
3
Ignitor Neutral
4
Combustion Air Blower Line Voltage
5
Not Used
6
Combustion Air Blower Neutral
TABLE 3
SureLight BOARD J58 (J1) TERMINAL
DESIGNATIONS
PIN # FUNCTION
1 Primary Limit In
2
Not Used
3
Roll Out Switch Out
4
Gas Valve 24V
5
Pressure Switch In
6
Primary Limit Out
7
Not Used
8
Roll Out Switch In
9
Ground
E)DQ 7LPH &RQWURO
The fan on time of 45 seconds is not adjustable. Fan off
time (time that the blower operates after the heat de-
mand has been satisfied) can be adjusted by flipping
the dip switches located on the SureLight integrated
control. The unit is shipped with a factory fan off setting
of 90 seconds. Fan off time will affect comfort and is ad-
justable to satisfy individual applications. See figure 4.
),*85( 
FAN-OFF TIME ADJUSTMENT
To adjust fan--off timing , flip dip switch to desired setting.
60sec. 90sec. 120sec. 180sec.
4-- Two--Stage Control (A86)
All G27M units are equipped with a two--stage control
board. The two--stage board acts as a go between from
the indoor thermostat to the SureLight ignition board.
The board can be utilized in three modes: with a
SINGLE--STAGE thermostat, a TWO--STAGE thermo-
stat or with a second--stage (high fire) delay called W2
TIMED. The two--stage board is equipped with a jumper
(see figure 5) which changes operating modes and a
jumper which adjusts second--stage heat delay during
W2 TIMED mode. While in the single--stage thermostat
mode, the unit will always operate on second--stage
heat. The combustion air blower (B6) will operate on
high speed and indoor blower (B3) will operate on heat-
ing speed. While in the two--stage thermostat mode the
unit will operate on first--stage heat (low fire). The com-
bustion air blower (B6) and indoor blower will operate
on low speed. The unit will switch to second--stage heat
(high fire) on call from the indoor thermostat W2. While
in the W2 TIMED mode the unit will fire on first--stage
heat (low fire) with the combustion air blower (B6) and

Page 7
indoor blower (B3) operating on low speed. After a set
time delay the unit switches to second–stage heat (high
fire). The combustion air blower and indoor blower
also switch to second–stage heat mode.
TWO–STAGE CONTROL BOARD
MODE OF
OPERATION
JUMPER
W2 TIMED ON
DELAY JUMPER
FIGURE 5
B–Blower Motor Assembly
All G27M units use direct drive blower motors. All mo-
tors used are 120V permanent split capacitor motors to
ensure maximum efficiency. See table 4 for ratings.
Table 4
G27M BLOWER RATINGS 120V
BLOWER MOTOR HP CAP
G27M2–60 1/4 5MF 370V
G27M3–75 1/3 5MF 370V
G27M4–100 1/2 7.5MF 370V
G27M5–120 3/4 40MF 370V
Secondary Limit Switches
[(1)S21 and (2)S21]
The secondary limit switches [(1)S21 and (2)S21] on
G27M units are located in the blower compartment on
the back side of the blower housing (see figure 6). When
excess heat is sensed in the blower compartment, the
limit will open. If the limit is tripped, the SureLight control
energizes the supply air blower and closes the gas valve.
The limit automatically resets when unit temperature
returns to normal. Two limits are supplied in each fur-
nace. Figure 7 shows both switches. Both switches are
N.C. SPST auto-reset switches which actuate on a tem-
perature rise. The switches are factory set and cannot be
adjusted. The (1)S21 limit shown on the top opens at
140_F + 5_F (60.0_C + 2.8_C) on a temperature rise and
closes at 110_F + 10_F (43.3_C + 5.6_C). The (2)S21 limit
shown on the bottom opens at 165_F + 5_F (73.9_C +
2.8_C) on a temperature rise and closes at 135_F + 8_F
(57.2_C + 4.4_C).
FIGURE 6
SUPPLY AIR BLOWER
AND SECONDARY LIMITS
Front
Bottom
Right
Left
Top Back
BLOWER
MOTOR
To Remove Blower From Unit: Remove Bolts and
Wiring Jackplugs. Then Slide Out Front of Unit.
MOTOR
CAPACITOR
SECONDARY
LIMITS (S21)
INSULATING COVER (s)
FIGURE 7
SECONDARY LIMIT SWITCHES
FOR G27M SERIES UNITS
SPADE CONNECTORS
INSULATING COVER
LIMIT
LIMIT
(1)S21
(2)S21

Page 8
FIGURE 8
HEATING COMPONENTS (shown in horizontal position)
HEAT EXCHANGER
COMBUSTION AIR
BLOWER (B6)
MANIFOLD
ROLLOUT SWITCH (2)
(S47)
COMBUSTION AIR
PROVE SWITCHES
(S18 and S102)
GAS VALVE (GV1)
BURNERS
PRIMARY
LIMIT (S10)
ORIFICES
Front
Right
Top
Bottom
Left
Back
G27M BURNER ASSEMBLY (shown in upflow position)
FIGURE 9
Top
SureLight
IGNITOR
FLAME
SENSOR
MANIFOLD
ORIFICE
GAS
VALVE
(GV1)
HEAT EXCHANGER
BURNER
UPPER
BURNER
MOUNTING
RAIL
LOWER
BURNER
MOUNTING
RAIL
BURNER
BOX
BURNER BOX
TOP
ROLLOUT
SWITCHES
(S47)
Front Right
Bottom
Left Back
PRIMARY LIMIT (S10)
VEST PANEL

Page 9
C–Heating Components (Figures 8 and 9 )
Heating components are located inside the front louvered
panel. Inside the heating compartment are the combus-
tion air blower (B6), flame rollout switches (S47), primary
limit control (S10), ignitor, flame sensor, gas valve (GV1),
and the combustion air blower prove switches (S18 and
S102).
1–Combustion Air Blower (B6)
All G27M units use a two–stage combustion air blower to
move air through the burners and heat exchanger during
heating operation. The blower uses a 120VAC motor. The
motor operates during all heating operation and is con-
trolled by the SureLight integrated control A3. The blower
operates for 15 seconds before burner ignition (pre-
purge) and for 5 seconds after the gas valve closes (post-
purge). The combustion air blower operates on low speed
during first–stage heat (low fire), then switches to high
speed for second–stage heat (high fire).
2–Flame Rollout Switches (S47)
Flame rollout switch is a high temperature limit switch
located on top of the burner box (see figure 11). Each
furnace is equipped with two identical switches. One
switch is located over the leftmost burner and the other
switch is located over the rightmost burner. The switch is
a N.C. SPST manual-reset switch which opens at 260_F +
12_F (126.7_C + 6.7_C) on a temperature rise. The switch
is factory set and cannot be adjusted. To manually reset
a tripped switch, push the reset button located on the
control. The switches are connected in series and moni-
tored by the SureLight control A3. When S47 senses
rollout, the control immediately stops ignition and
closes the gas valve. If unit is running and flame rollout
is detected, the gas valve will close and ignition control
will be disabled. Rollout can be caused by a restricted
heat exchanger or a blocked flue.
FIGURE 10
ROLLOUT SWITCH (S47)
MANUAL
RESET BUTTON
3–Primary Limit Control (S10)
The primary limit (S10) is located in the middle of the
heating vestibule panel (see figure 9). S10 is the same
type of limit as (1) S21 or (2) S21 as shown in figure 7.
G27M–60 and –75 units use the same type limit as (1) S21.
G27M–100 and –120 units use the same type limit as (2)
S21. When excess heat is sensed in the heat exchanger,
the limit will open. If the limit is tripped, the SureLight
control energizes the supply air blower and closes the
gas valve. The limit automatically resets when unit
temperature returns to normal. The switch is factory set
and cannot be adjusted. The switch has different set-
points for each unit model number. However, the set
point will be printed on the side of the limit.
4–Gas Valve (GV1)
The G27M uses a gas valve manufactured by White
Rodgers. The valve is a two-stage internally redundant
to assure safety shut–off. If the gas valve must be re-
placed, the same type valve must be used.
24VAC terminals and gas control knob are located on top
of the valve. All terminals on the gas valve are connected
to wires from the SureLight integrated control. 24V ap-
plied to the terminals energizes the valve.
Inlet and outlet pressure taps are located on the valve. A
regulator adjustment screw is located on the side of the
valve. Refer to figure 11 for location of valve features.
NOTE–Do not attempt to regulate low fire manifold
pressure.
FIGURE 11
HIGH FIRE
MANIFOLD
PRESSURE
ADJUSTMENT
MANIFOLD
PRESSURE
OUTLET
WHITE RODGERS 36E SERIES GAS VALVE
INLET
PRESSURE

Page 10
5–SureLight Ignitor
Figure 12 and 14 shows the position of the SureLight
hot surface ignitor in perspective with the rest of the
burner assembly. The hot surface ignitor lights the two
rightmost burners and the burners cross-light to the left. A
flame retention ring in the end of each burner is used to
maintain correct flame length and shape and to keep the
flame from lifting off the burner head. The hot surface
ignitor is rated at 1800_F (982_C) in 17 seconds with a
120VAC 4 amp power source.
FIGURE 12
BURNER
IGNITOR LOCATION
SIDE VIEW END VIEW
0.26 in. (7 mm)
0.06 in. (1.5 mm) 0.47 in. (12 mm) 0.04 in. (1 mm)
0.26 in. (7 mm)
0.06 in.
(1.5 mm)
IGNITOR
6–Flame Sensor
Figure 14 shows the position of the flame sensor in per-
spective with the rest of the burner assembly. The
flame sensor uses flame rectification to sense com-
bustion. Figure 13 shows the gap between tip of the
electrode and the burner surface.
FLAME SENSOR TO BURNER GAP
BURNER
FLAME
SENSOR
SIDE VIEW END VIEW
1/4 in. 7 mm
1/32 in. 0.79 mm
3/16 in.
4.7 mm
11/16 in. 18 mm
1/32 in. 0.79 mm
FIGURE 13
BURNER
FLAME RETENTION RING
UPPER BURNER
MOUNTING RAIL
LOWER BURNER
MOUNTING RAIL
MANIFOLD ORIFICE
Right Left
Top
Bottom
FLAME SENSOR
SURELIGHT IGNITOR
SureLight Ignitor and Sensor Location
(Early Models, Ignitor On Bottom)
FLAME SENSOR
FLAME RETENTION RING
UPPER BURNER
MOUNTING
RAIL ORIFICE
SURELIGHT
IGNITOR
SureLight Ignitor and Sensor Location
(Late Models, Ignitor On Top)
FIGURE 14

Page 11
FIGURE 15
PROVE SWITCH
COMBUSTION AIR BLOWER
PROVE SWITCH
Normally Open
Closes on Negative Pressure
PROVE
SWITCHES
sensing tube
Ts the two switches
together and attaches
to side of blower
Top
Bottom
Left
Right
Back
Front
7--Combustion Air Blower Prove
(Pressure) Switches (S128 and S102)
G27M series units are equipped with two combustion air
prove switches. S128 (low fire) and S102 (high fire) are lo-
cated on the vestibule panel. The switches T together
and are connected to the combustion air blower housing
by a flexible silicone hose. The switches monitor air pres-
sure in the combustion air blower housing.
Both switches are N.O. SPST auto-reset pressure
switches electrically connected to the integrated con-
trol A3. The switches prevent burner operation if the
combustion air blower is not operating properly and
in case of flue blockage.
On start-up, the S128 and S102 switches sense whe --
ther the combustion air blower is operating. The
switches close a circuit to the burner control when pres-
sure inside the combustion air blower decreases below a
given setpoint (see table 5). The pressure sensed by the
switches is relative to atmospheric pressure. If the flue be-
comes obstructed during operation, and either switch
senses a loss of negative pressure (pressure approaches
atmospheric pressure) the switch opens the circuit to the
burner control and gas valve. Either switch, if opened, will
open the circuit. The switch trip pressures are different de-
pending on unit size. The trip pressures are printed on the
side of the switches.
All the switches provide factory set and are not field ad-
justable. The switches provide a safety shut-down control
in the furnace and must not be bypassed for any reason.
TABLE 5
MODEL
NUMBER S102 HIGH S128 LOW
G27M3--60 .52W.C. + 0.05W.C.
(129.37 Pa + 12.43Pa) 0.29W.C + 0.05W.C. (72.10Pa + 12.43Pa)
G27M--75 0.56W.C. + 0.05W.C.
(139.23Pa + 12.43Pa) 0.29W.C. + 0.05W.C. (72.05Pa + 12.43Pa)
G27M--100 0.65W.C. + 0.03W.C.
(161.61Pa + 7.5Pa) 0.43W.C. + 0.05W.C. (107Pa + 12.43Pa)
G27M--120 0.68W.C. + 0.03W.C.
(169Pa + 7.53Pa) 0.53W.C. + 0.05W.C. (131.77Pa + 12.43Pa)
NOTE: All measurements are negative.
II--PLACEMENT AND INSTALLATION
Make sure unit is installed in accordance with the G27M
installation instructions.
III--START-UP
A--Preliminary and Seasonal Checks
1 -- Inspect electrical wiring, both field and factory
installed for loose connections. Tighten as required.
2 -- Check voltage at disconnect switch. Voltage must be
within range listed on the nameplate. If not, consult
the power company and have voltage condition
corrected before starting unit.
B--Heating Start-Up
1 -- Set thermostat to OFF position. Close manual knob
on gas valve.
2 -- Wait 5 minutes.
3 -- Open manual knob on gas valve, replace the front
access panel and turn on unit electrical supply.
WARNING
Shock and burn hazard.
G27M units are equipped with a hot surface
ignition system. Do not attempt to light manually.
4 -- Set fan switch to AUTO or ON and move system
selection switch to HEAT. Adjust thermostat to a
setting above room temperature.
5 -- If unit does not light the first time, it will attempt
four more ignitions before locking out.
6 -- If lockout occurs, repeat steps 1, 2, 3 and 4.
7 -- On start-up check operation of both high and low
fire. Refer to section IV item E for manifold pressure
check procedure.
C--Safety or Emergency Shutdown
Turn off unit power. Close manual and main gas valves.
D--Extended Period Shutdown
Turn off thermostat or set to UNOCCUPIED mode.
Close all gas valves (both internal and external to unit)
to guarantee no gas leak into combustion chamber.
Turn off power to unit. All access panels, covers and
vent caps must be in place and secured.

Page 12
IV–HEATING SYSTEM SERVICE CHECKS
A–A.G.A. / C.G.A. Certification
All G27M units are A.G.A. / C.G.A. design certified with-
out modifications. Refer to the G27M Installation
Instruction for Information.
B–Gas Piping
Gas supply piping should not allow more than 0.5”W.C.
drop in pressure from unit with gas valve off and with
unit operating at second–stage heat (high fire). Supply
gas pipe must not be smaller than unit gas connection.
Refer to table 6 if pressure drop exceeds 0.5”W.C. or if
gas piping is suspect. Do not operate furnace below
nameplate minimum line pressure.
C–Testing Gas Piping
IMPORTANT
In case emergency shutdown is required, turn off
the main shut-off valve and disconnect the main
power to unit. These controls should be properly
labeled by the installer.
When pressure testing gas lines, the gas valve must be
disconnected and isolated. Gas valves can be damaged
if subjected to more than 0.5psig (14” W.C.). See figure
16 . If the pressure is equal to or less than 0.5psig
(14”W.C.), use the manual shut–off valve before
pressure testing to isolate furnace from gas supply.
FIGURE 16
MANUAL MAIN SHUT–OFF VALVE
GAS VALVE WILL NOT HOLD TEST
PRESSURE IN EXCESS OF 0.5
PSIG (14”W.C.)
GAS VALVE CAP
GAS PIPING TEST PROCEDURE
FIELD PROVIDED
LINE PRESSURE TAP
When checking piping connections for gas leaks, use
preferred means. Kitchen detergents can cause harmful
corrosion on various metals used in gas piping. Use of a
specialty gas leak detector is strongly recommended. It
is available through Lennox under part number
31B2001. See Corp. 8411–L10, for further details.
Do not use matches, candles, flame or any other source
of ignition to check for gas leaks.
D–Testing Gas Supply Pressure
When testing supply gas pressure, connect test gauge to
inlet pressure tap (field provided). See figure 16. Check
gas line pressure with unit firing at maximum rate. Low
pressure may result in erratic operation or underfire. High
pressure can result in permanent damage to gas valve or
overfire. For natural gas units, operating pressure at unit
gas connection must be a minimum of 5.0” W.C. but no
more than 13.0”W.C. at any time.
TABLE 6
GAS PIPE CAPACITY – FT3/HR (kL/HR)
Nominal
IPiSi
Internal
Di
Length of Pipe–Feet(m)
Iron Pipe Size
–Inches(mm)
Diameter
–Inches(mm) 10
(3.048)
20
(6.096)
30
(9.144)
40
(12.192)
50
(15.240)
60
(18.288)
70
(21.336)
80
(24.384)
90
(27.432)
100
(30.480)
1/4
(6.35) .364 (9.246) 43 (1.13) 29 (.82) 24 (.68) 20 (.57) 18 (.51) 16 (.45) 15 (.42) 14 (.40) 13 (.37) 12 (.34)
3/8
(9.53) .493 (12.522) 95 (2.69) 65 (1.84) 52 (1.47) 45 (1.27) 40 (1.13) 36 (1.02) 33 (.73) 31 (.88) 29 (.82) 27 (.76)
1/2
(12.7) .622 (17.799) 175 (4.96) 120 (3.40) 97 (2.75) 82 (2.32) 73 (2.07) 66 (1.87) 61 (1.73) 57 (1.61) 53 (1.50) 50 (1.42)
3/4
(19.05) .824 (20.930) 360 (10.19) 250 (7.08) 200 (5.66) 170 (4.81) 151 (4.28) 138 (3.91) 125 (3.54) 118 (3.34) 110 (3.11) 103 (2.92)
1
(25.4) 1.049 (26.645) 680 (919.25) 465 (13.17) 375 (10.62) 320 (9.06) 285 (8.07) 260 (7.36) 240 (6.80) 220 (6.23) 205 (5.80) 195 (5.52)
1–1/4
(31.75) 1.380 (35.052) 1400 (39.64) 950 (26.90) 770 (21.80) 660 (18.69) 580 (16.42) 530 (15.01) 490 (13.87) 460 (13.03) 430 (12.18) 400 (11.33)
1–1/2
(38.1) 1.610 (40.894) 2100 (59.46) 460 (41.34) 1180 (33.41) 990 (28.03) 900 (25.48) 810 (22.94) 750 (21.24) 690 (19.54) 650 (18.41) 620 (17.56)
2
(50.8) 2.067 (52.502) 3950 (111.85) 2750 (77.87) 2200 (62.30) 1900 (53.80) 1680 (47.57) 1520 (43.04) 1400 (39.64) 1300 (36.81) 1220 (34.55) 1150 (32.56)
2–1/2
(63.5) 2.469 (67.713) 6300 (178.39) 4350 (123.17) 3520 (99.67) 3000 (84.95) 2650 (75.04) 2400 (67.96) 2250 (63.71) 2050 (58.05) 1950 (55.22) 1850 (52.38)
3
(76.2) 3.068 (77.927) 11000 (311.48) 7700 (218.03) 6250 (176.98) 5300 (150.07) 4750 (134.50) 4300 (121.76) 3900 (110.43) 3700 (104.77) 3450 (97.69) 3250 (92.03)
4
(101.6) 4.026 (102.260) 23000 (651.27) 15800 (447.39) 12800 (362.44) 10900 (308.64) 9700 (274.67) 8800 (249.18) 8100 (229.36) 7500 (212.37) 7200 (203.88) 6700 (189.72)
NOTE–Capacity given in cubic feet of gas per hour (kilo liters of gas per hour) and based on 0.60 specific gravity gas.

Page 13
On multiple unit installations, each unit should be
checked separately, with and withoutother units oper-
ating. Supply pressure must be a minimum of 5.0
W.C. for each unit but no more than 13.0W.C. at any
time.
E--Check Manifold Pressure
After line pressure has been checked and adjusted,
check manifold pressure. Move pressure gauge to out-
let pressure tap located on unit gas valve (GV1).Checks
of manifold pressure are made as verification of proper
regulator adjustment. Manifold pressure for the G27M
can be measured at any time the gas valve is open and is
supplying gas to the unit. Normal manifold pressure
for natural gas units is 3.5 W.C. (870.2 Pa) for high
fire operation and 1.7 W.C. (422.7 Pa) for low fire.
IMPORTANT
For safety, connect a shut-off valve between the
manometer and the gas tap to permit shut off of
gas pressure to the manometer.
Thegas valveis factorysetand shouldnotrequiread-
justment. All gas valves are factory regulated.
Manifold Pressure Check Procedure:
1 -- Connect manometer to outlet pressure tap on gas
valve (see figure 17 for outlet tap location). Start
unit and allow 5 minutes for unit to reach steady
state.
2 -- While waiting for the unit to stabilize, notice the
flame. Flame should be stable and should not lift
from burner. Natural gas should burn blue.
3 -- After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to the normal
manifold pressure of 3.5 W.C. (870.2 Pa) for high
fire operation or 1.7 W.C. (422.7 Pa) for low fire.
NOTE--Shut unit off, turn gas valve to OFF position
and remove manometer as soon as an accurate
reading has been obtained. Take care to replace
pressure tap plug.
FIGURE 17
HIGH FIRE
MANIFOLD
PRESSURE
ADJUSTMENT
MANIFOLD
PRESSURE
OUTLET
WHITE RODGERS 36E SERIES GAS VALVE
F-- Proper Gas Flow (Approximate)
Furnace should operate at least 5 minutes before
checking gas flow. Determine time in seconds for two
revolutionsof gasthroughthemeter. (Tworevolutions
assures a more accurate time.) Divide by two and
compare to time in table 7 below. Seconds in table 7
arebasedona1cu.ft.dialandgasvalue of 1000 btus
for natural and 2500 btus for LP. Adjust manifold pres-
sure on gas valve to match time needed.
NOTE-- To obtain accurate reading, shut off all other
gas appliances connected to meter.
TABLE 7
Unit Seconds for
Natural Seconds for Propane
G27M2--60 60 150
G27M3--75 48 120
G27M4--100 36 90
G27M5--120 30 75
G--High Altitude Derate
Refer to table 8 for high fire manifold pressure settings
for installations at different altitudes. For low fire, no
manifold pressure adjustment is required.
TABLE 8
Manifold Absolute Pressure (Outlet) in. w.c.
UNIT
Altitude ---- feet (m) above sea level
UNIT
MODEL 0--2000
(0--610) 2000--4500 (610--1372) 4500--5500
(1372--1676)
5500--6500
(1676--1981)
6500--7500
(1981--2286)
G27M2--60
G27M3--75
G27M4--100
G27M5--120
3.5
(0.87) 3.5 (0.87)* 3.4 (0.85) 3.3 (0.82) 3.1 (0.77)
*No adjustment required.
NOTE -- In Canada, certification for installations at
elevations over 4500 feet (1372m) is the jurisdiction of
local authorities.
The combustion air pressure switches are factory set
and are not to be adjusted.
Factory-installed pressure switches are acceptable for
units installed at elevations up to 4500 feet (1372m).
Installations at higher elevations require replacement
of the high-fire pressure switch. Table 9 outlines pres-
sure switch requirements.
TABLE 9
UNIT MODEL PRESSURE SWITCH
PART NUMBER
G27M2--60A--1 80K4001
G27M3--75A--1 80K4301
G27M4--100A--1 18L2501
G27M5--120A--1 18L2501
IMPORTANT
For safety, shut un it off and remove manometer as
soon as an accurate reading has been obtained.
Take care to replace pressure tap plug.

FIGURE 18
STATIC PRESSURE
TEST
MANOMETER
G27M UNIT
Page 14
H--Flame Signal
A 20 microamp DC meter is needed to check the flame
signal on the SureLight ignition control. Use a flame sig-
nal transducer (part number 78H5401) available from
Lennox to measure the flame signal.
Flame (microamp) signal is an electrical current which
passes from the ignition control through the sensor
electrode during unit operation. Current passes from
the sensor through the flame to ground to complete a
safety circuit.
To Measure Flame Signal:
1 -- Remove front louvered panel and disconnect the
sensor wire from the flame sensor.
2 -- Place meter in series between flame sensor and
sensor wire. Connect the positive (+) lead of meter
to the sensor wire and the negative (--) lead of the
meter to the flame sensor. Refer to table 10 for
flame signal.
TABLE 10
FLAME SIGNAL IN MICROAMPS
NORMAL 0.8 or More
LOW 0.7 or Less
MINIMUM 0.15 or Less
V--TYPICAL OPERATING CHARACTERISTICS
A--Blower Operation and Adjustment
NOTE-- The following is a generalized procedure
and does not apply to all thermostat controls.
1 -- Blower operation is dependent on thermostat
control system.
2 -- Generally, blower operation is set at thermostat
sub--base fan switch. With fan switch in ON posi-
tion, blower operates continuously on low speed.
With fan switch in AUTO position, blower cycles
with demand or runs continuously while heating or
cooling circuit cycles.
3 -- In all cases, blower and entire unit will be off when
the system switch is in OFF position.
B--Temperature Rise
Temperature rise for G27M units depends on unit input,
blower speed, blower horsepower and static pressure as
marked on the unit rating plate. The blower speed must
be set for unit operation within the range of AIR TEMP.
RISE q F listed on the unit rating plate. Unit must also op-
erate on second--stage heat.
To Measure Temperature Rise:
1 -- Place plenum thermometers in the supply and return
air plenums. Locate supply air thermometer in the
first horizontal run of the plenum where it will not pick
up radiant heat from the heat exchanger.
2 -- Set thermostat to highest setting.
3 -- After plenum thermometers have reached their
highest and steadiest readings, subtract the two
readings. The difference should be in the range
listed on the unit rating plate. If the temperature is
too low, decrease blower speed. If temperature is
too high, first check the firing rate. Provided the firing
rate is acceptable, increase blower speed to reduce
temperature. To change blower speed taps see the
Blower Speed Taps section in this manual.
C--External Static Pressure
1 -- Measure tap locations as shown in figure 18.
2 -- Punch a 1/4 diameter hole
in supply and return air
plenums. Insert manome-
ter hose flush with inside
edge of hole or insulation.
Seal around the hose with
permagum. Connect the
zero end of the manometer
to the discharge (supply) side of the system. On
ducted systems, connect the other end of manometer
to the return duct as above. For systems with
non--ducted returns, leave the other end of the
manometer open to the atmosphere.
3 -- With only the blower motor running and the
evaporator coil dry, observe the manometer
reading. Adjust blower motor speed to deliver
theairdesiredaccordingtothejobrequirements.
4 -- External static pressure drop must not be more
than 0.5 W.C. (124 Pa).
5 -- Seal around the hole when the check is complete.
D--Blower Speed Taps Lealess Motors
--1 Models
Blower speed tap selection is accomplished by changing
the taps at the blower motor harness connector. Discon-
nect harness connector from motor to expose speed se-
lectors. See table 11 for blower speed tap selection.
To Change Blower Speed:
1 -- Turn off electric power to furnace.
2 -- Remove front panel and blower access door.
3 -- Disconnect blower motor harness from motor.
4 -- Select desired speeds for heating and cooling. (Red
= continues fan and low heat, Yellow = heating,
Black = cooling, White = common).
5 -- Depress harness connector tab to release wire ter-
minal. Select connector location for new speed (re-
fer to unit wiring diagram). Insert wire terminal un-
til it is securely in place. See figure 19.
6 -- Replace harness connector to motor.

Page 15
E--Blower Speed Taps Leaded Motors
--2 Models
Blower speed tap changes are made on the SureLight
control board. See figure 2. Unused taps must be se-
cured on dummy terminals PARK M1 and or PARK
M2 on the SureLight board. The heating tap is con-
nected to the ACB HEAT  terminal and the cooling tap
is connected to the ACB COOL terminal. The continu-
ous blower tap is connected to the ACB LOW termi-
nal.
To change existing heat tap, turn off power then switch
out speed tap on ACB HEAT with tap connected to
PARK M1 or PARK M2. See table 12 for blower mo-
tor tap colors for each speed.
TABLE 11
TABLE 12
FIGURE 19
BLOWER SPEED TAP SELECTION
Leadless Motors
HARNESS
CONNECTOR
MOTOR
DEPRESS TAB TO RELEASE
WIRE TERMINAL. SELECT
CONNECTOR LOCATION
FOR NEW SPEED (REFER TO
UNIT WIRING DIAGRAM).
INSERT WIRE UNTIL IT IS SE-
CURELY IN PLACE.
VI--MAINTENANCE
WARNING
Disconnect power before servicing unit.
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DIWHU VHUYLFLQJ
At the beginning of each heating season, the system
should be checked as follows:
Blowers
Check and clean blower wheels for any debris. Blower
motors are prelubricated for extended bearing life. No
further lubrication is needed.
Filters
All G27M filters are installed external to the unit. Filters
should be inspected monthly and cleaned or replaced
when necessary to assure proper furnace operation.
See table 13 for filter sizes. Replacement filters for
G27M--60/75 units must have a minimum velocity rat-
ing of 400 FPM. Replacement filters for G27M--100/120
units require a minimum velocity rating of 625 FPM.
TABLE 13
FILTER SIZE
MODEL NO. UPFLOW / HORIZ.
(1 filter) DOWNFLOW (2 filters)
G27M--60/75 16 x 20 x 1 16 x 20 x 1
G27M--100/120 20 x 20 x 1 16 x 20 x 1
WARNING
Blower door must be securely in place when blow-
er and burners are operating. Gas fumes, which
could contain carbon monoxide, can be drawn
into living space resulting in personal injury or
death.
Flue And Chimney
Check flue pipe, chimney and all connections for tight-
ness and to make sure there is no blockage. Draft at flue
collar should always be negative.
Burners
Burners and burner flame should be inspected at the
beginning of each heating season. Clean burners, if
necessary, as outlined below:
1 -- Turn off electrical and gas supply to unit.
2 -- Disconnect gas supply piping and remove gas
manifold.
3 -- Disconnect ignitor and flame sensor leads.
4 -- Remove burner tray and burners.
5 -- Clean burner retention ring with wire brush as
shown in figure 20.
6 -- Use test tube brush to clean inside of each burner.
7 -- Replace burners and burner tray, making sure
burners are properly seated in slots on tray and ori-
fice in manifold.

Page 16
8 -- Reinstall burner box and gas supply piping. Recon-
nect ignitor and flame sensor leads.
9 -- Carefully check all piping connections (factory and
field) for gas leaks. Use a leak detecting solution or
other preferred means.
10 --Restore electrical power and gas supply. Follow
lighting instructions on front of unit. Check burner
flame and adjust if necessary.
CAUTION
Some soaps used for leak detection are corrosive
to certain metals. Carefully rinse piping thorough-
ly after leak test has been completed. Do not use
matches, candles, flame or other sources of igni-
tion to check for gas leaks.
Heat Exchanger
NOTE--Use papers or protective covering in front of
furnace while cleaning furnace.
Due to dimples designed in the heatexchanger, clean-
ing is not recommended. Removal is for inspection
only.
Electrical
1 -- Check all wiring for loose connections.
2 -- Check for correct voltage at unit (unit operating).
3 -- Check amp--draw on blower motor.
Motor Nameplate__________Actual__________
FIGURE 20
CLEANING BURNERS
BOTTLE
BRUSH
RETENTION
RING
BURNER
REMOVE SCREWS (4)
SLIDE ENTIRE ASSEMBLY OUT OF CABINET
HEAT EXCHANGER REMOVAL (unit shown in horizontal position)
FIGURE 21

3DJH 
VII--WIRING DIAGRAM AND SEQUENCE OF OPERATION
TYPICAL G27M WIRING DIAGRAM
(--2 Model 60 Hz. Units Only)

3DJH 
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 valve is 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.

Page 19
HEATING SEQUENCE OF OPERATION
NORMAL HEATING MODE ABNORMAL HEATING MODE
CONTROL SELF–CHECK OKAY?
BURNER OFF?
(CONTINUOUS FLAME CHECK)
NORMAL OPERATION:
LED #1 –– SLOW FLASH
LED #2 –– FAST FLASH
YES
YES
YES
GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER DELAY OFF.
LED #1 ON
LED #2 ON
(RESET CONTROL BY TURNING MAIN POWER OFF.)
NO
NO
POLARITY REVERSED.
LED #1 –– FAST FLASH
LED #2 –– SLOW FLASH
NO
POWER ON
COMBUSTION AIR BLOWER ON FOR 1 SECOND.
POLARITY OKAY?
ROLLOUT SWITCH CLOSED?
THERMOSTAT CALLS FOR HEAT:
LED #1 –– FAST FLASH
LED #2 –– FAST FLASH
PRIMARY & SECONDARY LIMIT SWITCHES CLOSED?
PRESSURE SWITCH CONTACTS OPEN?
(HIGH AND LOW FIRE)
TWO–STAGE MODE
CALL FOR 1ST STAGE HEAT TWO–STAGE MODE
SINGLE STAGE T’STAT SINGLE–STAGE MODE HIGH HEAT ONLY
COMBUSTION AIR BLOWER
ON LOW SPEED (2.5 MINUTES)
LOW–FIRE PRESSURE
SWITCH CLOSED?
PREPURGE
15 SECONDS
COMBUSTION AIR BLOWER
ON HIGH SPEED (2.5 MINUTES)
HIGH–FIRE PRESSURE
SWITCH CLOSED?
GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER OFF DELAY.
LED #1 –– ON
LED #2 –– SLOW FLASH
GAS VALVE OFF. COMBUSTION AIR BLOWER ON.
INDOOR BLOWER ON.
LED #1 –– SLOW FLASH
LED #2 –– OFF
GAS VALVE OFF.
COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER OFF DELAY.
LED #1 OFF
LED #2 SLOW FLASH
(Sequence holds until pressure switch
closes or thermostat resets control.)
PRESSURE SWITCH WATCHGUARD
(5 MINUTES)
COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER OFF.
LED #1 OFF
LED #2 SLOW FLASH
NO
NO
COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER ON.
LED #1 –– SLOW FLASH
LED #2 –– ON
NO
NO
WATCHGUARD
60–MINUTE DELAY
YES
YES
NO
YES YES
GAS VALVE OFF. COMBUSTION AIR BLOWER ON.
INDOOR BLOWER ON.
LED# 1 ALTERNATING FAST FLASH
LED# 2 ALTERNATING FAST FLASH
CHECK FOR BROKEN IGNITOR OR OPEN
IGNITOR CIRCUIT
SIGNAL IMPROPER
GROUND AT LED.
SIGNAL HOLDS
UNTIL IS
PROPERLY
GROUNDED.
IS THERE
PROPER GROUND? IS VOLTAGE
ABOVE 75 VOLTS?
YESNO
LOW VOLTAGE SIGNAL AT LED HOLDS
UNTIL VOLTAGE RISES ABOVE 75 VOLTS
NO
IS COMBUSTION AIR BLOWER ENERGIZED?
(HTG ACC ENERGIZES WITH C.A.B)

Page 20
HEATING SEQUENCE CONTINUED
NORMAL HEATING MODE ABNORMAL HEATING MODE
IGNITION SEQUENCE BEGINS
(Ignition trial counter initiated.)
FLAME RECTIFICATION CURRENT
CHECK
(0.15 microamps)
W2 TIMED MODE
SINGLE STAGE T'STAT
SINGLE-STAGE MODE
HIGH HEAT ONLY
CFM" LED-DS4 BLINKS ONCE
FOR EVERY 100CFM. INDOOR
BLOWER ON LOW SPEED
PREPURGE
15 SECONDS
CFM" LED-DS4 BLINKS ONCE FOR
EVERY 100CFM. INDOOR BLOWER
ON HIGH HEATING SPEED
GAS VALVE
DE-ENERGIZED
NO
IGNITOR WARM-UP
(20 SECONDS)
TWO-STAGE MODE
CALL FOR 1st STAGE HEAT
GAS VALVE OPEN
HIGH AND LOW HEAT
(Ignitor remains energized
for another 1 second.)
MAIN GAS VALVE OPEN
LOW HEAT ONLY
(Ignitor remains energized
for another 1 second.)
MAIN GAS VALVE OPEN
LOW HEAT ONLY
(Ignitor remains energized
for another 1 second.)
SECOND STAGE
TIME ON BEGINS
4 SECONDS
FLAME PRESENT?
(Response time -- 2 seconds maximum.)
"HEAT" LED-DS2 LIT INDOOR BLOWER ON DELAY BEGINS
(45 seconds.)
W2 TIMED MODE
SINGLE STAGE T'STAT
SINGLE-STAGE MODE
HIGH HEAT ONLY
TWO-STAGE MODE
CALL FOR 1st STAGE HEAT
FLAME PRESENT?
(Continuous flame check)
FLAME SIGNAL
(0.7 microamps)
PRIMARY LIMIT SWITCH CLOSED?
ROLLOUT SWITCH CLOSED?
PRESSURE SWITCH CLOSED?
(CONTINUED ON NEXT PAGE)
CONTROL CHECKS RETRY
FOR IGNITION COUNT.
HAVE FOUR RETRIES
BEEN COMPLETED?
WATCHGUARD -- FLAME FAILURE.
GAS VALVE OFF.
COMBUSTION AIR BLOWER OFF.
INDOOR BLOWER OFF.
LED #1 & LED #2 ALTERNATE FLASH.
NO YES
LOW FLAME SIGNAL
LED #1 -- SLOW FLASH
LED #2 -- FAST FLASH
GAS VALVE DE-ENERGIZED.
COMBUSTION AIR BLOWER DE-ENERGIZED.
INDOOR BLOWER ON.
LED #1 - SLOW FLASH; LED #2 - ON.
GAS VALVE DE-ENERGIZED.
COMBUSTION AIR BLOWER DE-ENERGIZED.
INDOOR BLOWER OFF DELAY (Selected time)
LED #1 -- ON
LED #2 -- SLOW FLASH
WATCHGUARD 60MINUTE DELAY
(SIGNAL LIMIT OR IGNITION FAILURE)
GAS VALVE OFF. COMBUSTION AIR
AND INDOOR BLOWER OFF.
LED #1 AND LED #2
ALTERNATING SLOW FLASHES
NO
NO
NO
NO
HAS LIMIT RESET?
(See table 1 DIAGNOSTIC CODES for limit reset operation) YES
NO
CONTINUED ON NEXT PAGE
WATCHGUARD 60MINUTE DELAY
LIMIT SWITCH. GAS VALVE OFF.
COMBUSTION AIR AND INDOOR
BLOWER OFF. LED #1 SLOW FLASH
AND LED #2 ON. IS 60 MINUTE RESET
PERIOD COMPLETE?
YES