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G24M
User Manual
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User Manual
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Lennox G24M User Manual

G24M
Blowerwheelnomina
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Blowerwheelnomina
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diameterxwidt
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Nominalcoolin
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Nominalcoolin
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Down-flow
Corp. 9723-L12
Service Literature
G24M SERIES UNITS
Units are factory equipped for use with natural gas. A kit is availableforconversiontoLPGoperation.EarlymodelG24M units use electronic (direct spark) ignition. Late model G24M model units feature the Lennox SureLightT silicon-nitride ignition system. The G24MX unit meets the California Nitro­gen Oxides (NOx) Standards and California Seasonal Effi­ciency requirements. All units use a redundant gas valve to assure safety shut-off as required by A.G.A. or C.G.A.
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.
Model No. G24M2(X)-45 G24M2-60 G24M3(X)-60 G24M2-75 G24M3(X)-75
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) lA.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)
in. 9 x 7 10 x 7 9 x 7 10 x 7
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)
Catalog No.
Filter Kit
Down-flow Combustible Floor Base Sidewall Power Venting Kit 79J15 — 25 lbs. (11 kg) Hanging Bracket Kit
lAnnual 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 isfurnished with kit. ¡Filters arenot furnished with kitand must be ordered extra.
No. & Size of Filters — in. (mm)
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
b Optional Accessories (Must Be Ordered Extra) b
Revised 07-2001

SPECIFICATIONS

LB-69843A (32J01) — 3 lbs. (1 kg)
LB-79239A (67J91) — 10 lbs. (4 kg)
LB-69957 (46J66) — 15 lbs. (8 kg)
Single (32J02) TenPack (66K64)
(1) 16 x 20 x 1 (406 x 508 x 25)
(2) 16 x 20 x 1 (406 x 508 x 25)
Page 1
ã 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)
R
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 EF(EC) 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 wheelnomina
diameter x width
in. 12 x 8 12 x 9
mm 305 x 203 305 x 229
Blower motor output hp (W) 1/2 (373) 3/4 (560)
that can be added
cooling
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
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
H
Optional Accessories (Must Be Ordered Extra)
LPG/propane kit LB-69845L
Twinning Kit
Up--Flow/Horizontal Filter and Filter Rack Kits
c
No. & size of filters -- in. (mm)
Single (
32J02
) Ten Pack (
(1) 16 x 20 x 1 (406 x 508 x 25)
(38K84)
66K64
96J69
)
H
LB-69845K
5 lbs. (2 kg)
Single
(46J14)
Ten Pack
(1) 20 x 20 x 1 (508 x 508 x 25)
(81J14)
(66K65)
Catalog No. LB-69843A
o
Down-flow Filter Kit
Down-flow Combustible floor Base LB-79239A
Sidewall Power Venting Kit
Hanging Bracket Kit LB-69957
R
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 is furnished with kit.
o
Filters are not furnished with kit and must be ordered extra.
No. & Size of Filters in. (mm)
(2) 16 x 20 x 1 (406 x 508 x 25)
10lbs.(4kg) LB-79239B
(67J91)
79J15
(46J66)
3DJH
3 lbs. (1 kg)
(32J01)
25 lbs. (11 kg)
15 lbs. (8 kg)
(67J92)
10 lbs. (4 kg)
SPECIFICATIONS
Blower wheel nominal
g
Nominal cooling
No.& Size
External Static
Model No.
Input Btuh (kW) Output Btuh (kW)
R
A.F.U.E.
California Seasonal Efficiency
o
Flue size connection diameter in. (mm) round Temperature rise range EF(EC) 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
in. mm
Blower motor output hp (W)
Nominal coolin
that can be added
Tons 2, 2-1/2, 3, 3-1/2 or 4
kW 7.0, 8.8, 10.6, 12.3 or 14.1 Shipping weight lbs. (kg) 1 package Electrical characteristics
H
Optional Accessories (Must Be Ordered Extra)
LPG/propane kit Twinning Kit Up--Flow/Horizontal Filter and Filter Rack Kits
c
No. and size of filters -- in. (mm)
Catalog No.
w
Down-flow
Filter Rack
No. & Size of Filters
in.
mm
Down-flow Combustible Floor Base Sidewall Power Venting Kit
Hanging Bracket Kit
R
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 is furnished with kit.
o
2 in. x 5 in. (51 mm x127 mm) flue adaptor furnished with -140 input furnaces for connection to furnace induced draft blower.
w
Filters are not furnished with kit and must be ordered extra.
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 12x9
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
H
Single
(46J14)
Ten Pack
LB-69845K
96J69
(66K65)
(81J14)
5 lbs. (2 kg)
(1) 20 x 20 x 1 (508 x 508 x 25)
LB-69843A
(32J01)
3 lbs. (1 kg)
(1) 20 x 20 x 1 (508 x 508 x 25)
(2)16x20x1
(2) 406 x 508 x 25
LB-79239B
(67J92)
10 lbs. (4 kg)
25 lbs. (11 kg)
79J15
LB-69957
(46J66)
15 lbs. (8 kg)
G24M4/5-140
140,000 (41.0) 114,800 (33.6)
80.0%
Not Available
190 (86)
Single
(58J93)
(66K66)
LB-79239C
12 lbs. (5 kg)
Ten Pack
(67J93)

BLOWER DATA

G24M2-45, G24M2-60 AND G24M2-75 BLOW
ER PERFORMANCE
External Static
Pressure
High Medium-High Medium-Low Low
Air Volume at Various Blower Speeds
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
3DJH
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
3DJH
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)
3DJH
G24M
PARTS IDENTIFICATION
COMBUSTION AIR
BLOWER
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
FIGURE 1
HEATING COMPONENTS (shown in horizonta
BLOWER ASSEMBLY
l position)
PRIMARY LIMIT
Bottom
Front
Left
Back
Top
GAS VALVE
COLLECTOR
FIGURE 2
Page 6
Right
HEAT EXCHANGER
COMBUSTION AIR BLOWER
(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
Top
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 connec-
tions 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
Top
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 reĆ moving 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 reĆ quired for bottom air return. Indentations on side of units, show where side return opening should be cut during installaĆ tion.
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 reĆ moved 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 secĆ ondary.
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 (f igur e 5) and ignition control board (f igur e 6 wit h 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 termiĆ nals rated at (4) four amps. See table 2 for troubleshootĆ ing diagnostic codes. Table 3 and 4 show jack plug termiĆ nal designations. Units equipped with the SureLight board can be used with either electronic or electro-meĆ chanical thermostats without modification. The SurĆ eLight 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 temperaĆ ture which will successfully light the burner, thus increasĆ ing the life of the ignitor.
a-flame sensor
The flame sensor uses flame rectification to sense comĆ bustion. 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 24VACHOT 24VACRTN FLAME SENSE
Blower - Cooling Speed (Line Volt) Blower - Heating Speed (Line Volt) Alternate Blower Speeds (Dead) Continuous Low Speed Blower Accessory Terminal (Line Volt) 120VACHot to Transformer 120VACHot Input Heat Only Accessory (Line Volt) 120VACNeutrals 24VACHot from Transformer 24VACReturn from Transformer Flame Sense Terminal
NORMAL FLAME SIGNAL > 0.7 MICROAMPS LOW FLAME SIGNAL < 0.7 MICROAMPS MINIMUM FLAME SIGNAL < 0.15 MICROAMPS
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
11/16in. 18 mm
¦ 1/32 in. 0.79
mm
NOTE - Slowflash equals1 Hz (oneflash persecond). Fast flashequals 3Hz (threeflashes per second).Drop outflame sense current< 0.15microAmps
Page 9
TABLE 3
SureLight BOARD J156 (J2) TERMINAL
PIN # FUNCTION
1 Ignitor
2 3 4 5 6
DESIGNATIONS
Not Used
Ignitor Neutral
Combustion Air Blower Line Voltage
Not Used
Combustion Air Blower Neutral
TABLE 4
SureLight BOARD J58 (J1) TERMINAL
PIN # FUNCTION
1 Primary Limit In
2 3 4 5 6 7 8 9
DESIGNATIONS
Gas Valve Common Roll Out Switch Out
Gas Valve 24V
Pressure Switch In
Pressure Switch and Primary Limit Out
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 thegas 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 opensfor a 4-secondtrial for ignition.The ignitor staysen­ergized for the first second of the 4-second trial. G24M units with 63K89, 24L85 or 56L83: the ignitor stays ener­gizedthefirstsecondofthe4secondtrial.G24Munitswith board 97L48: ignitor stays energized during the 4-second trialuntilflameissensed.Ifignitionisnotproved during the 4-secondperiod,thecontrolwill try four more times withan inter purge and warm-up time between trials of 35 sec­onds.After a totalof five trialsfor ignition (including the ini­tial trial), the control goes into Watchguard-Flame Failure mode.Aftera 60-minute reset period, the control willbegin the ignition sequence again. The SureLight control board has an added feature that prolongs the life of the ignitor. After a successful ignition, theSureLightcontrolutilizeslesspower to energize the ig­nitoron successive callsfor heat. Thecontrol continues to rampdown the voltageto the ignitor untilit finds the lowest amount of power that will provide a successful ignition. Thisamount of poweris used for255 cycles. Onthe 256th callfor heat, thecontrol will againramp 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 theheatingcycle if the prove switchisclosed (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 func- tions 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 igni- tion 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 diagnos- tic 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 be- gins flashing at a fast rate and the combustion air blowerisenergized.The LED flashes different codes to indicate problem conditions. The diagnostic LED lights red (not flashing) to indicate control board failure. T able 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 com- bustion products and to introduce fresh air for com- bustion. If the combustion air prove switch does not close, the combustion air blower continues to run in- definitely (until the prove switch closes).
Post-Purge
After a demand, the combustion air blower continues to operate for 5 seconds (post-purge) before stopping. Post-purge 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. T rial for ignition lasts for 7 seconds. At the same time, the control begins monitoring the flame sensor.Iftheflame 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 com- bustion air blower and de-energize the gas valve. Flame current should be between 1and5microamps 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 mo- mentarily turning off power to the unit.
DANGER
Shock hazard. Spark related components contain high voltage.
Disconnect power before servicing. Control is not field repairable. Ifcontrol is inoperable, simply re- place entire control.
Can cause injury or death. Unsafe operation will result if repair is attempted.
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