Lennox G24M Service Manual

G24M

Blower wheel nominal

g

Nominal cooling

Corp. 9723−L12

Service Literature

G24M SERIES UNITS

G24M series units are mid−efficiency gas furnaces manufac­tured with tubular steel heat exchangers formed of alumi­nized steel. G24M units are available in heating capacities of
45,000 to 140,000 Btuh and cooling applications up to 5
tons. Refer to Engineering Handbook for proper sizing.

Units are factory equipped for use with natural gas. A kit is
available for conversion to LPG operation. Early model G24M
units use electronic (direct spark) ignition. Late model G24M
model units feature the Lennox SureLight silicon−nitride
ignition system. The G24MX unit meets the California Nitro­gen Oxides (NO
ciency requirements. All units use a redundant gas valve to
assure safety shut−off as required by A.G.A. or C.G.A.

Units may be installed in upflow, downflow or horizontal posi­tion. The heat exchanger is designed for upright or horizon­tal use only. When the unit is installed in the downflow posi­tion, the heat exchanger is field removed and reinstalled so it
is upright when the cabinet 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 simply dis­connecting gas, unplugging wiring harness and spark wires
and removing four screws holding the heat exchanger in
place. Then the heat exchanger slides out of the cabinet.

Input Btuh (kW) 45,000 (13.2) 60,000 (17.6) 75,000 (22.0)
Output Btuh (kW) 36,900 (10.8) 49,200 (14.4) 61,700 (18.1)
A.F.U.E. 80.1% 80.5% 80.5% 80.1% 80.0%

California Seasonal Efficiency 75.4% 76.4% 75.9% 76.8% 76.8%
Flue size connection diameter  in. (mm) round 3 (76) 4 (102)
Temperature rise range  F (C) 30 - 60 (17 - 33) 45 - 75 (25 - 42)
High static certified by A.G.A./C.G.A.  in wg. (Pa) .50 (125)
Gas Piping Size I.P.S. Natural or LPG/propane 1/2 (13)

Blower wheel nominal

Blower motor output  hp (W) 1/4 (187) 1/3 (224) 1/4 (187) 1/3 (224)

Shipping weight  lbs. (kg) 1 package 130 (59) 135 (61)
Electrical characteristics 120 volts  60 hertz  1 phase (less than 12 amps) All models

LPG/propane kit LB-69845L (38K84)
Twinning Kit 96J69  5 lbs. (2 kg)
Up−Flow/Horizontal Filter and Filter Rack Kits

No. & sizeoffilters− in. (mm)

Down-flow Combustible Floor Base
Sidewall Power Venting Kit 79J15  25 lbs. (11 kg)
Hanging Bracket Kit

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.

Polyurethane frame type filter is furnished with kit.
Filters are not furnished with kit and must be ordered extra.

diameter x width

Nominal coolin

that can be added

Down-flow

Filter Kit

) Standards and California Seasonal Effi-

x

Model No. G24M2(X)-45 G24M2-60 G24M3(X)-60 G24M2-75 G24M3(X)-75

in. 9 x 7 10 x 7 9 x 7 10 x 7
mm 229 x 178 254 x 178 229 x 178 254 x 178

Tons 1, 1-1/2 or 2 2, 2-1/2 or 3 1, 1-1/2 or 2 2, 2-1/2 or 3
kW 3.5, 5.3 or 7.0 7.0, 8.8 or 10.6 3.5, 5.3 or7.0 7.0, 8.8 or 10.6

 Optional Accessories (Must Be Ordered Extra) 

Catalog No.
No. & Size of Filters  in. (mm)

Revised 07−2001

All specifications in this manual are subject to change. Pro­cedures outlined in this manual are presented as a recom­mendation only and do not supersede or replace local or
state codes. In the absence of local or state codes, the
guidelines and procedures outlined in this manual (except
where noted) are recommended only and do not constitute
code.

SPECIFICATIONS

LB-79239A (67J91)  10 lbs. (4 kg)

Single (32J02) Ten Pack (66K64)

(1) 16 x 20 x 1 (406 x 508 x 25)

LB-69843A (32J01)  3 lbs. (1 kg)

(2) 16 x 20 x 1 (406 x 508 x 25)

LB-69957 (46J66)  15 lbs. (8 kg)

Page 1

© 1998 Lennox Industries Inc.

Litho U.S.A.

SPECIFICATIONS

Bl

l

Nominal

Model No. G24M4(X)-75 G24M3/4(X)-100 G24M4/5(X)-100

Input Btuh (kW) 75,000 (22.0) 100,000 (29.3)

Output Btuh (kW) 61,700 (18.1) 82,000 (24.0)

A.F.U.E. 80.0% 80.1% 80.0%

California Seasonal Efficiency 76.3% 76.5% 77.0%

Flue size connection diameter  in. (mm) round 4 (102)

Temperature rise range  F (C) 45 - 75 (25 - 42) 35 - 65 (19 - 36)

High static certified by A.G.A./C.G.A.  in wg. (Pa) .50 (125)

Gas Piping Size I.P.S. Natural or LPG/propane 1/2 (13)

ower wheel nomina

diameter x width

Blower motor output  hp (W) 1/2 (373) 3/4 (560)

that can be added

Shipping weight  lbs. (kg) 1 package 140 (64) 175 (79) 175 (79)

Electrical characteristics 120 volts  60 hertz  1 phase (less than 12 amps) All models

LPG/propane kit LB-69845L (38K84) LB-69845K (81J14)

Twinning Kit 96J69  5 lbs. (2 kg)

Up−Flow/Horizontal Filter and Filter Rack Kits

No. & sizeoffilters− in. (mm)

Down-flow

Filter Kit

Down-flow Combustible floor Base LB-79239A (67J91)  10 lbs. (4 kg) LB-79239B (67J92)  10 lbs. (4 kg)

cooling

Catalog No. LB-69843A (32J01)  3 lbs. (1 kg)

No. & Size of Filters  in. (mm)

in. 12 x 8 12 x 9

mm 305 x 203 305 x 229

Tons 2, 2-1/2, 3, 3-1/2 or 4 3-1/2, 4, 5 or 6

kW 7.0, 8.8, 10.6, 12.3 or 14.1 12.3, 14.1, 17.6 or 21.1

 Optional Accessories (Must Be Ordered Extra) 

Single (32J02) Ten Pack (66K64)

(1) 16 x 20 x 1 (406 x 508 x 25)

(2) 16 x 20 x 1 (406 x 508 x 25)

Single (46J14) Ten Pack (66K65)

(1) 20 x 20 x 1 (508 x 508 x 25)

Sidewall Power Venting Kit 79J15  25 lbs. (11 kg)

Hanging Bracket Kit LB-69957 (46J66)  15 lbs. (8 kg)

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.

Polyurethane frame type filter is furnished with kit.
Filters are not furnished with kit and must be ordered extra.

Page 2

SPECIFICATIONS

Blower wheel nominal

g

Nominal cooling

No. & Size

External Static

Model No.

Input Btuh (kW)

Output Btuh (kW)

A.F.U.E.

California Seasonal Efficiency

Flue size connection diameter  in. (mm) round

Temperature rise range  F (C)

High static certified by A.G.A./C.G.A.  in wg. (Pa)

Gas Piping Size I.P.S. Natural or LPG/propane − in (mm)

Blower wheel nominal

diameter x width

Blower motor output  hp (W)

Nominal coolin

that can be added

Shipping weight  lbs. (kg) 1 package

Electrical characteristics

LPG/propane kit

Twinning Kit

Up−Flow/Horizontal Filter and Filter Rack Kits
No. and sizeoffilters − in. (mm)

Catalog No.

Down-flow

Filter Rack

Down-flow Combustible Floor Base

Sidewall Power Venting Kit

Hanging Bracket Kit

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.

 Polyurethane frame type filter is furnished with kit.
2 in. x 5 in. (51 mm x127 mm) flue adaptor furnished with -140 input furnaces for connection to furnace induced draft blower.
Filters are not furnished with kit and must be ordered extra.

No. & Size
of Filters

in.

mm

Tons 2, 2-1/2, 3, 3-1/2 or 4

kW 7.0, 8.8, 10.6, 12.3 or 14.1

 Optional Accessories (Must Be Ordered Extra) 

in.

mm

G24M3/4-120 G24M4/5(X)-120

120,000 (35.2)

98,400 (28.8)

80.0% 80.1%

Not Available 75.5%

4 (102) 5 (127)

45 - 75 (25 - 42)

.50 (125) .65 (162)

1/2 (13)

12 x 8 12 x 9

305 x 203 305 x 229

1/2 (373) 3/4 (560)

3-1/2, 4, 5 or 6

12.3, 14.1, 17.6 or 21.1

175 (79)

120 volts  60 hertz  1 phase (less than 12 amps) All models

LB-69845K (81J14)

96J69  5 lbs. (2 kg)

Single (46J14) Ten Pack (66K65)

(1) 20 x 20 x 1 (508 x 508 x 25)

LB-69843A (32J01)  3 lbs. (1 kg)

(2) 16 x 20 x 1

(2) 406 x 508 x 25

LB-79239B (67J92)  10 lbs. (4 kg)

79J15  25 lbs. (11 kg)

LB-69957 (46J66)  15 lbs. (8 kg)

Single (58J93) Ten Pack (66K66)

(1) 20 x 20 x 1 (508 x 508 x 25)

G24M4/5-140

140,000 (41.0)

114,800 (33.6)

80.0%

Not Available

190 (86)

LB-79239C (67J93)

 12 lbs. (5 kg)

BLOWER DATA

G24M2-45, G24M2-60 AND G24M2-75 BLOWER PERFORMANCE

External Static

Pressure

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 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table

High Medium-High Medium-Low Low

Air Volume at Various Blower Speeds

Page 3

BLOWER DATA

External Static

External Static

G24M3-60 AND G24M3-75 BLOWER PERFORMANCE

Air Volume at Various Blower Speeds

Pressure

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 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table

High Medium-High Medium-Low Low

G24M4-75, G24M3/4-100 AND G24M3/4-120 BLOWER PERFORMANCE

External Static

Pressure

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 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table

High Medium-High Medium Medium-Low Low

Air Volume at Various Blower Speeds

Page 4

BLOWER DATA

External Static

G24M4/5-100, G24M4/5-120 AND G24M4/5-140 BLOWER PERFORMANCE

External Static

Pressure

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 in. (25 mm) cleanable filter (not furnished) in place. Also see Filter Air Resistance table

High Medium-High Medium Medium-Low Low

Air Volume at Various Blower Speeds

FILTER AIR RESISTANCE

cfm (L/s) in. w.g. (Pa)

0 (0) 0.00 (0)

200 (95) 0.01 (2)

400 (185) 0.03 (7)

600 (280) 0.04 (10)

800 (375) 0.06 (15)

1000 (470) 0.09 (22)

1200 (560) 0.12 (30)

1400 (655) 0.15 (37)

1600 (750) 0.19 (47)

1800 (845) 0.23 (57)

2000 (935) 0.27 (67)

2200 (1030) 0.33 (82)

2400 (1125) 0.38 (95)

2600 (1220) 0.44 (110)

Page 5

G24M

PARTS IDENTIFICATION

COMBUSTION AIR

INDUCER

FRONT LOUVERED

PANEL

TRANSFORMER

HEAT EXCHANGER

FLUE BOX

PRESSURE

SWITCH

PRIMARY

LIMIT

LOW PRESSURE SWITCH

(Propane Only)

BURNER

ASSEMBLY

VENT ADAPTER

ASSEMBLY

NOx

TURBULATOR

SECONDARY

LIMITS

CABINET TOP

G24M

CABINET

CABINET

BOTTOM

ROLLOUT SWITCH (2)

COMBUSTION AIR

PROVE SWITCH

ORIFICES

MANIFOLD

BURNERS

CONTROL

BOARD

DOOR INTERLOCK SWITCH

BLOWER ASSEMBLY

FIGURE 1

HEATING COMPONENTS (shown in horizontal position)

PRIMARY LIMIT

Bottom

Front

Left

Back

To p

GAS VALVE

COLLECTOR

FIGURE 2

Page 6

Right

HEAT EXCHANGER

COMBUSTION AIR INDUCER

(shown in upflow position with SureLight ignition system)

G24M BURNER ASSEMBLY

BURNER BOX

TOP

UPPER

BURNER

MOUNTING

RAIL

LOWER

BURNER

MOUNTING

RAIL

FLAME

SENSOR

ORIFICE

MANIFOLD

SURELIGHT

IGNITOR

ROLLOUT

SWITCHES

BURNER

VEST PANEL

GAS VALVE

BURNER BOX

PRIMARY LIMIT

Left

Front

HEAT EXCHANGER

To p

Back

Right

Bottom

FIGURE 3

G24M BLOWER DOOR COMPONENTS − BLOWER ACCESS

To Access Blower:

4

J135/
P135

J1/P1

1− Turn off power to unit and disconnect L1 and

L2 line voltage power.

2− Disconnect thermostat wiring connections

from furnace control board.
3− Remove screws (2) from blower panel.
4− Disconnect J135 from P135 from limit

control.
5− Disconnect J43 from blower motor.
6− Remove blower panel and lift from unit.

5

To p

Back

Right

6

2

Left

Front

Bottom

3

1

FIGURE 4

Page 7

I−UNIT COMPONENTS (Figures 1, 2, 3)

G24M 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 to the blower access
door.

G24M units are factory equipped with bottom return air panels
in place. The panels are designed to be field removed as re­quired for bottom air return. Indentations on side of units,
show where side return opening should be cut during installa­tion.

A−Blower Door Components (Figure 4)

Electrical burner control and blower control components
are located on the outside surface of the blower access
door. Jackplugs allow the blower door to be easily re­moved for blower service.

Located on the blower door are the unit transformer (T1),
the furnace control (A3) and door interlock switch (S51).
Furnace control (A3) combines the function of a burner
ignition control and blower control.

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 30VA with a 120V primary and a 24V sec­ondary.

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.

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.

3− SureLight Ignition System (A3)

Late Model G24M Units

Late model G24M units are equipped with the Lennox
SureLight ignition system. The system consists of ignitor
(figure 5) and ignition control board (figure 6 with control
terminal designations in table 1). The board and ignitor
work in combination to ensure furnace ignition and ignitor
durability. The SureLight integrated board controls all
major furnace operations. The board also features two
LED lights for troubleshooting and two accessory termi­nals rated at (4) four amps. See table 2 for troubleshoot­ing diagnostic codes. Table 3 and 4 show jack plug termi­nal designations. Units equipped with the SureLight
board can be used with either electronic or electro−me­chanical thermostats without modification. The SureLight
ignitor is made of durable silicon−nitride. Ignitor longevity
is also enhanced by voltage ramping by the control board.
The board finds the lowest ignitor temperature which will
successfully light the burner, thus increasing the life of the
ignitor.

a−flame sensor

The flame sensor uses flame rectification to sense com­bustion. During operation, flame is sensed by current
passed through the flame and sensing electrode. Figure 7
shows the gap between tip of the electrode and the burner
surface.

SURELIGHT IGNITOR

UPPER BURNER
MOUNTING RAIL

SURELIGHT IGNITOR BURNER ASSEMBLY

FLAME RETENTION RING

FIGURE 5

Page 8

ORIFICE

FLAME SENSOR

SURELIGHT CONTROL BOARD

LATE MODEL G24M UNITS

TABLE 1

SURELIGHT CONTROL TERMINAL DESIGNATIONS

ACB COOL
ACB HEAT
PARK
ACB LOW
ACC
TX
HOT
HTG ACC
NEUTRALS
24VAC HOT
24VAC RTN
FLAME SENSE

Blower − Cooling Speed (Line Volt)
Blower − Heating Speed (Line Volt)
Alternate Blower Speeds (Dead)
Continuous Low Speed Blower
Accessory Terminal (Line Volt)
120VAC Hot to Transformer
120VAC Hot Input
Heat Only Accessory (Line Volt)
120VAC Neutrals
24VAC Hot from Transformer
24VAC Return from Transformer
Flame Sense Terminal

NORMAL FLAME SIGNAL > 0.7 MICROAMPS
LOW FLAME SIGNAL <
MINIMUM FLAME SIGNAL <

FLAME SENSOR TO BURNER GAP

1/4 in. 7 mm

 1/32 in. 0.79

BURNER

SIDE VIEW END VIEW

FIGURE 6

TABLE 2

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 FAST

FLASH

SLOW FLASH ON

OFF SLOW FLASH

ALTERNATING SLOW

FLASH

SLOW FLASH OFF

ON SLOW FLASH

ON
ON

OFF

FAST FLASH SLOW FLASH

SLOW FLASH FAST FLASH

ALTERNATING FAST

FLASH

SIMULTANEOUS

SLOW FLASH

SIMULTANEOUS FAST

ALTERNATING SLOW

ALTERNATING FAST

FLASH

FLASH

ON

OFF

ON

FLASH

Also signaled during cooling and continues fan.

Normal operation − signaled when heating demand initiated at thermostat.

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 or 97L48: Limit must close within 3 minutes or board

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.

Flame sensed without gas valve energized.

Rollout switch open. OR: 9 pin connector improperly attached.

Circuit board failure or control wired incorrectly.

Main power polarity reversed. Switch line and neutral.

Low flame signal. Measures below .7 microAmps. Replace flame sense rod.

Improper main ground or line voltage below 75 volts; OR: Broken ignitor; OR:

Power − Normal operation

goes into one hour limit Watchguard.

Watchguard − burners fail to ignite.

Open ignitor circuit.

mm

3/16 in.

4.7 mm

FLAME

SENSOR

FIGURE 7

0.7 MICROAMPS

0.15 MICROAMPS

11/16 in. 18 mm

 1/32 in. 0.79

mm

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 9

TABLE 3

SureLight BOARD J156 (J2) TERMINAL

DESIGNATIONS

PIN # FUNCTION

1 Ignitor

2

3

4

Combustion Air Blower Line Voltage

5

6

Combustion Air Blower Neutral

Not Used

Ignitor Neutral

Not Used

TABLE 4

SureLight BOARD J58 (J1) TERMINAL

DESIGNATIONS

PIN # FUNCTION

1 Primary Limit In

2

3

4

5

6

Pressure Switch and Primary Limit Out

7

8

9

Gas Valve Common

Roll Out Switch Out

Gas Valve 24V

Pressure Switch In

Not Used

Roll Out Switch In

Ground

ELECTROSTATIC DISCHARGE (ESD)

Precautions and Procedures

CAUTION

Electrostatic discharge can affect electronic
components. Take precautions during furnace
installation and service to protect the furnace’s
electronic controls. Precautions will help to
avoid control exposure to electrostatic dis­charge by putting the furnace, the control and
the technician at the same electrostatic poten­tial. Neutralize 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.

not proven within 2−1/2 minutes, the control goes into
Watchguard−Pressure Switch mode for a 5−minute re−set
period.

NOTE − The G24M furnace contains electronic com­ponents that are polarity sensitive. Make sure that the
furnace is wired correctly and is properly grounded.

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. G24M
units with 63K89, 24L85 or 56L83: the ignitor stays ener­gized the first second of the 4 second trial. G24M units with
board 97L48: ignitor stays energized during the 4−second
trial until flame is sensed. If ignition is not proved during the
4−second period, the control will try four more times with an
inter purge and warm−up time between trials of 35 sec­onds. After a total of five trials for ignition (including the ini­tial trial), the control goes into Watchguard−Flame Failure
mode. After a 60−minute reset period, the control will begin
the ignition sequence again.
The SureLight control board has an added feature that
prolongs the life of the ignitor. After a successful ignition,
the SureLight control utilizes less power to energize 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.

c−Fan Time Control

The fan on time of 45 seconds is not adjustable. Fan off
time (time that the blower operates after the heat demand
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 adjustable to satisfy
individual applications. See figure 8.

NOTEIf fan off" time is set too low, residual heat in
heat exchanger may cause primary limit S10 to trip re­sulting in frequent cycling of blower. If this occurs, ad­just blower to longer time setting.

b−Electronic Ignition

On a call for heat the SureLight control monitors the com­bustion air inducer prove switch. The control will not begin
the heating cycle if the prove switch is closed (by−passed).
Once the prove switch is determined to be open, the com­bustion air inducer is energized. When the differential in
the prove switch is great enough, the prove switch closes
and a 15−second pre−purge begins. If the prove switch is

FAN-OFF TIME ADJUSTMENT

60sec. 90sec. 120sec. 180sec.

To adjust fan−off timing, flip dip switch to desired setting.

FIGURE 8

Page 10

4− Ram Control (A3)

Early Model G24M Units

The furnace control combines burner ignition functions
with blower control functions.

Early model G24M units utilize a furnace control manufac­tured by RAM Electronics Corporation. The RAM" board is a
printed circuit board which controls the blower, gas valve,
combustion air blower and ignition spark. It also monitors the
flame, limit and gas valve operation. The control has a non−
adjustable, factory preset on" fan timing (45 seconds). Fan
off" timings are adjustable. The board utilizes both 120 and
24VAC. See figure 9. The board is also equipped with a
diagnostic LED for use when troubleshooting the unit.

When the furnace is idle (blower off and no heating or cool­ing demand), the diagnostic LED flashes at a slow steady
rate. On a call for heat, the diagnostic LED begins flashing
at a fast rate and the combustion air blower is energized.
The LED flashes different codes to indicate problem con­ditions. The diagnostic LED lights red (not flashing) to indi­cate control board failure. Table 7 shows how to interpret the
other LED modes.

Pre-Purge

On a call for heat, the combustion air blower begins oper­ating. If the combustion air prove switch closes, the com­bustion air blower continues to operate for 45 seconds
(pre-purge) before allowing ignition. Pre-purge allows the
heat exchanger to be cleared of combustion products and
to introduce fresh air for combustion. If the combustion air
prove switch does not close, the combustion air blower
continues to run indefinitely (until the prove switch closes).

Post-Purge

After a demand, the combustion air blower continues to
operate for 5 seconds (post-purge) before stopping. Post­purge allows the heat exchanger to be cleared of combus­tion products.

Ignition Control

The ignition control is a direct spark ignition control module
integral to the furnace control. See figure 9. When there is a
call for heat, the control delays ignition until combustion air
blower operation has been proved and pre-purge period
has elapsed. It then opens the gas valve and generates a
spark to ignite the burners. Trial for ignition lasts for 7 sec­onds. At the same time, the control begins monitoring the
flame sensor. If the flame current is too weak (less than 1
microamp) or if the burners do not ignite (within the 7 sec­ond ignition trial), the control will shut off the spark ignitor
and the combustion air blower and de-energize the gas
valve. Flame current should be between 1 and 5 micro­amps to keep the gas valve open. See figure 21.

The control will attempt to ignite the burners up to two more
times. Each time the control restarts the ignition sequence,
it begins with a 45 second pre-purge. If flame is not sensed
after the third trial, the control locks out. Lockout means that
the control shuts off the gas valve, spark and combustion air
blower for 60 minutes. At the end of 60 minutes the control
completely resets and will attempt ignition up to three times.
The control can be manually reset before the end of 60 min­utes by momentarily turning off power to the unit.

DANGER

Shock hazard.
Spark related components contain high voltage.

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 11

SPARK OUTPUT

LINE VOLTAGE

TERMINAL

CONNECTIONS

EARLY MODEL G24M FURNACE CONTROL (A3)

24VAC VOLT

TERMINAL

CONNECTIONS

See Table 6 for
Terminal Func-

tions

DIAGNOSTIC

LED

TABLE 5

Furnace Control A3 Limit Response During Operation

Response

Condition

Loss of Flame
Sensed Before
End of 45 sec­ond Blower On

Delay

(3 or Fewer

Trials

for Ignition)

Loss of Flame

Sensed After

45 second
Blower On

Delay (3 or

Fewer Trials

for Ignition)

Loss of Flame

Sensed (More

Than 3 Trials

for Ignition)

Flame Sensed

Without

Demand

Primary or Sec-

ondary Limit

Open

Rollout Switch

Open

Combustion Air

Prove Switch

Open

Combustion

Air

Blower

On

On

Off Off Off 2 Flashes

On Off On 5 Flashes

On Off On 4 Flashes

On Off On 4 Flashes

On Off On 3 Flashes

Gas

Valve

On

(Spark

Starts

Within 0.8

seconds)

Off Then

On With

Spark

After Pre-

Purge

Supply

Air

Blower

On Fast Flash

On Fast Flash

Diagnostic

LED

PLUG

P20

FAN-OFF TIMING

SWITCHES

THERMOSTAT

CONNECTIONS

FIGURE 9

When flame is sensed, the indoor blower starts after a 45
second delay. Gas valve remains open and blower contin­ues to run until demand stops, flame sensor senses loss of
flame, a limit opens or the prove switch opens. If any of
these events occur during a thermostat demand, the gas
valve closes and the diagnostic LED registers the error
condition (table 5).

Blower Control and Timings

DANGER

Electrical Shock Hazard.
This control contains field adjustable switches
and also contains line voltage. Make sure power
is disconnected before making any field adjust­ments or performing any service procedure.

NOTEIf fan off" time is set too low, residual heat in
heat exchanger may cause primary limit S10 to trip re­sulting in frequent cycling of blower. If this occurs, ad­just blower to longer time setting.

Fan ON" timing (time that the burners operate before the
supply air blower starts) is fixed at 45 seconds and cannot
be adjusted.

Fan OFF" timings (time that the blower operates after a
heating or cooling demand has been satisfied) are deter­mined by the arrangement of switches on the furnace con­trol board. See figure 9. To adjust fan off " timings, gently
reposition the switches to a new timing position. Figure 10

Page 12

shows the various fan off" timings and how switches should
be positioned. Unit is shipped with a factory fan off" setting of
180 seconds. Fan off" time will affect comfort and efficiency
and is adjustable to satisfy individual applications. The fan
off" timing is initiated after a heating or cooling demand but
not after a blower demand (that is, when indoor thermostat
switch is changed from ON to AUTO and heating/cooling de­mand is not present, the blower stops immediately).

TABLE 6

FURNACE CONTROL A3 TERMINAL DESIGNATIONS

Terminal Type Function

24VAC HOT 1/4" Spade 24VAC In From Transformer

GND 1/4" Spade To Cabinet Ground

Y Screw Strip Cooling Demand
G Screw Strip Blower Demand
R Screw Strip 24VAC to Thermostat

W Screw Strip Heating Demand

C Screw Strip 24VAC Common

120VAC HOT 1/4" Spade Line Voltage In

120VAC RTN 1/4" Spade Line Voltage Neutral

120VAC TX 1/4" Spade Line Voltage Out To Transformer

CMB J20/P20 Pin 1

CMB RTN J20/P20 Pin 2

ACB HEAT 1/4" Spade

ACB LOW 1/4" Spade

ACB COOL 1/4" Spade

VLV HOT J20/P20 Pin 13 24VAC to Gas Valve

VLV RTN J20/P20 Pin 9

PSW IN

HIL IN

HIL OUT

RO OUT J20/P20 Pin 7 24VAC Out To Rollout Switches

RO IN J20/P20 Pin 15 24VAC In From Rollout Switches

SPARK

ELECTRODE

FS J20/P20 Pin 12 Flame Microamp Sensing

J20/P20

Pin 10

J20/P20

Pin 11

J20/P20

Pin 14

Male Spark

Plug Type

FAN-OFF TIME ADJUSTMENT SWITCHES

LOCATED ON FURNACE CONTROL (A3)

Switched 120VAC to

Combustion Air Blower

120VAC Common

Combustion Air Blower

Switched 120VAC to

Blower Heating Tap

120VAC Output to Supply Air

Blower for Continuous Operation

During No Demand

Switched 120VAC to Blower

Cooling Tap

24VAC Common From

Gas Valve

24VAC In From Pressure Switch

Switch Open: Prohibits Ignition

Switch Closed: Allows Ignition

24VAC In From Limits

Limit Open: Closes Gas Valve

Limits Closed: Allows Ignition

24VAC to Limit Train
and Pressure Switch

High Voltage Out To

Spark Electrode

Fan-Off Timings
Switch
12

Off

On On

Off
OnOff
OffOn

Timing
Seconds

90
120
180
240

FIGURE 10

Diagnostic LED

The furnace control is equipped with a diagnostic LED
used for troubleshooting the unit and the control. LED
functions are shown in table 7.

TABLE 7

Furnace Control A3 Diagnostic LED

LED State Meaning Remedy

Steady On Control Failure Replace Control

Slow Flash

Fast Flash

Two

Flashes

Three

Flashes

Four

Flashes

Five

Flashes

Normal Operation and

No Call For Heat

Normal Operation with

Call For Heat

Control Lockout

Pressure Switch Open

Open Limit

Flame Sensed and Gas

Valve Not Energized.

- - - -

- - - -

Failed to Sense or Sustain

Flame. Check Gas Valve,

Burners, Spark Electrode
and Wire, Flame Sensor.

Replace Control If All OK.

Failed to Prove Combus-

tion Blower Operation or

Blocked Vent. Repair or

Replace as Necessary.

Check Primary Limit, Rol-

lout Switches and Secon­dary Limits. Find source of
Overtemperature. If all OK,
Reset or Replace Limits as

Necessary.

Check Gas Valve. If OK,

Check Flame Sensor.

B−Blower Motors and Capacitors

All G24M units use direct drive blower motors. All motors
used are 120V permanent split capacitor motors to ensure
maximum efficiency. See table8 for ratings.

TABLE

BLOWER MOTOR HP

G24M BLOWER RATINGS 120V 1PH

G24MQ2
G24MQ3
G24MQ3/4
G24MQ4
G24MQ4/5
G24MQ5/6

SUPPLY AIR BLOWER

AND SECONDARY LIMITS

To Remove Blower From Unit: Remove Bolts and

Wiring Jackplugs. Then Slide Out Front of Unit.

8

1/4 5MFD 370V
1/3
1/2
1/2 7.5MFD 370V
3/4 40MFD 370V
3/4 40MFD 370V

MOTOR

CAPACITOR

FIGURE 11

5MFD 370V

7.5MFD 370V

SECONDARY

LIMITS (S21)

Left

Front

Bottom

CAP

To p

Back

BLOWER

MOTOR

Right

Page 13

C−Combustion Air Inducer (B6)

All G24M units use a combustion air inducer to move air
through the burners and heat exchanger during heating
operation. Some early model G24M units are equipped
with a blower that uses a PSC (Permanent Split Capacitor)
120VAC motor. PSC motors use run capacitors. Other ear­ly and late model G24M units are equipped with a blower
that uses a shaded pole 120V motor. The motor operates
during all heating operation and is controlled by furnace
control A3. For G24M units equipped with the Ram ignition
system, the blower will operate for 45 seconds before
burner ignition (pre-purge) and for 5 seconds after the gas
valve closes (post-purge). For G24M units equipped with
the SureLight ignition system, the blower will operate for
15 seconds before burner ignition (pre purge) and for 5
seconds after the the gas valve closes (post purge).

A pressure switch connected to the combustion air blower
housing is used to prove combustion air blower operation.
The switch monitors air pressure in the blower housing. Dur­ing normal operation, the pressure in the housing is nega­tive. If pressure becomes less negative (signifying an ob­struction) the pressure switch opens. When the pressure
switch opens, the furnace control (A3) immediately closes
the gas valve to prevent burner operation.

D−Flame Rollout Switches (S47)

Flame rollout switch is a high temperature limit located on
top of the burner box. 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 limit is a N.C. SPST manual-reset limit con­nected in series with the ignition control A3. When S47
senses rollout, the ignition 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
blocked flue or lack of combustion air. The switch is facto­ry set and cannot be adjusted. The setpoint will be printed
on the side of the limit. The switch can be manually reset.
To manually reset a tripped switch, push the reset button
located on the control.

ROLLOUT SWITCH (S47)

E−Primary Limit Control (S10)

LIMIT CONTROL FOR

G24M SERIES UNITS

THIS TYPE AUTO-RESET LIMIT

IS USED FOR THE PRIMARY LIMIT (S10) AND

RIGHT SECONDARY LIMIT (S21)

(see FIGURE 11)

FIGURE 13

The primary limit (S10) on G24M units is located in the middle
of the heating vestibule panel. When excess heat is sensed in
the heat exchanger, the limit will open. If the limit is tripped, the
furnace 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 may have different setpoints for
each unit model number. However, the setpoint will be
printed on the side of the limit.

F−Secondary Limit Controls (S21)

The secondary limit (S21) on G24M units is located in the
blower compartment in the back side of the blower housing.
When excess heat is sensed in the blower compartment, the
limit will open. If the limit is tripped, the furnace 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. Two limits
are supplied in each furnace and each limit is a different
style (figures 13 and 14). The setpoint will be printed on the
side of the limit. If stick limit (figure 14) suffers from nuisance
trips and the furnace is in the horizontal position, replace
with limit kit no. 50L98.

SECONDARY LIMIT CONTROL

(S21)

INSULATING COVER

LIMIT

FOR G24M SERIES UNITS

SPADE CONNECTORS

THIS TYPE AUTO-RESET LIMIT IS

USED FOR THE LEFT SECONDARY

LIMIT (S21) (see FIGURE 11)

INSULATING COVER (s)

FIGURE 14

LIMIT

FIGURE 12

MANUAL

RESET BUTTON

G−Spark Electrode and Flame Sensor

Early Model G24M Units

Figure 15 shows the arrangement of flame sensor, spark
electrode and burners. The ignition control uses direct
spark to ignite the rightmost burner and the burners cross­light to the left. The flame sensor uses flame rectification to
sense combustion. 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.
Figure 16 shows the gap between tip of the electrodes and

the burner surface.

Page 14

1/8"(+1/64")

TYPICAL BURNER/ELECTRODE ORIENTATION

UPPER BURNER
MOUNTING RAIL

view looking at flame end of burners

BURNER

MANIFOLD ORIFICE

To p

Right Left

Bottom

GROUND

SPARK ELECTRODE

FLAME RETENTION RING

FIGURE 15

SPARK ELECTRODE TO BURNER GAP

11/16 in. 18 mm

 1/32 in. 0.79

mm

END VIEW

BURNER

SIDE VIEW

1/4 in. 7 mm

 1/32 in. 0.79

mm

FLAME

SENSOR

3/16 in.

4.7 mm

FIGURE 16

H−Gas Valve

The G24M uses a gas valve manufactured by Honeywell
(figure 17) or White Rodgers (figure 18). The valve is inter­nally redundant to assure safety shut−off. If the gas valve
must be replaced, 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 electronic ignition control. 24V applied to the
terminals energizes the valve.

Inlet and outlet pressure taps are located on the valve. A regu­lator adjustment screw is located on the valve. Refer to figure
17 or 18 for location of valve features.

An LPG changeover kit is available from Lennox. The kit in­cludes burner orifices and a regulator conversion kit.

I−Combustion Air Blower Prove

(Pressure) Switch (S64)

G24M series units are equipped with a combustion air
prove switch located on the vestibule panel. The switch is
connected to the combustion air blower housing by means
of a flexible silicone hose. It monitors air pressure in the
combustion air blower housing.

LOWER BURNER

MOUNTING RAIL

FLAME SENSOR

(SOME UNITS MAY HAVE

SENSOR LOCATED IN
MIDDLE OF BURNER)

TYPICAL ACCESS TO REGULATOR

FOR ADJUSTMENT AND L.P. CHANGEOVER

(Honeywell VR8205 series gas valve shown)

INLET

PRESSURE

TAP

GAS VALVE SHOWN IN OFF POSITION

PRESSURE

REGULATOR

ADJUSTING SCREW

(White)

ON

OFF

CAP SCREW

(Black)

SPRING

Tapered End

Down
(Red)

PRESSURE TAP

GAS INLET

FIGURE 17

WHITE RODGERS

GAS VALVE

KNOB SHOWN

IN OFF POSITION

GAS INLET

PRESSURE

TAP

36E GAS VALVE

REGULATOR

COVER SCREW

PRESSURE

OUTLET

PRESSURE

TAP

INLET

TAP

Page 15

FIGURE 18

The switch is a single-pole single-throw pressure switch
electrically connected to the furnace control. The purpose
of the switch is to prevent burner operation if the combus­tion air blower is not operating.

PROVE SWITCH

COMBUSTION AIR BLOWER

PROVE SWITCH

Normally Open

Closes on Negative Pressure

Sensing Tube

Attaches to Top

Side Of Blower

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 burner ac-

cess door and turn on unit electrical supply.

To p

PROVE

SWITCH

Back

Left

Right

Front

Bottom

FIGURE 19

On start-up, the switch senses that the combustion air
blower is operating. It closes a circuit to the furnace control
when pressure inside the combustion air blower increases
above switch setting (negative) w.c. The pressure sensed
by the switch is relative to atmospheric pressure. If the flue
becomes obstructed during operation, the switch senses a
loss of negative pressure (pressure becomes more equal
with atmospheric pressure) and opens the circuit to the
furnace control and gas valve. The switch trip pressure is
different depending on unit model number. The trip pres­sure is printed on the side of the limit.

The switch is factory set and is not field adjustable. It is a
safety shut-down control in the furnace and must not be
bypassed for any reason.

II−PLACEMENT AND INSTALLATION

Make sure unit is installed in accordance with installation
instructions and applicable codes.

WARNING

Shock and burn hazard.

G24M units are equipped with either a direct spark
or hot surface ignition system. Do not attempt to
light manually.

4 − Set fan switch to AUTO or ON and move system selec-

tion switch to HEAT. Adjust thermostat to a setting

above room temperature.
5 − If unit does not light the first time, the SureLight control

will attempt four more ignitions, the Ram control will at-

tempt two more ignitions before locking out.
6 − If lockout occurs, repeat steps 1, 2, 3 and 4.

C−Safety or Emergency Shutdown

Turn off unit power. Close manual and main gas valves.

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.

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 16

IV−HEATING SYSTEM SERVICE CHECKS

A−A.G.A./C.G.A. Certification

All units are A.G.A. design certified without modifications.
Refer to the G24M Operation and Installation Instruction
Manual for Information.

B−Gas Piping

Gas supply piping should not allow more than 0.5"W.C.
drop in pressure between gas meter and unit. Supply gas
pipe must not be smaller than unit gas connection.

Compounds used on gas piping threaded joints should be
resistant to action of liquefied petroleum gases.

C−Testing Gas Piping

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 20. 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.

pressure can result in permanent damage to gas valve or
overfire. For natural gas units, operating pressure at unit
gas connection must be between 5.0" W.C. and 13.0" W.C.
For L.P. gas units, operating pressure at unit gas connec­tion must be between 10.0" W.C. and 13.0" W.C.

On multiple unit installations, each unit should be checked
separately, with and without other units operating. Supply
pressure must fall within range listed in previous para­graph.

E−Check Manifold Pressure

After line pressure has been checked and adjusted, check
manifold pressure. Move pressure gauge to outlet pres­sure tap located on unit gas valve (GV1). Checks of man­ifold pressure are made as verification of proper regulator
adjustment. Manifold pressure for the G24M 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 in. w.c. For LP/propane gas the correct manifold pres­sure is 9.5 in. w.c.

GAS PIPING TEST PROCEDURE

MANUAL MAIN SHUT−OFF VALVE

WILL NOT HOLD TEST PRESSURE

IN EXCESS OF 0.5 PSIG (14"W.C.)

GAS VALVE

FIELD PROVIDED

LINE PRESSURE

CAP

TAP

FIGURE 20

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 20. Check
gas line pressure with unit firing at maximum rate. Low
pressure may result in erratic operation or underfire. High

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.

TABLE 9

GAS VALVE REGULATION

Unit (Fuel)

L.P. 9.5 +

The gas valve is factory set and should not require adjust­ment. All gas valves are factory regulated. See table 9.

Manifold Adjustment Procedure:

1 − Connect a test gauge to outlet pressure tap on gas

valve. 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. L.P. gas should burn

mostly blue with some orange streaks.

3 − After allowing unit to stabilize for 5 minutes, record

manifold pressure and compare to values given in

table 9.

NOTE−Shut unit off and remove manometer as soon as
an accurate reading has been obtained. Take care to
replace pressure tap plug.

Operating Pressure (outlet) in. W.C.

3.5 +0 −0.3Natural

0.5

Page 17

F− Proper Gas Flow

0 4500

4500 5500

5500 6500

6500 7500

Furnace should operate at least 5 minutes before check­ing gas flow. Determine time in seconds for two revolu­tions of gas through the meter. (Two revolutions assures
a more accurate time.) Divide by two and compare to time
in table 10 below. Adjust manifold pressure on gas valve
to match time needed.

NOTE− To obtain accurate reading, shut off all other
gas appliances connected to meter.

TABLE 10

GAS METER CLOCKING CHART

Seconds for One Revolution

G24M

Unit

Natural LP

1 cu ft

Dial

2 cu ft

Dial

1 cu ft

Dial

2 cu ft

DIAL

−45 80 160 200 400

−60 60 120 150 300

−75 48 96 120 240

−100 36 72 90 180

−120 30 60 75 150

−140 26 52 64 128

Natural−1000 btu/cu ft LP−2500 btu/cu ft

NOTE− To obtain accurate reading, shut off all other
gas appliances connected to meter.

G−High Altitude Derate

NOTE−In Canada, certification for installation at alti­tudes over 4500 ft. (1372m) above sea level is the juris­diction of the local authorities.

G24M−1 through −9 Models

This unit does not require gas pressure adjustment, or
pressure switch change when operating at elevations of 0
to 7500 ft. (0 to 2248m). Check gas line pressure with unit
firing. The minimum pressure as shown on the nameplate
for natural and propane gases must be maintained. No ori­fice change is required.

NOTE−This is the only permissible field derate for this
appliance.

G24M−10 and −11 Models

Table 11 shows manifold pressure settings for installations
at different altitudes. Refer to table 12 for pressure switch
replacement for models at elevations of 4500 feet (1372m)
and greater.

TABLE 11

ALTITUDE

feet (m)

0 − 4500

(0 − 1372)

4500 − 5500

(1372 − 1676)

5500 − 6500

(1676 − 1981)

6500 − 7500

(1372 − 2286)

GAS FUEL

Natural 3.5 (0.87)

Propane/LP 9.5 (2.36)

Natural 3.4 (0.86)

Propane/LP 9.2 (2.29)

Natural 3.3 (0.82)

Propane/LP 8.9 (2.21)

Natural 3.2 (0.80)

Propane/LP 8.6 (2.14)

MANIFOLD
PRESSURE

in. W.C. (kPa)

TABLE 12

Unit Model

Pressure Switch Part Number

G24M−45 No Change

G24M−60 No Change

G24M−75 88J8001

G24M−100 18L2401

G24M−120 18L2401

G24M−140 No Change

IMPORTANT

For safety, shut unit off and remove manometer as
soon as an accurate reading has been obtained.
Take care to replace pressure tap plug.

H−Flame Signal

A microamp DC meter is needed to check the flame signal
on the primary ignition control.

Flame (microamp) signal is an electrical current which
passes from the furnace 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 − Place meter in series between furnace control and

sensor wire. Connect the positive (+) lead of meter to

the ignition control sensor connection and the nega-

tive (−) lead of the meter to the sensor wire. See figure

21.

2 − Set thermostat for a heating demand and check flame

signal with unit operating. For G24M series with a RAM

control, a reading of 1 to 5 microamps DC should occur.

The furnace control must see at least 1.0 microamps in

order to keep the gas valve energized. G24M units with

the SureLight control should read 0.7 microamps or

more.

Page 18

FLAME SIGNAL TEST

D.C. MICROAMP

METER

FLAME

SENSOR

IGNITION

CON­TROL

SENSOR

WIRE

RAM CONTROL

SHOWN

SENSE"

TERMINAL

FIGURE 21

Flame signal from the Ram control may rise above 5 micro­amps for the first few seconds after ignition then level off within
the range.

WARNING

Fire and explosion hazard.
These instructions MUST be followed exactly.
Can cause a fire or explosion resulting in property
damage, personal injury or loss of life.

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 position, 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 G24M 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
°F" listed in the unit rating plate.

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 ra-

diant heat from the heat exchanger.
2 − Set thermostat to highest setting.

3 − After plenum thermometers have reached their high-

est 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, in-

crease 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 22.

2 − Punch a 1/4" diameter hole

in supply and return air ple-

nums. Insert manometer

hose flush with inside edge

of hole or insulation. Seal

around the hose with per-

magum. Connect the zero

end of the manometer to the

discharge (supply) side of the system. On ducted sys-

tems, connect the other end of manometer to the re-

turn duct as above. For systems with non−ducted re-

turns, 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 blow-

er motor speed to deliver the air desired according to

the job requirements.
4 − External static pressure drop must not be more than

0.5" W.C.

5 − Seal around the hole when the check is complete.

STATIC PRESSURE

TEST

MANOMETER

G24M UNIT

FIGURE 22

Page 19

D−Blower Speed Taps Leadless Motors

Blower speed tap selection is accomplished by changing the
taps at the blower motor harness connector. Disconnect har­ness connector from motor to expose speed selectors.
Blower speed selections are listed in table 13.

To Change Blower Speed:

1 − Turn off electric power to furnace.

2 − Remove front panel and blower access door. See

figure 4.

3 − Disconnect blower motor harness from motor.

4 − Select desired speeds for heating and cooling (refer to

unit wiring diagram).

5 − Depress harness connector tab to release wire termi-

nal. Select connector location for new speed (refer to
unit wiring diagram). Insert wire terminal until it is se­curely in place. See figure 23.

6 − Replace harness connector to motor.

BLOWER SPEED TAP SELEC-

TION

LEADLESS MOTORS

HARNESS

CONNECTOR

SureLight board. The heating tap is connected to the "ACB
HEAT " terminal and the cooling tap is connected to the
"ACB COOL" terminal. The continuous blower tap is con­nected to the "ACB LOW" terminal.
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 14 for blower motor tap col­ors for each speed.

TABLE 13

TABLE 14

DEPRESS TAB TO RELEASE
WIRE TERMINAL. SELECT
CONNECTOR LOCATION
FOR NEW SPEED (REFER TO
UNIT WIRING DIAGRAM). IN­SERT WIRE UNTIL IT IS SE­CURELY IN PLACE.

MOTOR

FIGURE 23

E−Blower Speed Taps Leaded Motors

Blower speed tap changes are made on the SureLight
control board. See figure 6. Unused taps must be secured
on dummy terminals "PARK M1" and or "PARK M2" on the

VI−MAINTENANCE

At the beginning of each heating season, the system
should be checked as follows:

A−Filters

A filter must be used in order to ensure long life and proper
operation. The filter is located in the return air duct or return
air register. Filters must be cleaned or replaced when dirty
to assure proper unit operation.

B−Heat Exchanger and Burners

NOTE−Use papers or protective covering in front of fur­nace while cleaning furnace.

Due to dimples designed in the heatexchanger, cleaning is
not recommended. Removal is for inspection only.

Page 20

To clean burners:

1 − Turn off both electrical and gas power supplies to fur-

nace.

2 − Disconnect gas supply piping and remove gas man-

ifold.

3 − Disconnect ignitor and flame sensor leads.

4 − Remove burner tray and burners.

5 − Clean burner retention ring with wire brush.

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 orifice in man­ifold.

8 − Reinstall burner box and gas supply piping. Reconnect

ignitor and flame sense 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 light-

ing instructions on front of unit.

CAUTION

Potential for gas leaks, fire or explosion.
Some soaps used for leak detection are corrosive
to certain metals. Carefully clean piping thor­oughly after leak detection has been completed.
Can cause damage to piping resulting in gas
leaks, fire or explosion.

C−Supply Air Blower

1 − Check and clean blower wheel.
2 − Motors used on the Lennox G24M series units are

permanently lubricated and need no further lu-

brication.

D−Flue and Chimney

Flue must conform to all AGA/GAMA venting require­ments. Flue pipe deteriorates from the inside out and must
be disconnected in order to check thoroughly. Check flue
pipe, chimney and all connections for tightness and to
make sure there is no blockage or leaks.

E−Electrical

1 − Check all wiring for loose connections.
2 − Check for correct voltage.
3 − Check amp−draw on blower motor.

HEAT EXCHANGER REMOVAL (unit shown in horizontal position)

REMOVE SCREWS (4)

Bottom

Left

Back

To p

Front

Right

SLIDE ENTIRE ASSEMBLY OUT OF CABINET

FIGURE 24

Page 21

VII−WIRING, OPERATION SEQUENCE
&TROUBLESHOOTING
A−Field Wiring, Thermostat Connections

FIELD INSTALLED CLASS II 24V
FIELD INSTALLED LINE VOLTAGE

THERMOSTAT

TO COMPRESSOR

CONTACTOR

TYPICAL G24M FIELD WIRING DIAGRAM

COMBUSTION

PRESSURE SWITCH

GWRY

FLAME ROLLOUT SWITCHES

PRIMARY LIMIT

TRANSFORMER

GWYCR

SURELIGHT CONTROL

AIR BLOWER

GAS

VALV E

SECONDARY

LIMITS

DOOR

INTERLOCK

SWITCH

(FURNISHED BY INSTALLER)

BLACK

WHITE

L1 N

GND

FIGURE 25

G24M and CONDENSING UNIT

THERMOSTAT DESIGNATIONS

(Refer to specific thermostat and outdoor unit.)

G24M

Condensing UnitThermostat

Furnace

HS UNIT

Y

COOLING

COMPRESSOR

Y

COMPRESSOR

INDOOR BLOWER

G

G

HEAT

W

W

POWER

R

COMMON

C

R

C

COMMON

HS UNIT

COMMON

FIGURE 26

Page 22

B−Early Model G24M Series With Ram Ignition Control

5

6

2

3

1

8

4

1 − When disconnect is closed, 120V is routed through

door interlock switch (S51) to feed the line voltage
side of the furnace control (A3) and transformer T1 pri­mary. Door interlock switch must be closed for A3 and
T1 to receive voltage.

2 − T1 supplies 24VAC to terminal 24VAC" on A3. In turn,

terminal R" of A3 supplies 24VAC to terminal RC" of
the indoor thermostat (not shown).

3 − When there is a call for heat, W1 of the thermostat en-

ergizes W of the furnace control with 24VAC.

4 − CMB BLWR of the blower control energizes the com-

bustion air blower (B6). When the combustion air
blower nears full speed, combustion air prove switch
(S18) closes.

5 − When S18 closes, assuming the flame rollout switch

7

(S47), primary limit (S10) and secondary limits (S21)

are closed, the furnace control begins a 45 second

time-delay (pre-purge).

6 − At the end of the pre-purge cycle, the furnace control

simultaneously opens the gas valve and sends high

voltage to the spark electrode.

7 − When flame is sensed, the furnace control begins a 45

second delay before energizing the indoor blower.

8 − When heat demand is satisfied, W1 of the thermostat

de-energizes W of the furnace control and the furnace

control immediately de-energizes the gas valve. The

combustion air blower runs for 5 seconds (post-purge)

before being de-energized. Also, the indoor blower

runs for a designated period (90−240 seconds) as set

by switches on furnace control.

Page 23

C−Ram Ignition Control Troubleshooting Guide

RAM CONTROL

HEATING TROUBLE SHOOTING SEQUENCE

NORMAL HEATING MODE ABNORMAL HEATING MODE

LED: SLOW FLASH RATE

THERMOSTAT CALLS FOR HEAT

LED: FAST FLASH RATE

FLAME OFF?

YES

CONTROL SELF−CHECK OKAY?

YES

HI LIMIT SWITCH CLOSED?

YES

PRESSURE SWITCH OPEN?

YES

COMBUSTION AIR BLOWER ON?

PRESSURE SWITCH CLOSED?

YES

PREPURGE (45 seconds)

IGNITION TRIAL (7 seconds) −− START IGNITION

SPARK, OPEN MAIN GAS VALVE. INCREMENT

TRIAL REGISTER. HAS FLAME BEEN

REGISTERED WITHIN 7 SECONDS?

YES

SYSTEM FAN ON.

(Fixed 45−second delay)

NO

NO

NO

NO

NO

NO

LOCKOUT: MAIN VALVE OFF

COMB. AIR AND SYSTEM FANS ON

RESET THERMOSTAT

LED: 5 flash if flame sensed with valve off

LED: Steady on if control board failure

CHECK TO SEE IF MAIN VALVE IS OFF.

SEQUENCE HOLDS UNTIL HI LIMIT CLOSES.

LED: 4 flashes.

CHECK TO SEE IF MAIN VALVE IS OFF.

SEQUENCE HOLDS UNTIL PRES. SWITCH OPENS.

LED: 3 flashes.

CHECK TO SEE IF MAIN VALVE IS OFF.

SEQUENCE HOLDS UNTIL PRES. SWITCH CLOSES.

LED: Fast flash rate.

MAIN VALVE AND SPARK ARE OFF.

INCREMENT TRIAL REGISTER

THREE TRIALS COMPLETED?

YES

IGNITION RESET PERIOD

COMBUSTION & SYSTEM FANS OFF.

IS 60 MIN. RESET PERIOD COMPLETE?

LED: Fast flash rate.

NO

YES

FLAME SENSE OKAY?

YES

FLAME CONTINUOUSLY CHECKED?

ROLLOUT SWITCHES CLOSED

YES

HI LIMIT SWITCH CLOSED?

YES

PRESSURE SWITCH CLOSED?

YES

THERMOSTAT OPENS

LED: SLOW FLASH RATE

COMB. AIR BLOWER OFF (5 sec. delay)

FAN OFF (After selected 90, 120,

180, or 240 delay)?

NO

NO

NO

NO

MAIN VALVE AND SPARK OFF.

NO

START IGNITION SPARK (IN 0.8 SEC. MAXIMUM).

HAS FLAME BEEN DETECTED WITHIN 7 SEC.?

YES

LOCKOUT. MAIN VALVE OFF.

COMBUSTION AND SYSTEM FANS ON.

RESET OR REPLACE ROLLOUT SWITCH.

LED: 4 flashes.

MAIN VALVE OFF. COMBUSTION & SYSTEM FANS ON.

SEQUENCE HOLDS UNTIL SWITCH CLOSES.

LED: 4 flashes for hi limit.

LED: 3 flashes for pressure switch.

SWITCH CLOSED

YES

COMBUSTION FAN ON. SYSTEM FAN OFF.

(AFTER DELAY TO OFF INTERVAL COMPLETED.)

LED: Fast flash rate.

Page 24

RAM CONTROL

COOLING TROUBLE SHOOTING SEQUENCE

REMAINS UNCHANGED THROUGHOUT COOLING CYCLE.

THERMOSTAT CALLS FOR COOLING.

COMPRESSOR CONTACTOR AND SYSTEM FAN

ENERGIZED AT COOLING SPEED AFTER

1 SECOND DELAY. ACC TERMINAL ENERGIZED.

SYSTEM FAN AND ACC TERMINAL OFF AFTER

COMPLETING SELECTED DELAY INTERVAL

LED: Slow flash rate.

THERMOSTAT OPENS.

COMPRESSOR OFF.

(At dip switches).

MANUAL FAN TROUBLESHOOTING SEQUENCE

REMAINS UNCHANGED THROUGHOUT SEQUENCE.

MANUAL FAN SELECTION MADE AT THERMOSTAT.

CONTROL ENERGIZES SYSTEM FAN AT CONTINUOUS

THERMOSTAT CALLS FOR COOLING.

YES NO

SYSTEM FAN SWITCHED TO COOLING

SPEED. ACC TERMINAL REMAINS ENERGIZED.

THERMOSTAT OPENS.

AND ENERGIZES ACC TERMINAL. BOTH REMAIN ON

UNTIL MANUAL FAN IS SWITCHED OFF AT THERMOSTAT.

SLOW FLASH
FAST FLASH
2 FLASH
3 FLASH
4 FLASH
5 FLASH
STEADY ON

SPEED. ACC TERMINAL ENERGIZED.

SYSTEM FAN SWITCHES TO CONTINUOUS SPEED

NORMAL OPERATION. NO CALL FOR HEAT.
NORMAL OPERATION. CALL FOR HEAT.
SYSTEM LOCKOUT. FAILURE TO DETECT OR SUSTAIN FLAME.

.

PRESSURE SWITCH OPEN OR CLOSED.
HIGH LIMIT OR ROLLOUT SWITCH OPEN.
FLAME SENSED AND GAS VALVE NOT ENERGIZED.
CONTROL BOARD FAILURE.

LED: Slow flash rate.

THERMOSTAT CALLS FOR HEAT.

NO

LED CODES

YES

SYSTEM FAN SWITCHES TO HEATING SPEED AFTER

SELECTED DELAY. ACC TERM. REMAINS ENERGIZED.

THERMOSTAT OPENS.

SYSTEM FAN SWITCHED OFF AFTER DELAY.

ACC TERMINAL DE−ENERGIZED.

Page 25

D−Late Model G24M Series With SureLight Ignition Control

TYPICAL G24M DIAGRAM

(−11 MODEL SHOWN)

1 − When there is a call for heat, W1 of the thermostat ener-

gizes W of the furnace control with 24VAC.

2 − S10 primary limit switch and S47 rollout switch are closed.

Call for heat can continue.

3 − SureLight control energizes combustion air blower B6.

Combustion air blower runs until S18 combustion air prove
switch closes (switch must close within 2−1/2 minutes or
control goes into 5 minute Watchguard Pressure Switch
delay). Once S18 closes, a 15−second pre−purge follows.

4 − SureLight control energizes ignitor. A 20−second warm−up

period begins.

5 − Gas valve opens for a 4−second trial for ignition. Ignitor

stays energized the first second of trial. (Board 97L48 only:

ignitor energized during trial or until flame is sensed.
6 − Flame is sensed, gas valve remains open for the heat call.
7 − After 45−second delay, SureLight control energizes indoor

blower B3.
8 − When heat demand is satisfied, W1 of the indoor thermostat

de−energizes W of the SureLight control which de−ener-

gizes the gas valve. Combustion air blower B6 continues a

5−second post−purge period, and indoor blower B3 com-

pletes a selected OFF time delay.

Page 26

SURELIGHT CONTROL HEATING

SEQUENCE OF OPERATION

NORMAL HEATING MODE ABNORMAL HEATING MODE

COMBUSTION AIR BLOWER ON FOR 1 SECOND.

SIGNAL IMPROPER

GROUND AT LED.

SIGNAL HOLDS

UNTIL UNIT IS

PROPERLY

GROUNDED.

IS COMBUSTION AIR BLOWER ENERGIZED?

(HTG ACC TERMINAL IS ENERGIZED WITH C.A.B.)

15-SECOND COMBUSTION AIR BLOWER PREPURGE

INITIATED BY CLOSED PRESSURE SWITCH.

POWER ON

CONTROL SELF−CHECK OKAY?

TURN INDUCER ON FOR 1 SECOND.

IS POLARITY REVERSED?

NO

NO

ROLLOUT SWITCH CLOSED?

(CONTINUOUS FLAME CHECK)

THERMOSTAT CALLS FOR HEAT:

PRESSURE SWITCH OPEN?

HAS COMBUSTION AIR PRESSURE

SWITCH CLOSED IN 2.5 MINUTES?

CONTINUED NEXT PAGE

IS THERE

PROPER GROUND?

YES

BURNER OFF?

YES

NORMAL OPERATION:

LED #1 −− SLOW FLASH
LED #2 −− SLOW FLASH

LED #1 −− FAST FLASH
LED #2 −− FAST FLASH

YES

IS COMBUSTION AIR

YES

YES

YES

YES

YES

NO

YES

IS VOLTAGE

ABOVE 75 VOLTS?

YES

NO

NO

NO

GAS VALVE OFF. COMBUSTION AIR BLOWER ON.

GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.

(RESET CONTROL BY TURNING MAIN POWER OFF.)

NO

GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.

LED #1 −− ON. LED #2 −− SLOW FLASH.

SEQUENCE HOLDS UNTIL ROLLOUT SWITCH CLOSES.

GAS VALVE OFF. COMBUSTION AIR BLOWER ON.

INDOOR BLOWER ON HEATING SPEED.

NO

PRESSURE SWITCH IS IN WATCHGUARD MODE.

GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.

INDOOR BLOWER OFF WITH DELAY.

LED #1 −− OFF. LED #2 −− SLOW FLASH.

IS 5-MINUTE RESET PERIOD COMPLETE?

INDOOR BLOWER ON.

LED #1 ALTERNATING FAST FLASH

LED#2 ALTERNATING FAST FLASH

CHECK FOR BROKEN IGNITOR OR

OPEN IGNITOR CIRCUIT

INDOOR BLOWER DELAY OFF.

LED #1 ON
LED #2 ON

POLARITY REVERSED.

LED #1 −− FAST FLASH

LED #2 −− SLOW FLASH

LOW VOLTAGE SIGNAL AT LED HOLDS

UNTIL VOLTAGE RISES ABOVE 75 VOLTS.

INDOOR BLOWER OFF WITH DELAY.

LED #1 −− SLOW FLASH

LED #2 −− OFF

GAS VALVE OFF.

COMBUSTION AIR BLOWER OFF.

INDOOR BLOWER OFF WITH DELAY.

LED #1 OFF

LED #2 SLOW FLASH

(Sequence holds until pressure switch

closes or thermostat resets control.)

Page 27

NORMAL HEATING MODE ABNORMAL HEATING MODE

15-SECOND COMBUSTION AIR INDUCER PREPURGE

INITIATED BY CLOSED PRESSURE SWITCH.

YES

IGNITOR WARM-UP −− 20 SECONDS.

YES

4-SECOND TRIAL FOR IGNITION. GAS VALVE OPENS.

IGNITOR ENERGIZED FOR 1 SECOND AFTER VALVE

OPENS. NO FLAME SENSED DURING THIS PERIOD.

BOARD 97L48: IGNITOR ENERGIZED DURING TRIAL OR

FLAME SIGNAL ABOVE 0.7 MICROAMPS?

UNTIL FLAME SENSED.

YES

FLAME STABILIZATION PERIOD.

4 SECONDS

FLAME RECTIFICATION CURRENT

CHECK. CAN FLAME BE PROVEN WITHIN

4 SECONDS AFTER GAS VALVE OPENS?

(u0.15 microamps)

YES

FLAME PRESENT?

YES

YES

INDOOR BLOWER ON DELAY BEGINS

(45 seconds.)

YES

PRIMARY AND SECONDARY LIMIT

SWITCHES CLOSED?

YES

(See table 2 DIAGNOSTIC CODES for limit reset operation)

IS VOLTAGE ABOVE 75 VOLTS?

NO

IS THERE A PROPER GROUND?

YES

IS IGNITOR INTACT AND CONNECTED?

GAS VALVE OFF. COMBUSTION AIR INDUCER ON.

NO

HAS CONTROL FAILED TO SENSE FLAME FOR

FIVE CONSECUTIVE TRIES DURING A SINGLE

IGNITION WATCHGUARD MODE. GAS VALVE OFF.

LED 1 ALT. SLOW FLASH LED 2 ALT SLOW FLASH

IS 60-MINUTE RESET PERIOD COMPLETE?

NO

NO

GAS VALVE DE−ENERGIZED.
COMBUSTION AIR INDUCER

DE−ENERGIZED.INDOOR BLOWER

NO

ON UNTIL SWITCH CLOSES.LED #1

−− SLOW FLASH. LED #2 −− ON.
IS LIMIT SWITCH CLOSED?

HAS PRIMARY/SECONDARY LIMIT RESET.

YES

YES

INDOOR BLOWER OFF.

HEAT DEMAND?

COMBUSTION AIR INDUCER OFF.

INDOOR BLOWER OFF WITH DELAY

HAS CONTROL RESET IGNITION

SEQUENCE FOUR TIMES?

LOW FLAME SIGNAL

(Does not affect operation of control)

LED #1 −− SLOW FLASH

LED #2 −− FAST FLASH

YES

NO

NO

LIMIT SWITCH WATCH GUARD
MODE. GAS VALVE OFF.
COMB. AIR INDUCER OFF. IN­DOOR BLOWER OFF WITH
DELAY. LED#1−SLOW FLASH
LED#2− ON. IS 60MINUTE RE­SET PERIOD COMPLETE.?

LEDS SIGNAL
ALTERNATING

FAST FLASH

NO

NO

YES

NO

YES

YES

NO

YES

NO

ROLLOUT SWITCH CLOSED?

YES

COMBUSTION AIR PRESSURE

SWITCH CLOSED?

YES

THERMOSTAT DEMAND SATISFIED.

YES

LED #1 & #2 SIMULTANEOUS SLOW FLASHES.

YES

COMB. AIR INDUCER CONTINUES 5-SECOND POST PURGE

AFTER T’STAT DEMAND IS SATISFIED. INDOOR AIR BLOWER

COMPLETES SELECTED OFF" DELAY BEFORE SHUTTING OFF.

ACB.HEAT SPEED, HTG ACC TERM. AND ACC TERM. OFF.

NO

HAS CAB SWITCH CLOSED IN 2.5 MINUTES?

COMBUSTION AIR INDUCER POWER OFF.

SEQUENCE HOLDS UNTIL ROLLOUT SWITCH IS RESET

GAS VALVE DE−ENERGIZED.

COMBUSTION AIR INDUCER ON.

INDOOR BLOWER OFF WITH DELAY

LED #1 −− OFF. LED #2 −− SLOW FLASH.

5-MINUTE PRESSURE SWITCH

GAS VALVE POWER OFF.

INDOOR BLOWER OFF WITH DELAY

LED #1 −− ON. LED #2 −− SLOW FLASH.

YES

NO

WATCHGUARD MODE.

YES

Page 28

SURELIGHT CONTROL COOLING SEQUENCE

NORMAL COOLING MODE ABNORMAL COOLING MODE

POWER ON

IGNITION CONTROL MAIN POWER ON.

SIGNAL IMPROPER

GROUND AT LED.

SIGNAL HOLDS

UNTIL UNIT IS

PROPERLY

GROUNDED.

ROLLOUT SWITCH MONITORED CONTINUOUSLY.

CHECK FOR MAIN BURNER FLAME SENSE.

LED: SLOW FLASH RATE REMAINS UNCHANGED

CONTROL SELF DIAGNOSTIC CHECK.

IS CONTROL OPERATING NORMALLY?

YES

TURN INDUCER ON FOR 1 SECOND.

IS POLARITY REVERSED?

NO

NO

IS ROLLOUT SWITCH CLOSED?

IS MAIN BURNER FLAME OFF?

THROUGHOUT COOLING CYCLE.

THERMOSTAT CALLS FOR COOLING.

IS THERE

PROPER GROUND?

YES

YES

NO

YES

IS VOLTAGE

ABOVE 75 VOLTS?

NO

NO

GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.

INDOOR BLOWER OFF WITH NORMAL DELAY.

SIGNAL CIRCUIT BOARD FAILURE AT LED.

INTERRUPT MAIN POWER TO RESET CONTROL.

SIGNAL POLARITY REVERSED AT LED.

NO

GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.

INDOOR BLOWER OFF WITH NORMAL DELAY.

SIGNAL CIRCUIT BOARD FAILURE AT LED.

SEQUENCE HOLDS UNTIL ROLLOUT SWITCH CLOSES.

GAS VALVE OFF. COMBUSTION AIR BLOWER OFF.

INDOOR BLOWER OFF WITH NORMAL DELAY.

SIGNAL CIRCUIT BOARD FAILURE AT LED.

SEQUENCE HOLDS UNTIL FLAME IS NOT SENSED.

LOW VOLTAGE SIGNAL AT LET HOLDS

UNTIL VOLTAGE RISES ABOVE 75 VOLTS.

COMPRESSOR CONTACTOR AND SYSTEM FAN

ENERGIZED WITH 0-SECOND DELAY

ACB COOL SPEED AND ACC. TERM. ENERGIZED.

THERMOSTAT OPENS.

COMPRESSOR OFF.

ACB COOL SPEED AND ACC. TERM. OFF

WITH 0-SECOND DELAY.

Page 29

CONTINUOUS LOW SPEED FAN SEQUENCE OF OPERATION

LED: SLOW FLASH RATE REMAINS

UNCHANGED THROUGHOUT SEQUENCE.

MANUAL FAN SELECTION MADE AT THERMOSTAT.

CONTROL (G) ENERGIZES SYSTEM FAN AT ACB

LOW SPEED. ACC. TERMINAL IS ENERGIZED.

THERMOSTAT CALLS FOR HEAT (W).

NO YES

THERMOSTAT CALLS FOR COOLING.

YES

SYSTEM FAN SWITCHED TO ACB COOL

SPEED. ACC. TERM. REMAINS ON.

THERMOSTAT OPENS.

NO

HTG ACC. TERM. ENERGIZES

WITH COMB. AIR BLOWER.

BOARD NO. 63K8901

SYSTEM FAN DE−ENERGIZED. SYSTEM FAN

ENERGIZES ON ACB HEAT SPEED AFTER

NORMAL DELAY.

BOARD NO. 24L85, 56L83, 97L48

SYSTEM FAN SWITCHES TO ACB HEAT SPEED

AFTER NORMAL DELAY (fan remains energized)

THERMOSTAT OPENS. COMBUSTION AIR

AND HTG ACC TERMINAL DE−ENERGIZE.

BOARD 63K89: SYSTEM FAN OFF. ACC. TERM.

OFF. (AFTER OFF DELAY COMPLETED).

BOARDS 24L85, 56L83 AND 97L48:

SYSTEM FAN SWITCHES TO

LOW SPEED AFTER NORMAL DELAY.

(fan remains energized)

MANUAL FAN SELECTION MADE AT THERMOSTAT.

CONTROL (G) ENERGIZES SYSTEM FAN AT ACB

LOW SPEED. ACC. TERM. ENERGIZED.

Page 30

E−SureLight Ignition Control Troubleshooting Guide

code
LED#2 Slow Flash

ACTION 1

blower does not energize

blower does not energize

120V field connection.

p

on operation

UPON INITIAL POWER UP, REMOVE ALL THERMOSTAT DEMANDS TO THE UNIT

PROBLEM: 1 UNIT FAILS TO OPERATE IN THE COOLING, HEATING, OR CONTINUOUS FAN MODE

Condition Possible Cause Corrective Action / Comments

1.1

− Both diagnostic lights fail to light up.

LED#1−Off
LED#2−Off

1.2

− Diagnostic lights flash the roll−out

.

code.

LED#1−On,
LED#2−Slow Flash

1.3

− On initial power−up the comb. air
.

.

− Diagnostic lights flash the reverse

polarity code.

Main voltage 120V not supplied to
unit.

Miswiring of furnace or improper
connections.

Circuit breaker tripped or fails to
close.

Door interlock switch failure.

Transformer Failure.

Failed control board.

Roll−out switch open.

Roll−out switch failure.

Miswiring or improper connections at
roll−out switch.

Nine pin connector failure

120V main power polarity reversed.

1.1.1

1.1.2

1.1.3

1.1.4

1.1.5

1.1.6

1.2.1

1.2.2

1.2.3

1.2.4

1.3.1

ACTION 1 − Check 120V main voltage.

Determine cause of main power failure.

ACTION 1 − Check for correct wiring of 120V to
power make up box and transformer.

ACTION 2 − Check 24V wiring to control board.

ACTION 1 − Replace circuit breaker if it is reset

but does not have continuity.
ACTION 2 − If circuit breaker still trips, check for
short.

ACTION 1 − Check that door switch is activated
when door is closed.
ACTION 2 − Check wire connections to switch, re­place loose connectors.
ACTION 3 − Check continuity of switch in closed
position. Replace if malfunctioning

ACTION 1 − Check that transformer output is
24V. Replace if malfunctioning

ACTION 1 − If all the above items have been
checked, replace board.

ACTION 1 − Manually reset the roll−out switch
by pushing the top button.
ACTION 2 − Determine the cause of the roll−out
switch activation before leaving furnace.

ACTION 1 − Check continuity across roll−out
switch. Replace roll−out switch if switch is reset
but does not have continuity.

− Check wiring connections to switch.

ACTION 1 − Check 9−pin connector for proper

connection to control board.
ACTION 2 − Check continuity of the multi plug
pin.

ACTION 1 − Check the 120V has line and neutral
correctly input into control.
ACTION 2 − Reverse the line and neutral at the
120V field connection.

LED#1−Fast Flash,
LED#2−Slow Flash.

− On initial power up the combustion

air blower does not energize.

− Diagnostic lights flash normal power

on o

eration.

.

LED#1−Slow Flash
LED#2−Slow Flash

1.4

1.4.1

Open combustion air blower motor
circuit.

1.4.2

Failed combustion air blower motor.

Page 31

ACTION 1 − Check for 120V to combustion air
blower. If no power, check wire and connec­tions.

ACTION 1 − If power is present at blower, replace
blower.

PROBLEM 1: UNIT FAILS TO OPERATE IN THE COOLING, HEATING, OR CONTINUOUS FAN MODE

g

5

gg

g

Diagnostic lights flash the pressure

Condition Possible Cause Corrective Action / Comments

1.5

− On initial power−up the combustion

air blower remains energized.

− Diagnostic lights flash the improper

main ground.

LED#1−Alternating Fast Flash
LED#2−Alternating Fast Flash

1.5.1

Improper ground to the unit.

1.5.2

6−Pin connector is improperly at-

tached to the circuit board.

1.5.3

Line voltage is below 75V.

ACTION 1 − Check that the unit is properly
ground.
ACTION 2 − Install a proper main ground to the
unit

ACTION 1 − Check 6−pin connector for proper
installation. Correctly insert connector into con­trol.

ACTION 1 − Check that the line voltage is above
75V. Determine cause of voltage drop and sup­ply correct voltage to the control.

PROBLEM 2: UNIT OPERATES IN COOLING MODE BUT NOT IN THE HEATING MODE, WITH

COMBUSTION AIR BLOWER CYCLING 5 SECONDS ON 55 SECONDS OFF.

Condition Possible Cause Corrective Action / Comments

2.1

− On initial power−up the combustion

air blower remains energized.

− Diagnostic lights flash the im prop-

er main ground.

− Combustion air blower cycles

seconds on 55 seconds off.

LED#1−Alternating Fast Flash
LED#2−Alternating Fast Flash

2.1.1

Open ignitor circuit.

2.1.2

Broken or failed ignitor.

ACTION 1 − Check for correct wiring and loose
connections in the ignitor circuit. Check mult−
plug connections for correct installation.

ACTION 1 − Unplug ignitor and read resistance
across ignitor. If resistance does not read be­tween 10.9 and 19.7 ohms, replace the ignitor.

PROBLEM 3: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER DOES

NOT ENERGIZE

Condition Possible Cause Corrective Action / Comments

3.1

− Unit operates with a cooling or con-

tinuous fan demand.

− Combustion air blower will not start

with a Heating demand.

− Diagnostic lights flash the limit failure

mode.

LED#1−Slow Flash,
LED#2−On

3.2

− Unit operates with a cooling and con-

tinuous fan demand.

− Combustion air blower will not start

with a Heating demand.

nostic lights flash the pressure

− Dia

−

switch failure code.

LED#1−Off,
LED#2−Slow Flash

3.1.1

Primary or secondary (if equipped)

limit open.

3.1.2

Miswiring of furnace or improper con-

nections at limit switch(es).

3.2.1

Miswiring of furnace or improper con-

nections to combustion air blower.

3.2.2

Pressure switch stuck closed.

ACTION 1 − Check continuity across switch(es).
Switches reset automatically upon cool down.
ACTION 2 − Check for restrictions on blower inlet
air (including filter) and outlet air. Determine
cause for limit activation before placing unit back
in operation.

ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.

ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.

ACTION 1 − Check that the pressure switch is
open without the combustion air blower operat­ing. Replace if malfunctioning

Page 32

PROBLEM 3: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER DOES

g

LED#1 Off

NOT ENERGIZE (CONT.).

Condition Possible Cause Corrective Action/Comments

3.3

− Unit operates with a cooling and con-

tinuous fan demand.

− Combustion air blower will not start

with a Heating demand.

Miswiring of furnace or improper con-

nections to combustion air blower.

3.3.1

ACTION 1 − Check for correct wiring and loose

connections. Correct wiring and/or replace any
loose connections.

− Diagnostic lights flash the pressure

switch failure code 2.5 minutes after
heating demand.

ACTION 1 − If there is 120V to combustion air
blower and it does not operate, replace combus­tion air blower.

LED#1−Off,

3.3.2

Combustion air blower failure.

LED#2−Slow Flash

PROBLEM 4: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER

ENERGIZES, IGNITOR IS NOT ENERGIZED.

Condition Possible Cause Corrective Action/Comments

4.1

− Unit operates with a cooling and

continuous fan demand.

− Combustion air blower energizes

with a heating demand.

− Diagnostic lights flash the pressure

switch failure code 2.5 minutes after
heating demand.

LED#1−Off
LED#2−Slow Flash

Pressure switch does not close due

to incorrect routing of the pressure

Pressure switch does not close due

to obstructions in the pressure lines.

Pressure switch lines damaged

Pressure switch does not close due
to a low differential pressure across

the pressure switch.

Wrong pressure switch installed in

the unit, or pressure switch is out of

Miswiring of furnace or improper con-

nections at pressure switch.

Pressure switch failure.

4.1.1

switch lines.

4.1.2

4.1.3

4.1.4

4.1.5

calibration.

4.1.6

4.1.7

ACTION 1 − Check that the pressure switch lines

are correctly routed. Correctly route pressure
switch lines.

ACTION 1 − Remove any obstructions from the
the pressure lines and/or taps.

ACTION 1 − Check pressure switch lines for
leaks. Replace any broken lines.

ACTION 1 − Check the differential pressure
across the pressure switch. This pressure
should exceed the set point listed on the
switch.
ACTION 2 − Check for restricted inlet and ex­haust vent. Remove all blockage.
ACTION 3 − Check for proper vent sizing and
run length. See installation instructions.
ACTION 4 − Check voltage to thre combustion
air blower.

ACTION 1 − Check that the proper pressure
switch is installed in the unit. Replace pressure
switch if necessary.

ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.

ACTION 1 − If all the above modes of failure have
been checked, the pressure switch may have
failed. Replace pressure switch and determine if
unit will operate.

Page 33

PROBLEM 5: UNIT FAILS TO FIRE IN THE HEATING MODE, COMBUSTION AIR BLOWER

ignite code.

ENERGIZES, IGNITOR IS ENERGIZED. (CONT.)

Condition Possible Cause Corrective Action/Comments

5.1

− Unit operates with a cooling and

continuous fan demand.

− Combustion air blower energizes

with Heating demand.

− Ignitor is energized but unit fails to

light.

LED#1−Alternating Slow Flash
LED#2−Alternating Slow Flash

5.1.1

Check that gas is being supplied to

the unit.

5.1.2

Miswiring of gas valve or loose con-

nections at multi−pin control amp

plugs or valve.

5.1.3

Malfunctioning gas valve or ignition

control.

ACTION 1 − Check line pressure at the gas
valve. Pressure should not exceed 13" WC for
both natural and propane. Line pressure should
read a minimum 4.5" WC for natural and 8.0"WC
for propane.

ACTION 1 − Check for correct wiring and loose
connections. Correct wiring and/or replace any
loose connections.

ACTION 1 − Check that 24V is supplied to the
gas valve approximately 35 seconds after heat
demand is initiated.
ACTION 2 − Replace the valve if 24V is supplied
but valve does not open (check for excessive
gas line pressure before replacing gas valve).
ACTION 3 − Replace the control board if 24V is
not supplied to valve.

PROBLEM 6: BURNERS LIGHT WITH A HEATING DEMAND BUT UNIT SHUTS DOWN

PREMATURELY

Condition Possible Cause Corrective Action/Comments

6.1

− Burners fire with a heating de-

mand.

− Burners light but unit shuts off prior

to satisfying T−stat demand.

− Diagnostic lights flash the pressure

switch code.

6.1.1

Low pressure differential at the pres-

sure switch.

ACTION 1 − Check for restricted vent inlet or ex­haust. Remove all blockage.
ACTION 2: Check for proper vent sizing. See
installation instructions.

LED#1−Off
LED#2−Slow Flash

6.2

− Combustion air blower energizes

with a heating demand.

− Burners light but fail to stay lit.

− After 5 tries the control diagnostics

flash the watchguard burners failed to
ignite code.

LED#1−Alternating Slow Flash
LED#2−Alternating Slow Flash

6.2.1

Sensor or sense wire is improperly

installed.

6.2.2

Sensor or sense wire is broken.

6.2.3

Sensor or sensor wire is grounded to

the unit.

6.2.4

Control does not sense flame.

ACTION 1 − Check that sensor is properly lo­cated (page 10) and that the sense wire is
properly attached to both the sensor and the
control.

ACTION 1 − Check for a broken sensor.
ACTION 2 − Test continuity across the sense

wire. If wire or sensor are damaged replace
the component.

ACTION 1 − Check for resistance between the
sensor rod and the unit ground.
ACTION 2 − Check for resistance between the
sensor wire and the unit ground.

ACTION 3 − Correct any shorts found in circuit.

ACTION 1 − Check the microamp signal from

the burner flame. If the microamp signal is be­low 0.70 microamps, check the sense rod for
proper location or contamination.
ACTION 2 − Replace, clean, or relocate flame
sense rod. If rod is to be cleaned, use steel
wool or replace sensor. DO NOT CLEAN ROD
WITH SAND PAPER. SAND PAPER WILL
CONTRIBUTE TO THE CONTAMINATION
PROBLEM. NOTE: Do not attempt to bend
sense rod.

Page 34

PROBLEM 6: BURNERS LIGHT WITH HEATING DEMAND BUT UNIT SHUTS DOWN

ure.

flame failure

Condition Possible Cause Corrective Action/Comments

6.3

− Combustion air blower energizes

with a heating demand.

− Burners light.

− Roll−out switch trips during the

heating demand.

− Diagnostic lights flash roll−out fail-

ure.

LED#1−On
LED#2−Slow Flash

6.4

− Combustion air blower energizes

with a heating demand.

− Burners light roughly and the unit

fails to stay lit.

− Diagnostic lights flash watchguard

flame failure.

.

PREMATURELY (CONT.)

6.3.1

Unit is firing above 100% of the

nameplate input.

6.3.2

Gas orifices leak at the manifold con-

nection.

6.3.3

Insufficient flow through the heat ex-

changer caused by a sooted or re-

stricted heat exchanger.

6.3.4

Burners are not properly located in

the burner box.

6.4.1

Recirculation of flue gases. This con-

dition causes rough ignitions and op-

eration. Problem is characterized by

nuisance flame failures.

ACTION 1 − Check that the manifold pressure
matches value listed on nameplate. See installa­tion instructions for proper procedure.
ACTION 2 − Verify that the installed orifice size
match the size listed on the nameplate or instal­lation instructions.
ACTION 3 − Check gas valve sensing hose to in­sure no leaks are present.
ACTION 4 − Check the input rate to verify rate
matches value listed on nameplate.

ACTION 1 − Tighten orifice until leak is sealed.
NOTE: Be careful not to strip orifice threads. AC-
TION 2 − Check for gas leakage at the threaded

orifice connection. Use approved method for
leak detection (see unit instructions).

ACTION 1 − Check for sooting deposits or other
restrictions in the heat exchanger assembly.
Clean assembly as outlined in instruction manu­al.
ACTION 2 − For G24M gas furnaces, check for
proper combustion and flow. CO2 should mea­sure between 6.0% and 8.0% for NG and be­tween 6.5% and 8.5% for LP. CO should mea­sure below .04% (400PPM) in an air−free sample
of flue gases for either NG or LP.

ACTION 1 − Check that the burners are firing into
the center of the heat exchanger openings. Cor­rect the location of the burners if necessary.

ACTION 1 − Check for proper flow of exhaust
gases away from intake vent. Remove any ob­stacles in front of the intake and exhaust vent
which would cause recirculation.
ACTION 2 − Check for correct intake and ex­haust vent installation. See instructions

LED#1−Alternating Slow Flash
LED#2−Alternating Slow Flash

6.4.2

Improper burner cross−overs

ACTION 1 − Remove burner and inspect the
cross−overs for burrs, or any restriction or if
crossover is warped. Remove restriction or re­place burners.

Page 35

PROBLEM 6: BURNERS LIGHT WITH HEATING DEMAND BUT UNIT SHUTS DOWN

sense code

6.5

− Combustion air blower energizes

with a heating demand.

− Burners light.

− Diagnostic lights flash watch guard

flame failure.

− NOTE" Unit might go into 60 min-

ute Watchguard mode depending on
intermittent nature of sensor signal.

LED#1−Alternating Slow Flash
LED#2−Alternating Slow Flash

PROBLEM 7: CONTROL SIGNALS LOW FLAME SENSE DURING HEATING MODE

Condition Possible Cause Corrective Action/Comments

7.0

− Unit operates correctly but the

diagnostic lights flash low flame

.

LED#1−Slow Flash
LED#2−Fast Flash

PREMATURELY (CONT.)

6.5.1

Loose sensor wire connection

causes intermittent loss of flame sig-

nal.

7.1.1

Sense rod is improperly located on

the burner.

7.1.2

Sense rod is contaminated.

ACTION 1 − Check that the sensor is properly lo­cated.
ACTION 2 − Check that the sense wire is proper­ly attached to both the sensor and the control.
Pay extra attention to the pin connectors.

ACTION 1 − Check the sense rod for proper loca­tion on the burner. Properly locate the sense rod
or replace if rod cannot be located correctly.

ACTION 1 − Check sense rod for contamination
or coated surface. Clean the sense rod with steel
wool or replace sensor. DO NOT USE SAND PA­PER TO CLEAN ROD. SAND PAPER WILL
CONTRIBUTE TO THE CONTAMINATION
PROBLEM.

PROBLEM 8: INDOOR BLOWER FAILS TO OPERATE IN COOLING, HEATING, OR CONTINUOUS

FAN MODE

Condition Possible Cause Corrective Action/Comments

8.0

− Indoor blower fails to operate in

continuous fan, cooling, or heating
mode.

Miswiring of furnace or improper con-

nections at control or indoor blower

120V is not being supplied to the in-

door air blower or blower motor fail-

Defective control board

8.1.1

motor.

8.1.2

ure.

8.1.3

ACTION 1− Correct wiring and/or replace any

loose connections. Check for correct wiring and
loose connections.

ACTION 1 − Check for 120V at the various calls
for indoor blower by energizing "Y", "G", and "W"
individually on the low voltage terminal strip.
Note that when "W’ is energized, the blower is
delayed 45 seconds. If there is 120V to each mo­tor tap but the blower does not operate, replace
the motor.

ACTION 1 − If there is not 120V when "Y", "G", or
"W" is energized, replace the control.

PROBLEM 9: RF STATIC DURING TIME FOR IGNITION

Condition Possible Cause Corrective Action/Comments

9.0

− AM radio interference.

9.1.2

Ignitor operation

ACTION 1 − Call Technical Support, Dallas.

Page 36