Lennox ML193UH045P36B Installation and operation manual

Page 1 ©2010 Lennox Industries Inc.
Litho U.S.A.
Corp. 1025−L5 ML193UH
Service Literature
ML193UH SERIES UNITS
ML193UH series units are high−efficiency gas furnaces
manufactured with Lennox DuralokPlustaluminized
steel clamshell−type heat exchangers, with a stainless steel
condensing coil. ML193UH units are available in heating
input capacities of 44,000 to 132,000 Btuh (13 to 38.6 kW)
and cooling applications from 2 through 5 tons (7.0 through
17.6 kW). 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. All ML193UH
units are equipped with a hot surface ignition system. The
gas valve is redundant to assure safety shut−off as re-
quired by C.S.A.
The heat exchanger, burners and manifold assembly can be
removed for inspection and service. The maintenance section
gives a detailed description on how this is done.
All specifications 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.
WARNING
Electric shock hazard. Can cause injury
or death. Before attempting to perform
any service or maintenance, turn the
electrical power to unit OFF at discon-
nect switch(es). Unit may have multiple
power supplies.
Table of Contents
Specifications 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Optional Accessories 3. . . . . . . . . . . . . . . . . . . . . . . . . .
Blower Performance Data 4. . . . . . . . . . . . . . . . . . . . . .
I−Unit Components 7. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II Placement and Installation 14. . . . . . . . . . . . . . . . . . . .
III−Start−Up 35. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IV−Heating System Service Checks 36. . . . . . . . . . . . . .
V−Typical Operating Conditions 38. . . . . . . . . . . . . . . . . .
VI−Maintenance 39. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VII−Sequence of Operation and Flow Charts 41. . . . . . .
VIII−Integrated Control Troubleshooting 46. . . . . . . . . . .
WARNING
Improper installation, adjustment, alteration, service
or maintenance can cause property damage, person-
al injury or loss of life. Installation and service must
be performed by a licensed professional installer (or
equivalent), service agency or the gas supplier.
WARNING
Sharp edges.
Be careful when servicing unit to avoid sharp edges
which may result in personal injury.

Page 2
SPECIFICATIONS
Gas
Heating
Performance
Model No. ML193UH045P36B ML193UH070P24B ML193UH070P36B ML193UH090P36C
1AFUE 93% 93% 93% 93%
Input - Btuh 44,000 66,000 66,000 88,000
Output - Btuh 42,000 62,000 62,000 83,000
Temperature rise range - °F 25 - 55 50 - 80 40 - 70 50 - 80
Gas Manifold Pressure (in. w.g.)
Nat. Gas / LPG/Propane
3.5 / 10.0 3.5 / 10.0 3.5 / 10.0 3.5 / 10.0
High static - in. w.g. 0.50 0.50 0.50 0.50
Connections
in.
Intake / Exhaust Pipe (PVC) 2 / 2 2 / 2 2 / 2 2 / 2
Gas pipe size IPS 1/2 1/2 1/2 1/2
Condensate Drain Trap (PVC pipe) - i.d. 1/2 1/2 1/2 1/2
with eld supplied (PVC coupling) - o.d. 3/4 3/4 3/4 3/4
Indoor
Blower
Wheel nom. dia. x width - in. 10 x 8 10 x 8 10 x 8 10 x 8
Motor output - hp 1/3 1/5 1/3 1/3
Tons of add-on cooling 2.5 - 3 1.5 - 2 2.5 - 3 2 - 3
Air Volume Range - cfm 700 - 1600 390 - 1140 660 - 1615 695 - 1620
Electrical
Data
Voltage 120 volts - 60 hertz - 1 phase
Blower motor full load amps 6.1 3.1 6.1 6.1
Maximum overcurrent protection 15 15 15 15
Shipping Data lbs. - 1 package 122 125 127 143
NOTE - Filters and provisions for mounting are not furnished and must be eld provided.
1 Annual Fuel Utilization Efciency based on DOE test procedures and according to FTC labeling regulations. Isolated combustion system rating for non-weatherized
furnaces.
SPECIFICATIONS
Gas
Heating
Performance
Model No. ML193UH090P48C ML193UH110P48C ML193UH110P60C ML193UH135P60D
1AFUE 93% 93% 93% 93%
Input - Btuh 88,000 110,000 110,000 132,000
Output - Btuh 83,000 103,000 103,000 123,000
Temperature rise range - °F 40 - 70 50 - 80 40 - 70 45 - 75
Gas Manifold Pressure (in. w.g.)
Nat. Gas / LPG/Propane
3.5 / 10.0 3.5 / 10.0 3.5 / 10.0 3.5 / 10.0
High static - in. w.g. 0.50 0.50 0.50 0.50
Connections
in.
Intake / Exhaust Pipe (PVC) 2 / 2 2 / 2 2 / 2 2 / 2
Gas pipe size IPS 1/2 1/2 1/2 1/2
Condensate Drain Trap (PVC pipe) - i.d. 1/2 1/2 1/2 1/2
with eld supplied (PVC coupling) - o.d. 3/4 3/4 3/4 3/4
Indoor
Blower
Wheel nom. dia. x width - in. 10 x 10 10 x 10 11 ½ x 10 11 ½ x 10
Motor output - hp 1/2 1/2 1 1
Tons of add-on cooling 3 - 4 3 - 4 4 - 5 4 - 5
Air Volume Range - cfm 900 - 2025 850 - 2030 1210 - 2525 1340 - 2800
Electrical
Data
Voltage 120 volts - 60 hertz - 1 phase
Blower motor full load amps 8.2 8.2 11.5 11.5
Maximum overcurrent protection 15 15 15 15
Shipping Data lbs. - 1 package 146 155 161 178
NOTE - Filters and provisions for mounting are not furnished and must be eld provided.
1 Annual Fuel Utilization Efciency based on DOE test procedures and according to FTC labeling regulations. Isolated combustion system rating for non-weatherized
furnaces.

Page 3
GAS HEAT ACCESSORIES
Input
High Altitude
Pressure Switch Kit
Natural Gas to
LPG/Propane Kit
LPG/Propane
to Natural Gas Kit
Natural Gas
High Altitude
Orice Kit
4501 - 7500 ft. 7501 - 10,000 ft. 0 - 7500 ft. 0 - 7500 ft. 7501- 10,000 ft.
all models 74W90 74W91 69W73 73W81 73W37
OPTIONAL ACCESSORIES - MUST BE ORDERED EXTRA
“B” Width
Models
“C” Width
Models
“D” Width
Models
CABINET ACCESSORIES
Horizontal Suspension Kit - Horizontal only 51W10 51W10 51W10
Return Air Base - Upow only 50W98 50W99 51W00
CONDENSATE DRAIN KITS
Condensate Drain Heat Cable 6 ft. 26K68 26K68 26K68
24 ft. 26K69 26K69 26K69
50 ft. 26K70 26K70 26K70
Heat Cable Tape Fiberglass - 1/2 in. x 66 ft. 36G53 36G53 36G53
Aluminum foil - 2 in. x 60 ft. 16P89 16P89 16P89
Crawl Space Vent Drain Kit 51W18 51W18 51W18
CONTROLS
Twinning Kit 65W80 65W80 65W80
FILTER KITS
1Air Filter and
Rack Kit
Horizontal (end) Size of lter - in. 87L96 - 18 x 25 x 1 87L97 - 20 x 25 x 1 87L98 - 25 x 25 x 1
Side Return Single 44J22 44J22 44J22
Ten Pack 66K63 66K63 66K63
Size of lter - in. 16 x 25 x 1 16 x 25 x 1 16 x 25 x 1
NIGHT SERVICE KITS
Night Service Kit 51W03 51W03 51W03
TERMINATION KITS
See Installation Instructions for specic venting information.
Termination Kits -
Direct Vent
Applications Only
Concentric US - 2 in. 71M80 69M29 - - -
3 in. - - - 60L46 60L46
Canada - 2 in. 44W92 44W92 - - -
3 in. - - - 44W93 44W93
Flush-Mount 2, 2-1/2 or 3 in. 51W11 51W11 51W11
Wall - Close
Couple
US - 2 in. 22G44 - - - - - -
3 in. 44J40 44J40 44J40
Wall - Close
Couple WTK
Canada - 2 in. 30G28 - - - - - -
3 in. 81J20 81J20 81J20
Termination Kits -
Direct or Non-
Direct vent
Roof 2 in. 15F75 15F75 - - -
Wall Ring Kit 2 in. 15F74 315F74 - - -
Roof Termination Flashing Kit - Direct or
Non-Direct Vent (2 ashings)
2 in. 44J41 44J41 44J41
1 Cleanable polyurethane frame type lter.
2 Kits contain enough parts for two, non−direct vent installations.
3 Non−direct vent only.
NOTE - Termination Kits 44W92, 44W93, 30G28, 81J20 are certied to ULC S636 standard for use in Canada only.
INSTALLATION CLEARANCES - INCHES (MM)
Sides 10 inches (0 mm)
Rear 0 inches (0 mm)
Top/Plenum 1 inch (25 mm)
Front 0 inches (0 mm)
Front (service/alcove) 24 inches (610 mm)
Floor 2Combustible
NOTE − Air for combustion must conform to the methods outlined in the National Fuel Gas Code (NFPA 54/ANSI−Z223.1)
or the National Standard of Canada CAN/CSA−B149.1 Natural Gas and Propane Installation Code”.
NOTE − In the U.S. ue sizing must conform to the methods outlined in the current National Fuel Gas Code (NFPA 54/
ANSI−Z223.1) or applicable provisions of local building codes. In Canada ue sizing must conform to the methods
outlined in National Standard of Canada CAN/CSA−B149.1.
1Allow proper clearances to accommodate condensate trap and vent pipe installation.
2 Do not install the furnace directly on carpeting, tile, or other combustible materials other than wood ooring.

Page 4
BLOWER DATA
ML193UH070P24B PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Various Blower Speeds
High Medium-
High
Medium-
Low Low
cfm Watts cfm Watts cfm Watts cfm Watts
0.00 1140 455 920 365 765 295 710 265
0.10 1135 445 900 360 765 290 690 255
0.20 1125 430 895 350 755 285 680 255
0.30 1090 415 870 340 725 280 660 250
0.40 1065 405 870 325 715 270 635 245
0.50 1020 390 825 315 675 260 605 235
0.60 945 365 780 300 640 250 555 225
0.70 910 350 740 295 585 240 505 220
0.80 790 325 670 275 510 225 455 205
0.90 735 310 575 255 460 220 390 195
ML193UH090P36C PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Various Blower Speeds
High Medium-
High
Medium-
Low Low
cfm Watts cfm Watts cfm Watts cfm Watts
0.00 1620 745 1340 620 1130 500 965 405
0.10 1610 720 1335 595 1135 490 975 395
0.20 1565 695 1335 565 1145 465 970 385
0.30 1525 665 1300 555 1135 455 970 370
0.40 1485 635 1295 520 1110 435 960 355
0.50 1431 600 1260 495 1090 405 940 345
0.60 1365 570 1210 475 1035 390 900 330
0.70 1295 535 1155 445 995 365 860 305
0.80 1200 505 1065 415 930 340 745 270
0.90 1060 460 955 375 820 305 695 260
ML193UH090P48C PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Various Blower Speeds
High Medium-
High
Medium-
Low Low
cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2025 900 1690 780 1395 645 1200 540
0.10 1995 880 1705 760 1390 635 1205 525
0.20 1925 835 1675 715 1405 605 1205 505
0.30 1850 795 1640 680 1400 580 1170 490
0.40 1790 760 1575 645 1395 560 1170 475
0.50 1700 725 1540 625 1350 535 1140 450
0.60 1610 690 1455 580 1295 505 1100 430
0.70 1540 645 1365 550 1225 480 1030 405
0.80 1415 615 1265 510 1125 445 980 385
0.90 1270 565 1165 470 1060 420 900 350
ML193UH110P48C PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Various Blower Speeds
High Medium-
High
Medium-
Low Low
cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2030 905 1750 805 1425 665 1225 530
0.10 1950 865 1755 770 1430 630 1215 515
0.20 1935 840 1675 735 1455 610 1230 505
0.30 1885 810 1660 685 1410 585 1200 485
0.40 1830 780 1585 645 1385 570 1190 470
0.50 1750 740 1565 630 1320 535 1165 455
0.60 1660 695 1485 585 1280 505 1105 425
0.70 1540 665 1380 555 1230 485 1060 405
0.80 1420 615 1290 520 1140 445 945 375
0.90 1290 575 1175 470 1045 410 850 350
ML193UH045P36B PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Various Blower Speeds
High Medium-
High
Medium-
Low Low
cfm Watts cfm Watts cfm Watts cfm Watts
0.00 1600 700 1370 590 1160 475 1005 400
0.10 1600 685 1355 565 1155 465 1015 390
0.20 1550 650 1330 540 1150 445 1000 380
0.30 1480 625 1295 515 1140 430 975 365
0.40 1425 590 1280 490 1105 415 975 350
0.50 1355 565 1190 460 1085 395 940 335
0.60 1320 545 1165 435 1030 375 900 315
0.70 1225 500 1110 425 975 355 855 310
0.80 1135 480 1050 395 920 330 780 280
0.90 1025 445 950 360 800 295 700 255
ML193UH070P36B PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Various Blower Speeds
High Medium-
High
Medium-
Low Low
cfm Watts cfm Watts cfm Watts cfm Watts
0.00 1615 680 1355 570 1140 480 1030 400
0.10 1565 660 1365 545 1150 465 1020 385
0.20 1535 630 1340 525 1150 440 1025 375
0.30 1440 600 1300 500 1110 420 1000 350
0.40 1405 570 1255 470 1075 400 975 345
0.50 1340 535 1200 445 1045 380 945 330
0.60 1255 500 1125 420 995 355 875 310
0.70 1165 475 1080 395 935 335 820 290
0.80 1060 455 990 365 870 315 750 265
0.90 955 425 895 345 750 285 660 245

Page 5
BLOWER DATA
ML193UH110P60C PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Different Blower Speeds
Bottom Return Air, Side Return Air with Optional Return
Air Base, Return Air from Both Sides or Return Air from
Bottom and One Side.
Single Side Return Air − Air volumes in bold require eld
fabricated transition to accommodate 20 x 25 x 1 in. air lter
in order to maintain proper air velocity.
High Medium-High Medium-Low Low High Medium-High Medium-Low Low
cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2525 1560 2175 1165 1820 905 1465 725 2520 1545 2135 1150 1750 905 1445 720
0.10 2585 1545 2200 1135 1860 900 1475 710 2555 1545 2135 1115 1775 890 1470 715
0.20 2515 1505 2150 1110 1840 890 1490 705 2465 1480 2105 1085 1775 875 1465 705
0.30 2445 1445 2135 1065 1790 870 1500 690 2370 1430 2045 1055 1750 855 1460 690
0.40 2340 1385 2065 1035 1770 845 1500 675 2275 1375 1990 1010 1730 830 1460 680
0.50 2230 1350 1985 985 1755 810 1470 665 2185 1345 1930 970 1690 800 1460 655
0.60 2130 1295 1920 950 1685 785 1425 640 2060 1290 1850 935 1650 780 1420 635
0.70 2030 1250 1815 905 1640 760 1405 625 1930 1230 1760 900 1580 750 1355 610
0.80 1920 1190 1735 865 1560 725 1350 605 1825 1180 1660 855 1505 710 1290 585
0.90 1735 1135 1620 830 1450 685 1270 575 1665 1130 1520 810 1415 675 1210 560
ML193UH135P60D PERFORMANCE (Less Filter)
External
Static
Pressure
in. w.g.
Air Volume / Watts at Different Blower Speeds
Bottom Return Air, Side Return Air with Optional Return
Air Base, Return Air from Both Sides or Return Air from
Bottom and One Side.
Single Side Return Air − Air volumes in bold require eld
fabricated transition to accommodate 20 x 25 x 1 in. air lter
in order to maintain proper air velocity.
High Medium-High Medium-Low Low High Medium-High Medium-Low Low
cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts cfm Watts
0.00 2800 1715 2155 1160 1730 900 1375 695 2720 1685 2110 1135 1670 905 1355 705
0.10 2770 1665 2170 1145 1740 895 1415 700 2660 1650 2110 1115 1725 895 1390 700
0.20 2690 1635 2150 1110 1770 890 1450 700 2600 1585 2125 1090 1750 885 1450 695
0.30 2590 1560 2140 1080 1785 870 1455 695 2535 1525 2075 1065 1750 865 1465 685
0.40 2500 1535 2105 1055 1785 855 1475 690 2400 1490 2055 1030 1715 845 1460 680
0.50 2420 1465 2050 1025 1770 835 1465 665 2335 1420 2000 1005 1725 825 1455 660
0.60 2330 1410 2015 995 1720 810 1460 655 2270 1385 1950 970 1720 800 1445 650
0.70 2225 1370 1965 960 1690 785 1450 650 2175 1335 1895 950 1665 780 1430 635
0.80 2150 1335 1875 925 1655 755 1435 630 2075 1295 1840 910 1605 745 1400 620
0.90 2025 1290 1830 890 1575 720 1375 605 1975 1255 1755 875 1540 725 1340 590

Page 6
ML193UH PARTS IDENTIFICATION
FIGURE 1
TOP CAP
CABINET
BURNER BOX
ASSEMBLY
SIGHT
GLASS
DuralokPlusTM
HEAT EXCHANGER
ASSEMBLY
CONTROL BOX
(includes integrated itegrated control, transformer and interlock switch)
COMBUSTION AIR
INDUCER
BLOWER
ACCESS
DOOR
BURNER
ACCESS
PANEL
COMBUSTION
AIR PRESSURE
SWITCH
PRIMARY LIMIT
GAS VALVE
BLOWER
ASSEMBLY
FLEXIBLE NO−HUB
EXHAUST COLLAR
FLUE COLLAR
MANIFOLD
COLD END
HEADER BOX
BAG ASSEMBLIES
(shipping location)

Page 7
I−UNIT COMPONENTS
ML193UH unit components are shown in figure 1. The
combustion air inducer, gas valve and burners can be ac-
cessed by removing the burner access panel. The blower
and control box can be accessed by removing the blow-
er access door.
A−Control Box Components (Figure 2)
Unit transformer (T1) and integrated ignition control (A92)
are located in the control box. In addition, a door interlock
switch (S51) is located in the control box.
FIGURE 2
DOOR INTERLOCK
SWITCH (S51)
INTEGRATED IGNITION
CONTROL
(A92)
TRANSFORMER
(T1)
ML193UH Control Box
1. Transformer (T1)
A transformer located in the control box provides power to
the low voltage section of the unit. The transformers on all
models are rated at 40VA with a 120V primary and 24V
secondary.
2. Door Interlock Switch (S51)
A door interlock switch rated 14A at 120VAC is located on
the control box. The switch is wired in series with line volt-
age. When the blower door is removed the unit will shut
down.
3. Integrated Ignition Control (A92)
WARNING
Shock hazard.
Disconnect power before servicing. Control is not
field repairable. If control is inoperable, simply re-
place entire control.
Can cause injury or death. Unsafe operation will
result if repair is attempted.
The ignition control system consists of an integrated con-
trol (figure 4) ignitor (figure 6) and flame sensor (figure 6).
The integrated control and ignitor work in combination to
ensure furnace ignition and ignitor durability. The inte-
grated control, controls all major furnace operations. The
integrated control also features two LED lights (DS1 red
and DS2 green) for troubleshooting and two accessory
terminals rated at (1) one amp. The integrated control also
features a (3) amp fuse for overcurrent protection. Tables 1
and 2 show jack plug terminal designations. See table 3 for
troubleshooting diagnostic codes. The mini−nitride ignitor
is made from a non−porous, high strength proprietary ce-
ramic material that provides long life and trouble free
maintenance. The integrated control continuously moni-
tors line voltage and maintains the ignitor power at a con-
sistent level to provide proper lighting and maximum igni-
tor life.
TABLE 1
4−Pin Terminal Designation
PIN # FUNCTION
1Combustion Air Inducer Line
2Ignitor Line
3Combustion Air Inducer Neutral
4Ignitor Neutral
TABLE 2
12−Pin Terminal Designations
PIN # FUNCTION
1High Limit Output
2Not Used
324V Line
4Not Used
5Rollout Switch Out
624V Neutral
7High Limit Input
8 Ground
9Gas Valve Common
10 Pressure Switch In
11 Rollout Switch In
12 Gas Valve Out
Electronic Ignition (See Figure 5)
On a call for heat the integrated control monitors the com-
bustion air inducer prove switch. The integrated 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 differen-
tial in the prove switch is great enough, the prove switch
closes and a 15−second pre−purge begins. If the prove
switch is not proven within 2−1/2 minutes, the integrated
control goes into Watchguard−Pressure Switch mode for a
5−minute re−set period.

Page 8
After the 15−second pre−purge period, the ignitor warms up
for 20 seconds during which the gas valve opens at 19 sec-
onds for a 4−second trial for ignition. The ignitor remains
energized for the first 3 seconds during the 4 second trial. If
ignition is not proved during the 4−second period, the inte-
grated control will try four more times with an inter purge
and warm−up time between trials of 35 seconds. After a to-
tal of five trials for ignition (including the initial trial), the inte-
grated control goes into Watchguard−Flame Failure mode.
After a 60−minute reset period, the integrated control will
begin the ignition sequence again.
The integrated control has an added feature of ignitor pow-
er regulation to maintain consistent lighting and longer igni-
tor life under all line voltage conditions.
Fan Control
The fan on time of 30 seconds is not adjustable. The fan off
delay (amount of time that the blower operates after the
heat demand has been satisfied) may be adjusted by
changing the jumper position across the five pins on the
integrated control. The unit is shipped with a factory fan off
setting of 90 seconds. The fan off delay affects comfort and
is adjustable to satisfy individual applications. Adjust the
fan off delay to achieve a supply air temperature between
90° and 110°F at the moment that the blower is de−ener-
gized. Longer off delay settings provide lower return air
temperatures; shorter settings provide higher return air
temperatures. See figure 3.
FIGURE 3
FAN-OFF TIME IN SECONDS
To adjust fan−off timing, reposition jumper across pins
to achieve desired setting.
NO JUMPER
FIGURE 4
INTEGRATED CONTROL
(Automatic Hot Surface Ignition System)
TERMINAL DESIGNATIONS
HUM
LINE
XFMR
EAC
COOL
HEAT
PARK
FLAME
NEUTRALS
Humidifier (120VAC)
Input (120VAC)
Transformer (120VAC)
Electronic Air Cleaner (120VAC)
Blower − Cooling Speed (120VAC)
Blower − Heating Speed (120VAC)
Dead terminals to park alternate spd taps
Flame sensor
Neutral terminals (120VAC)
3 AMP, 32 VAC FUSE
BLOWER OFF
DELAY JUMPER
LED 1 LED 2

Page 9
The integrated control is equipped with two LED lights for troubleshooting. The diagnostic codes are listed below in table 3.
TABLE 3
DIAGNOSTIC CODES
Make sure to Identify LED’S Correctly.
LED #1 (Red) LED #2 (Green) DESCRIPTION
SIMULTANEOUS
SLOW FLASH
SIMULTANEOUS
SLOW FLASH
Power on − Normal operation.
Also signaled during cooling and continuous fan.
SIMULTANEOUS
FAST FLASH
SIMULTANEOUS
FAST FLASH Normal operation − signaled when heating demand initiated at thermostat.
SLOW FLASH ON Primary or secondary limit switch open. Limit must close within 3 minutes or unit
goes into 1 hour Watchguard.
OFF SLOW FLASH
Pressure prove switch open.
OR: Blocked inlet/exhaust vent;
OR: Pressure switch closed prior to activation of combustion air inducer.
ALTERNATING
SLOW FLASH
ALTERNATING
SLOW FLASH
Watchguard 1 hour −− burners failed to ignite or lost flame 5 times during single
heating demand.
SLOW FLASH OFF Flame sensed without gas valve energized.
ON SLOW FLASH Rollout switch open. OR: 12-pin connector improperly attached.
ON
ON
OFF
ON
OFF
ON
Circuit board failure or control wired incorrectly.
FAST FLASH SLOW FLASH Main power polarity reversed. Switch line and neutral.
SLOW FLASH FAST FLASH Low flame signal. Measures below 1.5 microamps. Replace flame sense rod.
ALTERNATING
FAST FLASH
ALTERNATING
FAST FLASH
Improper main ground.
OR: Line voltage below 90 volts.
NOTE − Slow flash rate equals 1 Hz (one flash per second). Fast flash rate equals 3 Hz (three flashes per second).
Minimum flame sense current = 0.5 microAmps.
FIGURE 5
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉ
ÉÉ
DEMAND
CAI
GAS VALVE
ON
OFF
ÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉÉ
ÉÉ
ÉÉ
ÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉ
IGNITOR
15 Sec.
Pre −Purge
20 sec.
Ignitor Warmup
*4
Sec.Trial
for Ign. Post
Purge
5 SEC
**Blower on time will be 45 seconds after gas valve is energized. Blower off time will depend on OFF TIME" Setting.
INDOOR BLOWER
ÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉ
**Blower On"
Delay
End of
Heat Demand Blower
Off
Time
*Ignitor will energize the first 3 seconds of the 4 second trial for ignition

Page 10
B−Heating Components
Combustion air inducer (B6), primary limit control (S10),
SureLight ignitor, burners, flame rollout switch (S47), gas
valve (GV1), combustion air prove switch (S18), and clam-
shell heat exchangers are located in the heating compart-
ment. The heating compartment can be accessed by re-
moving the burner access panel.
1. Ignitor (Figure 6)
ML193UH units use a mini−nitride ignitor made from a pro-
prietary ceramic material. Ignitor longevity is enhanced by
controlling the voltage to the ignitor. Due to this feature of
the integrated control, voltage cannot be measured. To
check ignitor, measure its resistance. A value of 50 to 450
ohms indicates a good ignitor.
2. Flame Sensor (Figure 6)
A flame sensor is located on the left side of the burner sup-
port. The sensor is mounted on the front burner box plate
and the tip protrudes into the flame envelope of the left−
most burner. The sensor can be removed for service with-
out removing any part of the burners. During operation,
flame is sensed by current passed through the flame and
sensing electrode. The ignition control allows the gas valve
to remain open as long as flame signal is sensed.
NOTE − The ML193UH furnace contains electronic
components that are polarity sensitive. Make sure that
the furnace is wired correctly and is properly grounded.
3. Flame Rollout Switches (Figure 6)
Flame rollout switches S47 are SPST N.C. high temperature
limits located on the top left and bottom right of the front buner
box plate. S47 is wired to the burner ignition control A92.
When either of the switches sense flame rollout (indicat-
ing a blockage in the combustion passages), the flame
rollout switch trips, and the ignition control immediately
closes the gas valve. Switch S47 in all ML193UH units is
factory preset to open at 210_F + 12_F (99_C + 6.7_C) on a
temperature rise. All flame rollout switches are manual reset.
FIGURE 6
ML193UH Burner Box Assembly
IGNITOR
FLAME SENSOR
ROLLOUT SWITCHES
GAS VALVE
BURNERS
ORIFICES
FRONT BURNER BOX PLATE