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INSTALLATION
E 2015 Lennox Industries Inc.
Dallas, Texas, USA
DOWNFLOW
INSTRUCTIONS
ML180DF
MERIT® SERIES GAS FURNACE
DOWNFLOW AIR DISCHARGE
507326-01
06/2015
Supersedes 507057-01
This is a safety alert symbol and should never be ignored.
When you see this symbol on labels or in manuals, be alert
to the potential for personal injury or death.
THIS MANUAL MUST BE LEFT WITH THE
HOMEOWNER FOR FUTURE REFERENCE
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.
Litho U.S.A.
AIR FLOW
Table of Contents
Unit Dimensions 2................................
ML180DF Gas Furnace 3..........................
Shipping and Packing List 3........................
Safety Information 3...............................
Use of Furnace as a Construction Heater 4...........
General 4........................................
Combustion, Dilution & Ventilation Air 5..............
Setting Equipment 9...............................
Filters 10..........................................
Duct System 11....................................
CAUTION
As with any mechanical equipment, personal injury
can result from contact with sharp sheet metal
edges. Be careful when you handle this equipment.
Venting 11........................................
Gas Piping 16.....................................
Electrical 18.......................................
Integrated Control 21...............................
Unit Start-Up 22...................................
Gas Pressure Adjustment 23........................
Proper Combustion 23..............................
High Altitude 23....................................
Other Unit Adjustments 24..........................
Service 26........................................
Repair Parts 28....................................
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Unit Dimensions - inches (mm)
3−1/8
(79)
9/16
(14)
D
RETURN AIR
OPENING
B
FLUE OUTLET
(Top)
9/16
(14)
19−7/16
(494)
9/16
(14)
TOP VIEW
A
B
9/16
(14)
3/4 (19)
Front Panel
27−3/4
(705)
19−7/16
(494)
9/16
(14)
ELECTRICAL INLET
(Either Side)
GAS PIPING INLET
(Either Side)
19−1/4
Supply
(489)
Air
3/4
(19)
3/4
(19)
C
Supply
Air
(838)
3/4
(19)
33
5−3/8 (137) Right
2−1/8 (54) Left
6−5/8 (168) Right
7−1/8 (181) Left
FRONT VIEW SIDE VIEW
Model No.
ML180DF045P24A
ML180DF045P36A
ML180DF070P36A
ML180DF090P36B
ML180DF090P48B
ML180DF110P60C 21 533 19-7/8 504 19-1/2 495 8 203
14-1/2 368 13-3/8 340 13 330 4-3/4 121
17-1/2 446 16-3/8 416 16 406 6-1/4 159
A B C D
in. mm in. mm in. mm in. mm
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ML180DF Gas Furnace
The ML180DF gas furnace is shipped ready for installation
in the downflow position fueled by natural gas. A conver
sion kit (ordered separately) is required for use in LP/Pro
pane gas applications.
Shipping and Packing List
Package 1 of 1 contains
1 - Assembled ML180DF unit
1 - Bag assembly containing the following:
2 - Screws
1 - Snap bushing
1 - Snap plug
1 - Wire tie
1 - Vent warning label
1 - Owner's manual and warranty card
The following items may be ordered separately:
1 - Thermostat
1 - LP/Propane changeover kit
1 - Combustible flooring base
1 - High altitude kit
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
Safety Information
DANGER
Danger of explosion.
There are circumstances in which odorant used with
LP/propane gas can lose its scent. In case of a leak,
LP/propane gas will settle close to the floor and may
be difficult to smell. An LP/propane leak detector
should be installed in all LP applications.
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.
CAUTION
As with any mechanical equipment, personal injury
can result from contact with sharp sheet metal
edges. Be careful when you handle this equipment.
Certifications
ML180DF units are CSA International certified.
In the USA, installation of gas furnaces must conform with
local building codes. In the absence of local codes, units
must be installed according to the current National Fuel
Gas Code (ANSI‐Z223.1). The National Fuel Gas Code is
available from the following address:
American National Standards Institute, Inc.
11 West 42nd Street
New York, NY 10036
Clearances
Adequate clearance must be made around the air open
ings into the vestibule area. In order to ensure proper unit
operation, combustion and ventilation air supply must be
provided according to the current National Fuel Gas Code.
Vent installations must be consistent with the National Fuel
Gas Code venting tables (in this instruction) and applicable
provisions of local building codes.
This furnace is CSA International certified for installation
clearances to combustible material as listed on the unit
nameplate and in the tables in figure 13. Accessibility and
service clearances must take precedence over fire protec
tion clearances.
Installed Locations
For installation in a residential garage, the furnace must be
installed so that the burner(s) and the ignition source are
located no less than 18 inches (457 mm) above the floor.
The furnace must be located or protected to avoid physical
damage by vehicles. When a furnace is installed in a public
garage, hangar, or other building that has a hazardous at
mosphere, the furnace must be installed according to rec
ommended good practice requirements and current Na
tional Fuel Gas Code.
NOTE - Furnace must be adjusted to obtain a temperature
rise within the range(s) specified on the unit nameplate.
Failure to do so may cause erratic limit operation and may
also result in premature heat exchanger failure.
This ML180DF furnace must be installed so that its electri
cal components are protected from water.
Installed in Combination with a Cooling Coil
When this furnace is used with cooling units, it shall be
installed in parallel with, or on the upstream side of, cooling
units to avoid condensation in the heating compartment.
With a parallel flow arrangement, a damper (or other
means to control the flow of air) must adequately prevent
chilled air from entering the furnace (see figure 1). If the
damper is manually operated, it must be equipped to pre
vent operation of either the heating or the cooling unit, un
less it is in the full HEAT or COOL setting.
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GAS UNIT
Dampers
(open during heating
operation only)
AIR HANDLER
Dampers
(open during cooling
operation only)
FIGURE 1
When installed, this furnace must be electrically grounded
according to local codes. In addition, in the United States,
installation must conform with the current National Electric
Code, ANSI/NFPA No. 70. The National Electric Code
(ANSI/NFPA No. 70) is available from the following ad
dress:
National Fire Protection Association
1 Battery March Park
Quincy, MA 02269
NOTE - This furnace is designed for a minimum continuous
return air temperature of 60°F (16°C) or an intermittent op
eration down to 55°F (13°C) dry bulb for cases where a
night setback thermostat is used. Return air temperature
must not exceed 85°F (29°C) dry bulb.
The ML180DF furnace may be installed in alcoves, closets,
attics, basements, garages, and utility rooms in the down
flow position.
This furnace design has not been CSA International
certified for installation in mobile homes, recreational
vehicles, or outdoors.
Use of Furnace as Construction Heater
Lennox does not recommend the use of ML180DF units as
a construction heater during any phase of construction.
Very low return air temperatures, harmful vapors and op
eration of the unit with clogged or misplaced filters will dam
age the unit.
ML180DF units may be used for heating of buildings or
structures under construction, if the following conditions
are met:
D The vent system must be permanently installed per
these installation instructions.
D A room thermostat must control the furnace. The use of
fixed jumpers that will provide continuous heating is not
allowed.
D The return air duct must be provided and sealed to the
furnace.
D Return air temperature range between 60°F (16°C) and
80°F (27°C) must be maintained.
D Air filters must be installed in the system and must be
maintained during construction.
D Air filters must be replaced upon construction comple
tion.
D The input rate and temperature rise must be set per the
furnace rating plate.
D One hundred percent (100%) outdoor air must be pro
vided for combustion air requirements during construc
tion. Temporary ducting may supply outdoor air to the
furnace. Do not connect duct directly to the furnace.
Size the temporary duct following these instructions in
section for Combustion, Dilution and Ventilation Air in a
confined space with air from outside.
D The furnace heat exchanger, components, duct sys
tem, air filters and evaporator coils must be thoroughly
cleaned following final construction clean-up.
D All furnace operating conditions (including ignition, in
put rate, temperature rise and venting) must be verified
according to these installation instructions.
General
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
In addition to the requirements outlined previously, the fol
lowing general recommendations must be considered
when installing a ML180DF furnace:
D Place the furnace as close to the center of the air dis
tribution system as possible. The furnace should also be
located close to the chimney or vent termination point.
D Do not install the furnace where drafts might blow direct
ly into it. This could cause improper combustion and un
safe operation.
D Do not block the furnace combustion air openings with
clothing, boxes, doors, etc. Air is needed for proper
combustion and safe unit operation.
D When the furnace is installed in an attic or other insu
lated space, keep insulation away from the furnace.
NOTE - The Commonwealth of Massachusetts stipu
lates these additional requirements:
D Gas furnaces shall be installed by a licensed plumb
er or fitter only.
D The gas cock must be “T handle” type.
D When a furnace is installed in an attic, the passage
way to and service area surrounding the equipment
shall be floored.
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WARNING
The State of California has determined that this prod
uct may contain or produce a chemical or chemicals,
in very low doses, which may cause serious illness
or death. It may also cause cancer, birth defects or
reproductive harm.
Combustion, Dilution & Ventilation Air
In the past, there was no problem in bringing in sufficient
outdoor air for combustion. Infiltration provided all the air
that was needed. In today's homes, tight construction prac
tices make it necessary to bring in air from outside for
combustion. Take into account that exhaust fans, ap
pliance vents, chimneys, and fireplaces force additional air
that could be used for combustion out of the house. Unless
outside air is brought into the house for combustion, nega
tive pressure (outside pressure is greater than inside pres
sure) will build to the point that a downdraft can occur in the
furnace vent pipe or chimney. As a result, combustion
gases enter the living space creating a potentially danger
ous situation.
In the absence of local codes concerning air for combus
tion and ventilation, use the guidelines and procedures in
this section to install ML180DF furnaces to ensure efficient
and safe operation. You must consider combustion air
needs and requirements for exhaust vents. A portion of this
information has been reprinted with permission from the
National Fuel Gas Code (ANSI‐Z223.1). This reprinted ma
terial is not the complete and official position of the ANSI on
the referenced subject, which is represented only by the
standard in its entirety.
CAUTION
Do not install the furnace in a corrosive or contami
nated atmosphere. Meet all combustion and ventila
tion air requirements, as well as all local codes.
All gas‐fired appliances require air for the combustion pro
cess. If sufficient combustion air is not available, the fur
nace or other appliances will operate inefficiently and un
safely. Enough air must be provided to meet the needs of
all fuel‐burning appliances and appliances such as ex
haust fans which force air out of the house. When fire
places, exhaust fans, or clothes dryers are used at the
same time as the furnace, much more air is necessary to
ensure proper combustion and to prevent a downdraft. In
sufficient air causes incomplete combustion which can re
sult in carbon monoxide.
CAUTION
Insufficient combustion air can cause headaches,
nausea, dizziness or asphyxiation. It will also cause
excess water in the heat exchanger resulting in rust
ing and premature heat exchanger failure. Excessive
exposure to contaminated combustion air will result
in safety and performance related problems. Avoid
exposure to the following substances in the
combustion air supply:
Permanent wave solutions
Chlorinated waxes and cleaners
Chlorine base swimming pool chemicals
Water softening chemicals
De‐icing salts or chemicals
Carbon tetrachloride
Halogen type refrigerants
Cleaning solvents (such as perchloroethylene)
Printing inks, paint removers, varnishes, etc.
Hydrochloric acid
Cements and glues
Antistatic fabric softeners for clothes dryers
Masonry acid washing materials
In addition to providing combustion air, fresh outdoor air di
lutes contaminants in the indoor air. These contaminants
may include bleaches, adhesives, detergents, solvents
and other contaminants which can corrode furnace compo
nents.
The requirements for providing air for combustion and ven
tilation depend largely on whether the furnace is installed in
an unconfined or a confined space.
Unconfined Space
An unconfined space is an area such as a basement or
large equipment room with a volume greater than 50 cubic
feet (1.42 m
bined input rating of all appliances installed in that space.
This space also includes adjacent rooms which are not
separated by a door. Though an area may appear to be un
confined, it might be necessary to bring in outdoor air for
combustion if the structure does not provide enough air by
infiltration. If the furnace is located in a building of tight
construction with weather stripping and caulking around
the windows and doors, follow the procedures in the air
from outside section.
Confined Space
A confined space is an area with a volume less than 50 cu
bic feet (1.42 m
combined input rating of all appliances installed in that
space. This definition includes furnace closets or small
equipment rooms.
When the furnace is installed so that supply ducts carry air
circulated by the furnace to areas outside the space
containing the furnace, the return air must be handled by
ducts which are sealed to the furnace casing and which ter
minate outside the space containing the furnace. This is
3
) per 1,000 Btu (.29 kW) per hour of the com
3
) per 1,000 Btu (.29 kW) per hour of the
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especially important when the furnace is mounted on a
platform in a confined space such as a closet or small
equipment room. Even a small leak around the base of the
unit at the platform or at the return air duct connection can
cause a potentially dangerous negative pressure condi
tion. Air for combustion and ventilation can be brought into
the confined space either from inside the building or from
outside.
EQUIPMENT IN CONFINED
CHIMNEY
OR GAS
VENT
FURNACE
NOTE - Each opening shall have a free area of at least one square
inch (645 mm
ing of all equipment in the enclosure, but not less than 100 square
inches (64516 mm
2
) per 1,000 Btu (.29 kW) per hour of the total input rat
2
).
SPACE ALL AIR FROM INSIDE
WATER
HEATER
OPENINGS
(To Adjacent
Room)
FIGURE 2
Air from Inside
If the confined space that houses the furnace adjoins a
space categorized as unconfined, air can be brought in by
providing two permanent openings between the two
spaces. Each opening must have a minimum free area of 1
square inch (645 mm
2
) per 1,000 Btu (.29 kW) per hour of
total input rating of all gas-fired equipment in the confined
space. Each opening must be at least 100 square inches
(64516 mm
2
). One opening shall be within 12 inches (305
mm) of the top of the enclosure and one opening within 12
inches (305 mm) of the bottom. See figure 2.
Air from Outside
If air from outside is brought in for combustion and ventila
tion, the confined space must have two permanent open
ings. One opening shall be within 12 inches (305 mm) of the
top of the enclosure and one opening within 12 inches (305
mm) of the bottom. These openings must communicate di
rectly or by ducts with the outdoors or spaces (crawl or at
tic) that freely communicate with the outdoors or indirectly
through vertical ducts. Each opening shall have a minimum
free area of 1 square inch (645 mm
2
) per 4,000 Btu (1.17
kW) per hour of total input rating of all equipment in the en
closure. See figures 3 and 4. When communicating with
the outdoors through horizontal ducts, each opening shall
have a minimum free area of 1 square inch (645 mm
2
) per
2,000 Btu (.56 kW) per total input rating of all equipment in
the enclosure. See figure 5.
When ducts are used, they shall be of the same cross-sec
tional area as the free area of the openings to which they
connect. The minimum dimension of rectangular air ducts
shall be no less than 3 inches (75 mm). In calculating free
area, the blocking effect of louvers, grilles, or screens must
be considered. If the design and free area of protective cov
ering is not known for calculating the size opening required,
it may be assumed that wood louvers will have 20 to 25 per
cent free area and metal louvers and grilles will have 60 to
75 percent free area. Louvers and grilles must be fixed in
the open position or interlocked with the equipment so that
they are opened automatically during equipment opera
tion.
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CHIMNEY OR
GAS VENT
FURNACE
(Inlet Air from Crawlspace and Outlet Air to Ventilated Attic)
VENTILATION LOUVERS
(Each end of attic)
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM OUTSIDE
OUTLET
AIR
WATER
HEATER
CHIMNEY
OR GAS
VENT
FURNACE
VENTILATION
LOUVERS
(For unheated crawl space)
NOTE-The inlet and outlet air openings shall each have a free area of at least one square inch (645
2
mm
) per 4,000 Btu (1.17 kW) per hour of the total input rating of all equipment in the enclosure.
INLET
AIR
FIGURE 3
EQUIPMENT IN CONFINED SPACE
ALL AIR FROM OUTSIDE
(All Air Through Ventilated Attic)
VENTILATION LOUVERS
(Each end of attic)
OUTLET
AIR
INLET AIR
(Ends 12 in.
above bottom)
FURNACE
CHIMNEY
OR GAS
VENT
CONFINED SPACE
WATER
HEATER
OUTLET AIR
INLET AIR
EQUIPMENT IN
ALL AIR FROM
OUTSIDE
NOTE-The inlet and outlet air openings shall each have a
free area of at least one square inch (645 mm
Btu (1.17 kW) per hour of the total input rating of all equip
ment in the enclosure.
FIGURE 4
WATER
HEATER
2
) per 4,000
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NOTE - Each air duct opening shall have a free area of at least
one square inch (645 mm
2
) per 2,000 Btu (.59 kW) per hour of
the total input rating of all equipment in the enclosure. If the
equipment room is located against an outside wall and the air
openings communicate directly with the outdoors, each open
ing shall have a free area of at least one square inch (645 mm
per 4,000 Btu (1.17 kW) per hour of the total input rating of all
other equipment in the enclosure.
FIGURE 5
2
)
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Downflow Installation
Downflow unit installs in three ways: on non-combustible
flooring, on combustible flooring using a downflow com
bustible flooring base, or on a reverse-flow cooling cabinet.
Do not drag the unit across the floor.
Installation on Non-Combustible Flooring (Figure 6)
1 - Cut floor opening keeping in mind clearances listed on
unit rating plate. Also keep in mind gas supply connec
tions, electrical supply, flue and air intake connections
and sufficient installation and servicing clearances.
See table 1 for correct floor opening size.
2 - Flange warm air plenum and lower the plenum into the
opening.
3 - Set the unit over the plenum and seal the plenum to
the unit.
4 - Ensure that the seal is adequate.
CAUTION
The furnace and downflow combustible flooring
base shall not be installed directly on carpeting, tile,
or other combustible material other than wood floor
ing.
ML180DF UNIT
SUPPLY AIR PLENUM
PROPERLY
SIZED FLOOR
OPENING
DOWNFLOW
COMBUSTIBLE
FLOORING BASE
ML180DF UNIT
OPENING
SUPPLY AIR
PLENUM
FIGURE 6
TABLE 1
NON-COMBUSTIBLE FLOOR OPENING SIZE
Cabinet Width
A (14.5”) 19-3/4 502 13-1/4 337
B (17.5”) 19-3/4 502 16-1/4 413
C (21”) 19-3/4 502 19-3/4 502
NOTE - Floor opening dimensions listed are 1/4 inch (6 mm) larger than
the unit opening. See unit dimensions on page 2.
Front to Rear Side to Side
in. mm in. mm
Installation on Combustible Flooring (Figure 7)
1 - When unit is installed on a combustible floor, a down
flow combustible flooring base must be installed be
tween the furnace and the floor. The base must be or
dered separately.
See table 2 for opening size to cut in floor.
FIGURE 7
TABLE 2
COMBUSTIBLE FLOOR OPENING SIZE
Cabinet Width
A (14.5”)
B (17.5”)
C (21”)
Front to Rear Side to Side
in. mm in. mm
22 559 15-3/4 400
22 559 18-3/4 476
22 559 22-3/4 578
2 - After opening is cut, set combustible flooring base into
opening.
3 - Check sealing strips on combustible flooring base to
make sure they are properly glued and positioned.
4 - Lower supply air plenum into downflow combustible
flooring base until plenum flanges seal against the
strips.
NOTE - Be careful not to damage sealing strips. Check
for a tight seal.
5 - Set the furnace over the plenum.
6 - Ensure that the seal between the furnace and plenum
is adequate.
Installation on Cooling Cabinet (Figure 8)
1 - Refer to reverse-flow coil installation instructions for
correctly sized opening in floor and installation of cabi
net.
NOTE - Downflow combustible flooring kit is not used
2 - When cooling cabinet is in place, set and secure the
furnace according to the instructions that are provided
with the cooling coil. Secure the furnace to the cabinet.
3 - Seal the cabinet and check for air leaks.
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Setting Equipment
ML180DF UNIT
COOLING COIL
PROPERLY
SIZED FLOOR
OPENING
PLENUM
FIGURE 8
Return Air Opening -- Downflow Units
The following steps should be taken when installing ple
num:
1 - Bottom edge of plenum should be flanged with a
hemmed edge (See figure 9).
PLENUM
SECURE FROM
OUTSIDE CABINET
Side View
HEMMED EDGE
SEALING STRIP
(Field Provided)
CABINET SIDE PANEL
FIGURE 9
2 - Sealing strip should be used.
3 - In all cases, plenum should be secured to top flanges
of furnace with sheet metal screws.
WARNING
Do not install the furnace on its front, back or in the
horizontal position. See figure 12. Do not connect the
return air ducts to the back of the furnace. Doing so
will adversely affect the operation of the safety con
trol devices, which could result in personal injury or
death.
Install the ML180DF gas furnace as shipped in the down
flow position only. Do not install the furnace hori
zontally.
Select a location that allows for the required clearances
that are listed on the unit nameplate. Also consider gas
supply connections, electrical supply, vent connection, and
installation and service clearances [24 inches (610 mm) at
unit front]. The unit must be level.
NOTE - Units with 1/2 hp blower motors are equipped with
three flexible legs and one rigid leg. See figure 11. The rigid
leg is equipped with a shipping bolt and a flat white plastic
washer (rather than the rubber mounting grommet used
with a flexible mounting leg). The bolt and washer must
be removed before the furnace is placed into opera
tion. After the bolt and washer have been removed, the rig
id leg will not touch the blower housing.
UNITS WITH 1/2 HP BLOWER MOTOR
RIGID LEG
remove shipping bolt and washer
SECURE FROM IN
SIDE CABINET
FIBERGLASS
SEALING STRIP
(Field Provided)
PLENUM
HEMMED EDGE
CABINET SIDE
PANEL
FIGURE 10
4 - In closet installations, it may be impossible to install
sheet metal screws from the outside. In this case,
make plenum with a removable front and install
screws from the inside (See figure 10).
5 - Make certain that an adequate seal is made.
Side View
FIGURE 11
WARNING
The blower access panel must be securely in place
when the blower and burners are operating. Gas
fumes, which could contain carbon monoxide, can
be drawn into living space resulting in personal inju
ry or death.
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Front
Back
FIGURE 12
Downflow Application
Allow for clearances to combustible materials as indicated
on the unit nameplate. Minimum clearances for closet or al
cove installations are shown in figure 13.
Downflow Application Installation Clearances
Top
Horizontal
CAUTION
If this unit is being installed in a space serviced by
an exhaust fan, power exhaust fan, or other device
which may create a negative pressure in the space,
take care when sizing the inlet air opening. The in
let air opening must be sized to accommodate the
maximum volume of exhausted air as well as the
maximum volume of combustion air required for
all gas appliances serviced by this space.
Left Side
Bottom
Type of Vent
Connector
Top 1 in. (25 mm) 1 in. (25 mm)
*Front 2-1/4 in. (57 mm)** 2-1/4 in. (57 mm)
Back 0 0
Sides 0† 0
Vent 6 in. (152 mm) 1 in. (25 mm)
Floor NC†† NC††
*Front clearance in alcove installation must be 24 in. (610 mm).
Maintain a minimum of 24 in. (610 mm) for front service access.
** 4-1/2 in. if a single wall vent pipe is used.
†Left side requires 3 in. if a single wall vent is used on 14 -1/2 in.
cabinets, or 2 in. if a single wall vent is used on 17 - 1/2 in. cabinets.
††The furnace may be installed on a combustible wood floor if an
optional additive base is installed between the furnace and the
combustible floor.
Type C Type B1
Right Side
FIGURE 13
WARNING
Improper installation of the furnace can result in per
sonal injury or death. Combustion and flue products
must never be allowed to enter the return air system
or the living space. Use screws and joint tape to seal
the return air system to the furnace.
In platform installations with bottom return air, the
furnace should be sealed airtight to the return air ple
num. A door must never be used as a portion of the
return air duct system. The base must provide a
stable support and an airtight seal to the furnace. Al
low absolutely no sagging, cracks, gaps, etc.
The return and supply air duct systems must never
be connected to or from other heating devices such
as a fireplace or stove, etc. Fire, explosion, carbon
monoxide poisoning, personal injury and/or proper
ty damage could result.
Filters
This unit is not equipped with a filter or rack. A field-pro
vided high-velocity filter is required for the unit to operate
properly. Table 3 lists recommended filter sizes.
A filter must be in place any time the unit is operating.
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TABLE 3
Cabinet Width
A (14-1/2”) 14 x 25 x 1 (1)
B (17-1/2”) 16 x 25 x 1 (1)
C (21”) 20 x 25 x 1 (1)
Return Air Filter Size (inches)
Duct System
Use industry‐approved standards (such as those pub
lished by Air Conditioning Contractors of America or Ameri
can Society of Heating, Refrigerating and Air Conditioning
Engineers) to size and install the supply and return air duct
system. This will result in a quiet and low‐static system that
has uniform air distribution.
NOTE - Do not operate the furnace in the heating mode
with an external static pressure that exceeds 0.5 inches
w.c. Higher external static pressures may cause erratic lim
it operation.
Ensure that you have made a seal between the supply air
plenum and the furnace and between the furnace and the
return air plenum.
Return Air Plenum
The venting tables in this manual were extracted from the
National Fuel Gas Code (NFPA 54 / ANSI Z223.1) and are
provided as a guide for proper vent installation. Proper ap
plication, termination, construction and location of vents
must conform to local codes having jurisdiction. In the ab
sence of local codes, the NFGC serves as the defining doc
ument.
Refer to the tables and the venting information contained in
these instructions to properly size and install the venting
system.
IMPORTANT
Once the venting system is installed, attach the “Dis
connected Vent” warning sticker to a visible area of
the plenum near the vent pipe. The warning sticker
is provided in the bag assembly. Order kit 66W04 for
additional stickers.
WARNING
Asphyxiation hazard. The exhaust vent for this fur
nace must be securely connected to the furnace flue
transition at all times.
Return air must not be drawn from a room where this
furnace, or any other gas-fueled appliance (i.e., water
heater), or carbon monoxide-producing device (i.e.,
wood fireplace) is installed. When return air is drawn
from a room, a negative pressure is created in the room. If a
gas appliance is operating in a room with negative pres
sure, the flue products can be pulled back down the vent
pipe and into the room. This reverse flow of the flue gas
may result in incomplete combustion and the formation of
carbon monoxide gas. This toxic gas might then be dis
tributed throughout the house by the furnace duct system.
Venting
A 4-inch diameter flue transition is factory‐installed on all
models. Modifying or removing the flue transition will
cause the unit to operate unsafely and will void the unit
certification. The vent connector does not require insula
tion.
The ML180DF series units are classified as fan-assisted
Category I furnaces when vertically vented according to
the latest edition of National Fuel Gas Code (NFPA 54 /
ANSI Z223.1). A fan-assisted Category I furnace is an ap
pliance equipped with an integral mechanical means to ei
ther draw or force combustion products through the com
bustion chamber and/or heat exchanger.
NOTE - Use these instructions as a guide. They do not su
persede local codes. This furnace must be vented accord
ing to all local codes, these installation instructions, and the
provided venting tables in these instructions
VENT CONNECTION
FLUE TRANSITION
COLLAR
FIGURE 14
Use self-drilling sheet metal screws or a mechanical fas
tener to firmly secure the vent pipe to the round collar of the
flue transition. If self-drilling screws are used to attach the
vent pipe, it is recommended that three be used. Drive one
self-drilling screw through the front and one through each
side of the vent pipe and collar. See figure 14.
Install the first vent connector elbow at a minimum of six
inches (152 mm) from the furnace vent outlet.
Masonry chimneys used to vent Category I central fur
naces must be either tile‐lined or lined with a listed metal
lining system or dedicated gas vent. Unlined masonry
chimneys are prohibited. See figures 15 and 16 for com
mon venting.
“DISCONNECTED VENT”
WARNING
STICKER
Page 11
Page 12
Common Venting Using Tile-Lined Interior Masonry Chimney and Combined Vent Connector
MINIMUM LENGTH = AS SHORT AS PRACTICAL.
NOTE- Refer to provided venting
tables for installations.
FOR MAXIMUM LENGTH SEE NOTE TO LEFT
OTHER
APPLIANCE
FIGURE 15
Venting Using a Masonry Chimney
The following additional requirements apply when a lined
masonry chimney is used to vent this furnace.
A chimney with one or more sides exposed to the outside of
the structure is considered to be an exterior chimney.
An exterior masonry chimney that is not tile-lined must be
lined with B1 vent or a listed insulated flexible metal vent.
An exterior tile-lined chimney that is sealed and capped
may be lined with a listed uninsulated flexible metal vent.
If the existing chimney will not accommodate a listed metal
liner, either the chimney must be rebuilt to accommodate
one of these liners or an alternate approved venting meth
od must be found.
Insulation for the flexible vent pipe must be an encapsu
lated fiberglass sleeve recommended by the flexible vent
pipe manufacturer. See figure 16.
Common Venting Using Metal-Lined Masonry Chimney
SEALED
MAX. LENGTH
-- SEE NOTE
5 ft. (1.5 m)
minimum
VENT CONNECTOR
4 in. (102 mm)
minimum
OTHER
APPLIANCE
NOTE 1 - Refer to the provided venting tables for installations.
BELOW.
MIN. LENGTH -- AS
SHORT AS PRACTICAL
FURNACE
EXTERIOR
CHIMNEY WITH
METAL
LINER
PERMANENTLY
SEALED FIREPLACE
OPENING
FIGURE 16
INTERIOR TILE-LINED
MASONRY CHIMNEY
NOTE - the chimney must be properly
sized per provided venting tables or
lined with listed metal lining system.
VENT
CONNECTOR
FURNACE
PERMANENTLY
SEALED FIREPLACE
OPENING
DO NOT insulate the space between the liner and the
chimney wall with puffed mica or any other loose gran
ular insulating material
IMPORTANT
SINGLE appliance venting of a fan‐assisted furnace
into a tile‐lined masonry chimney (interior or outside
wall) is PROHIBITED. The chimney must first be lined
with either type B1 vent or an insulated single wall
flexible vent lining system which has been sized ac
cording to the provided venting tables and the vent
pipe manufacturer's instructions.
A fan-assisted furnace may be commonly vented into an
existing lined masonry chimney if the following conditions
are met:
S The chimney is currently serving at least one drafthood
equipped appliance
S The vent connectors and chimney are sized according
to the provided venting tables.
If type B1 double‐wall vent is used inside a chimney, no oth
er appliance can be vented into the chimney. The outer wall
of type B1 vent pipe must not be exposed to flue products.
A type B1 vent or masonry chimney liner shall terminate
above the roof surface with a listed cap or a listed roof as
sembly according to the terms of their respective listings
and the vent manufacturer's instructions.
When inspection reveals that an existing chimney is not
safe for the intended purpose, it shall be rebuilt to conform
to nationally recognized standards, lined or relined with
suitable materials, or replaced with a gas vent or chimney
suitable for venting ML180DF series units. The chimney
passageway must be checked periodically to ensure that it
is clear and free of obstructions.
Do not install a manual damper, barometric draft regulator,
or flue restrictor between the furnace and the chimney.
Page 12
Page 13
Never connect a Category I appliance to a chimney that is
servicing a solid-fuel appliance. If a fireplace chimney flue
is used to vent this appliance, the fireplace opening must
be permanently sealed.
A type B or listed chimney lining system that passes
through an unused masonry chimney flue is not considered
to be exposed to the outdoors.
General Venting Requirements
Vent all ML180DF furnaces according to these instruc
tions:
1 - Vent diameter recommendations and maximum allow
able piping runs are found in the provided venting
tables.
2 - In no case should the vent or vent connector diameter
be less than the diameter specified in the provided
venting tables.
3 - The minimum vent capacity determined by the sizing
tables must be less than the low fire input rating and the
maximum vent capacity must be greater than the high
fire input rating.
4 - Single appliance vents - If the vertical vent or tile‐lined
chimney has a larger diameter or flow area than the
vent connector, use the vertical vent diameter to de
termine the minimum vent capacity and the vent
connector diameter to determine the maximum vent
capacity. The flow area of the vertical vent, however,
shall not exceed 7 times the flow area of the listed ap
pliance categorized vent area, drafthood outlet area or
flue collar area unless designed according to approved
engineering methods.
5 - Multiple appliance vents - The flow area of the largest
section of vertical vent or chimney shall not exceed 7
times the smallest listed appliance categorized vent
area, drafthood outlet area or flue collar area unless
designed according to approved engineering meth
ods.
6 - The entire length of single wall metal vent connector
shall be readily accessible for inspection, cleaning,
and replacement.
7 - Single appliance venting configurations with zero lat
eral lengths (table 5) are assumed to have no elbows in
the vent system. For all other vent configurations, the
vent system is assumed to have two 90° elbows. For
each additional 90° elbow or equivalent (for example
two 45° elbows equal one 90° elbow) beyond two, the
maximum capacity listed in the venting table should be
reduced by 10% (0.90 x maximum listed capacity).
8 - The common venting tables (6 and 7) were generated
using a maximum horizontal vent connector length of
1-1/2 feet (.46 m) for each inch (25 mm) of connector
diameter as follows:
NOTE - Single Wall Vent Connectors may be used if the re
quirements set forth by the National Fuel Gas Code are
met. Please consult latest edition of NFPA 54/ANSI Z223.1
for proper sizing and application.
TABLE 4
Connector Diameter
inches (mm)
3 (76) 4-1/2 (1.37)
4 (102) 6 (1.83)
5 (127) 7-1/2 (2.29)
6 (152) 9 (2.74)
7 (178) 10-1/2 (3.20)
Maximum Horizontal
Connector Length feet (m)
9 - If the common vertical vent is offset, the maximum
common vent capacity listed in the common venting
tables should be reduced by 20%, the equivalent of two
90° elbows (0.80 x maximum common vent capacity).
The horizontal length of the offset shall not exceed
1‐1/2 feet (.46 m) for each inch (25 mm) of common
vent diameter.
10 - The vent pipe should be as short as possible with the
least number of elbows and angles required to com
plete the job. Route the vent connector to the vent us
ing the shortest possible route.
11 - A vent connector shall be supported without any dips
or sags and shall slope a minimum of 1/4 inch (6.4 mm)
per linear foot (305 mm) of connector, back toward the
appliance.
12 - Vent connectors shall be firmly attached to the furnace
flue collar by self-drilling screws or other approved
means, except vent connectors of listed type B vent
material which shall be assembled according to the
manufacturer's instructions. Joints between sections
of single wall connector piping shall be fastened by
screws or other approved means.
13 - When the vent connector used for Category I ap
pliances must be located in or pass through a crawl
space or other areas which may be cold, that portion of
the vent connector shall be constructed of listed
double‐wall type B vent material or material having
equivalent insulation qualities.
14 - All venting pipe passing through floors, walls, and ceil
ings must be installed with the listed clearance to com
bustible materials and be fire stopped according to lo
cal codes. In absence of local codes, refer to NFGC
(Z223.1).
15 - No portion of the venting system can extend into, or
pass through any circulation air duct or plenum.
16 - Vent connectors serving Category I appliances shall
not be connected to any portion of mechanical draft
systems operating under positive pressure such as
Category III or IV venting systems.
17 - If vent connectors are combined prior to entering the
common vent, the maximum common vent capacity
listed in the common venting tables must be reduced
by 10%, the equivalent of one 90° elbow (0.90 x maxi
mum common vent capacity).
18 - The common vent diameter must always be at least as
large as the largest vent connector diameter.
Page 13
Page 14
19 - In no case, shall the vent connector be sized more than
two consecutive table size diameters over the size of
the draft hood outlet or flue collar outlet.
20 - Do not install a manual damper, barometric draft regu
lator or flue restrictor between the furnace and the
chimney.
cated or common venting system, you must inspect the
venting system's general condition and look for signs
of corrosion. The existing vent pipe size must conform
to these instructions and the provided venting tables. If
the existing venting system does not meet these re
quirements, it must be resized.
21 - When connecting this appliance to an existing dedi
TABLE 5
Capacity of Type B Double-Wall Vents with Type B Double-Wall Connectors
Serving a Single Category I Appliance
Height
H
(feet)
6
8
10
15
20
30
Lateral
L
(feet)
0 0 78 0 152 0 251 0 375
2 13 51 18 97 27 157 32 232
4 21 49 30 94 39 153 50 227
6 25 46 36 91 47 149 59 223
0 0 84 0 165 0 276 0 415
2 12 57 16 109 25 178 28 263
5 23 53 32 103 42 171 53 255
8 28 49 39 98 51 164 64 247
0 0 88 0 175 0 295 0 447
2 12 61 17 118 23 194 26 289
5 23 57 32 113 41 187 52 280
10 30 51 41 104 54 176 67 267
0 0 94 0 191 0 327 0 502
2 11 69 15 136 20 226 22 339
5 22 65 30 130 39 219 49 330
10 29 59 40 121 51 206 64 315
15 35 53 48 112 61 195 76 301
0 0 97 0 202 0 349 0 540
2 10 75 14 149 18 250 20 377
5 21 71 29 143 38 242 47 367
10 28 64 38 133 50 229 62 351
15 34 58 46 124 59 217 73 337
20 48 52 55 116 69 206 84 322
0 0 100 0 213 0 374 0 587
2 9 81 13 166 14 283 18 432
5 21 77 28 160 36 275 45 421
10 27 70 37 150 48 262 59 405
15 33 64 44 141 57 249 70 389
20 56 58 53 132 66 237 80 374
30 NA NA 73 113 88 214 104 346
3 Inch 4 Inch 5 Inch 6 Inch
MIN MAX MIN MAX MIN MAX MIN MAX
NOTE - Single appliance venting configurations with zero lateral lengths are assumed to have no elbows in the vent system. For all other
vent configurations, the vent system is assumed to have two 90° elbows. For each additional 90° elbow or equivalent (for example two 45°
elbows equal one 90° elbow) beyond two, the maximum capacity listed in the venting table should be reduced by 10 percent (0.90 x maxi
mum listed capacity).
Vent and Connector Diameter - D (inches)
Appliance Input Rating in Thousands of Btu Per Hour
Page 14
Page 15
Vent
Height
H
(feet)
6
8
10
15
20
30
TABLE 6
Vent Connector Capacity
Type B Double-Wall Vents with Type B Double-Wall Connectors
Serving Two or More Category I Appliances
Connector
Rise
R
(feet)
1 22 37 35 66 46 106 58 164
2 23 41 37 75 48 121 60 183
3 24 44 38 81 49 132 62 199
1 22 40 35 72 49 114 64 176
2 23 44 36 80 51 128 66 195
3 24 47 37 87 53 139 67 210
1 22 43 34 78 49 123 65 189
2 23 47 36 86 51 136 67 206
3 24 50 37 92 52 146 69 220
1 21 50 33 89 47 142 64 220
2 22 53 35 96 49 153 66 235
3 24 55 36 102 51 163 68 248
1 21 54 33 99 46 157 62 246
2 22 57 34 105 48 167 64 259
3 23 60 35 110 50 176 66 271
1 20 62 31 113 45 181 60 288
2 21 64 33 118 47 190 62 299
3 22 66 34 123 48 198 64 309
3 Inch 4 Inch 5 Inch 6 Inch
MIN MAX MIN MAX MIN MAX MIN MAX
Vent and Connector Diameter - D (inches)
Appliance Input Rating in Thousands of Btu Per Hour
TABLE 7
Common Vent Capacity
Type B Double-Wall Vents with Type B Double-Wall Connectors
Serving Two or More Category I Appliances
Vent
Height
H
(feet)
6 92 81 140 116 204 161 309 248
8 101 90 155 129 224 178 339 275
10 110 97 169 141 243 194 367 299
15 125 112 195 164 283 228 427 352
20 136 123 215 183 314 255 475 394
30 152 138 244 210 361 297 547 459
FAN + FAN FAN + NAT FAN + FAN FAN + NAT FAN + FAN FAN + NAT FAN + FAN FAN + NAT
4 Inch 5 Inch 6 Inch 7 Inch
Appliance Input Rating in Thousands of Btu Per Hour
Common Vent Diameter - D (inches)
Page 15
Page 16
Removal of the Furnace from Common Vent
In the event that an existing furnace is removed from a
venting system commonly run with separate gas ap
pliances, the venting system is likely to be too large to prop
erly vent the remaining attached appliances.
Conduct the following test while each appliance is operat
ing and the other appliances (which are not operating) re
main connected to the common venting system. If the vent
ing system has been installed improperly, you must
correct the system as indicated in the general venting re
quirements section.
Resize the common venting system to the minimum
vent pipe size determined by using the appropriate
tables in Appendix G. (These are in the current stan
dards of the National Fuel Gas Code ANSI Z223.1.
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.
WARNING
CARBON MONOXIDE POISONING HAZARD
Failure to follow the steps outlined below for each
appliance connected to the venting system being
placed into operation could result in carbon mon
oxide poisoning or death.
The following steps shall be followed for each ap
pliance connected to the venting system being
placed into operation, while all other appliances
connected to the venting system are not in
operation:
1 - Seal any unused openings in the common venting sys
tem.
2 - Inspect the venting system for proper size and horizon
tal pitch. Determine that there is no blockage, restric
tion, leakage, corrosion, or other deficiencies which
could cause an unsafe condition.
3 - Close all building doors and windows and all doors be
tween the space in which the appliances remaining
connected to the common venting system are located
and other spaces of the building. Turn on clothes dry
ers and any appliances not connected to the common
venting system. Turn on any exhaust fans, such as
range hoods and bathroom exhausts, so they will oper
ate at maximum speed. Do not operate a summer ex
haust fan. Close fireplace dampers.
4 - Follow the lighting instructions. Turn on the appliance
that is being inspected. Adjust the thermostat so that
the appliance operates continuously.
5 - After the main burner has operated for 5 minutes, test
for leaks of flue gases at the draft hood relief opening.
Use the flame of a match or candle, or smoke from a
cigarette, cigar, or pipe.
6 - After determining that each appliance connected to the
common venting system is venting properly, (step 3)
return all doors, widows, exhaust fans, fireplace damp
ers, and any other gas-burning appliances to their pre
vious mode of operation.
7 - If a venting problem is found during any of the preced
ing tests, the common venting system must be modi
fied to correct the problem.
CAUTION
If a flexible gas connector is required or allowed by
the authority that has jurisdiction, black iron pipe
shall be installed at the gas valve and extend outside
the furnace cabinet. The flexible connector can then
be added between the black iron pipe and the gas
supply line.
Gas Supply
1 - This unit is shipped standard for left or right side instal
lation of gas piping. Connect the gas supply to the pip
ing assembly.
2 - When connecting the gas supply piping, consider fac
tors such as length of run, number of fittings, and fur
nace rating to avoid excessive pressure drop. Table 8
lists recommended pipe sizes for typical applications.
3 - The gas piping must not run in or through air ducts,
clothes chutes, gas vents or chimneys, dumb waiters,
or elevator shafts.
4 - The piping should be sloped 1/4 inch (6.4 mm) per 15
feet (4.57 m) upward toward the meter from the fur
nace. The piping must be supported at proper intervals
[every 8 to 10 feet (2.44 to 3.01 m)] with suitable hang
ers or straps. Install a drip leg inside vertical pipe runs
to the unit.
5 - A 1/8” N.P.T. plugged tap or pressure post is located
on the gas valve to facilitate test gauge connection.
See figure 22.
6 - In some localities, codes may require the installation of
a manual main shut‐off valve and union (furnished by
the installer) external to the unit. The union must be of
the ground joint type.
IMPORTANT
Compounds used on threaded joints of gas piping
must be resistant to the actions of liquified petro
leum gases.
NOTE - If emergency shutoff is necessary, shut off the
main manual gas valve and disconnect main power to the
furnace. The installer should properly label these devices.
Page 16
Page 17
TABLE 8
3
Gas Pipe Capacity - ft
Nominal
Iron Pipe
Size
inches
(mm)
1/2
(12.7)
3/4
(19.05)
1
(25.4)
1-1/4
(31.75)
1-1/2
(38.1)
2
(50.8)
2-1/2
(63.5)
3
(76.2)
NOTE - Capacity given in cubic feet (m3 ) of gas per hour and based on 0.60 specific gravity gas.
Internal
Diameter
inches
(mm)
.622
(17.799)
.824
(20.930)
1.049
(26.645)
1.380
(35.052)
1.610
(40.894)
2.067
(52.502)
2.469
(67.713)
3.068
(77.927)
10
(3.048)20(6.096)30(9.144)40(12.192)50(15.240)60(18.288)70(21.336)80(24.384)90(27.432)
172
(4.87)
360
(10.19)
678
(19.19)
1350
(38.22)
2090
(59.18)
4020
(113.83)
6400
(181.22)
11300
(319.98)
118
(3.34)
247
(7.00)
466
(13.19)
957
(27.09)
1430
(40.49)
2760
(78.15)
4400
(124.59)
7780
(220.30)
95
(2.69)
199
(5.63)
374
(10.59)
768
(22.25)
1150
(32.56)
2220
(62.86)
3530
(99.95)
6250
(176.98)
81
(2.29)
170
(4.81)
320
(9.06)
657
(18.60)
985
(27.89)
1900
(53.80)
3020
(85.51)
5350
(151.49)
/hr (m3/hr)
Length of Pipe - feet (m)
72
(2.03)
151
(4.28)
284
(8.04)
583
(16.50)
873
(24.72)
1680
(47.57)
2680
(75.88)
4740
(134.22)
65
(1.84)
137
(3.87)
257
(7.27)
528
(14.95)
791
(22.39)
1520
(43.04)
2480
(70.22)
4290
(121.47)
60
(1.69)
126
(3.56)
237
(6.71)
486
(13.76)
728
(20.61)
1400
(39.64)
2230
(63.14)
3950
(111.85)
56
(1.58)
117
(3.31)
220
(6.23)
452
(12.79)
677
(19.17)
1300
(36.81)
2080
(58.89)
3670
(103.92)
52
(1.47)
110
(3.11)
207
(5.86)
424
(12.00)
635
(17.98)
1220
(34.55)
1950
(55.22)
3450
(97.69)
100
(30.480)
50
(1.42)
104
(2.94)
195
(5.52)
400
(11.33)
600
(17.00)
1160
(32.844)
1840
(52.10)
3260
(92.31)
MANUAL
MAIN SHUT-OFF
VALVE
GROUND
JOINT
UNION
Left Side Piping
(Standard)
DRIP LEG
AUTOMATIC
GAS VALVE
AUTOMATIC
GAS VALVE
(with manual
shut-off valve)
FIELD
PROVIDED
AND INSTALLED
NOTE - BLACK IRON PIPE ONLY TO BE ROUTED INSIDE OF CABINET
(with manual
shut-off valve)
FIGURE 17
MANUAL
MAIN SHUT-OFF
VALVE
GROUND
JOINT
UNION
DRIP LEG
Right Side Piping
(Alternate)
Page 17
Page 18
Leak Check
After gas piping is completed, carefully check all piping
connections (factory- and field-installed) for gas leaks. Use
a leak detecting solution or other preferred means.
NOTE - If emergency shutoff is necessary, shut off the
main manual gas valve and disconnect the main power to
the furnace. The installer should properly label these de
vices.
CAUTION
Some soaps used for leak detection are corrosive to
certain metals. Carefully rinse piping thoroughly af
ter leak test has been completed. Do not use
matches, candles, flame or other sources of ignition
to check for gas leaks.
The furnace must be isolated from the gas supply system
by closing its individual manual shut‐off valve during any
pressure testing of the gas supply system at pressures
greater than or equal to 1/2 psig (3.48 kPa, 14 inches w.c.).
The unit is equipped with a field make-up box on the left
hand side of the cabinet. The make-up box may be moved
to the right side of the furnace to facilitate installation. If the
make-up box is moved to the right side, clip the wire ties
that bundle the wires together. The excess wire must be
pulled into the blower compartment. Secure the excess
wire to the existing harness to protect it from damage.
INTERIOR MAKE-UP BOX INSTALLATION
(Right Side)
MAKE-UP
BOX
IMPORTANT
When testing pressure of gas lines, gas valve must
be disconnected and isolated. See figure 18. Gas
valves can be damaged if subjected to pressures
greater than 1/2 psig (3.48 kPa, 14 inches w.c.).
MANUAL MAIN
SHUT-OFF VALVE
WILL NOT HOLD
NORMAL TEST
PRESSURE
1/8 NPT PLUG
CAP
FIGURE 18
Electrical
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
ISOLATE
GAS VALVE
FURNACE
FIGURE 19
Refer to figure 20 for schematic wiring diagram and trou
bleshooting.
The power supply wiring must meet Class I restrictions.
Protected by either a fuse or circuit breaker, select circuit
protection and wire size according to unit nameplate.
CAUTION
Failure to use properly sized wiring and circuit
breaker may result in property damage. Size wiring
and circuit breaker(s) per Product Specifications
bulletin (EHB) and unit rating plate.
NOTE - Unit nameplate states maximum current draw.
Maximum over-current protection allowed is 15 AMP.
Holes are on both sides of the furnace cabinet to facilitate
wiring.
Install a separate (properly sized) disconnect switch near
the furnace so that power can be turned off for servicing.
Before connecting the thermostat, check to make sure the
wires will be long enough for servicing at a later date. Make
sure that thermostat wire is long enough to facilitate future
removal of blower for service.
CAUTION
Electrostatic discharge can affect elec
tronic components. Take precautions
to neutralize electrostatic charge by
touching your hand and tools to metal
prior to handling the control.
WARNING
Fire Hazard. Use of aluminum wire with this product
may result in a fire, causing property damage, severe
injury or death. Use copper wire only with this
product.
Page 18
Page 19
Complete the wiring connections to the equipment. Use the
provided unit wiring diagram. Use 18-gauge wire or larger
that is suitable for Class II rating for thermostat connec
tions.
Electrically ground the unit according to local codes or, in
the absence of local codes, according to the current Na
tional Electric Code (ANSI/NFPA No. 70). A green ground
wire is provided in the field make-up box.
NOTE - The ML180DF furnace contains electronic compo
nents that are polarity sensitive. Make sure that the furnace
is wired correctly and is properly grounded.
WARNING
Electric Shock Hazard. Can cause
injury or death. Unit must be properly
grounded in accordance with national
and local codes.
Accessory Terminals
One line voltage “EAC” 1/4” spade terminal is provided on
the furnace integrated control. See figure 21 for integrated
control configuration. This terminal is energized when the
indoor blower is operating. Any accessory rated up to one
amp can be connected to this terminal with the neutral leg
of the circuit being connected to one of the provided neutral
terminals. If an accessory rated at greater than one amp is
connected to this terminal, it is necessary to use an exter
nal relay.
One line voltage “HUM” 1/4” spade terminal is provided on
the furnace integrated control. See figure 21 for integrated
control configuration. This terminal is energized in the
heating mode when the combustion air inducer is operat
ing. Any humidifier rated up to one amp can be connected
to this terminal with the neutral leg of the circuit being con
nected to one of the provided neutral terminals. If a humidi
fier rated at greater than one amp is connected to this termi
nal, it is necessary to use an external relay relay.
Generator Use - Voltage Requirements
The following requirements must be kept in mind when
specifying a generator for use with this equipment:
S The furnace requires 120 volts +
volts to 132 volts).
S The furnace operates at 60 Hz +
63 Hz).
S The furnace integrated control requires both polarity
and proper ground. Both polarity and proper grounding
should be checked before attempting to operate the
furnace on either permanent or temporary power.
S Generator should have a wave form distortion of less
than 5% THD (total harmonic distortion).
Thermostat
Install the room thermostat according to the instructions
provided with the thermostat. See figure 20 for thermostat
designations. If the furnace is being matched with a heat
pump, refer to the FM21 installation instruction or appropri
ate dual fuel thermostat instructions.
Indoor Blower Speeds
1 - When the thermostat is set to “FAN ON,” the indoor
blower will run continuously on the heating speed
when there is no cooling or heating demand.
2 - When the ML180DF is running in the heating mode,
the indoor blower will run on the heating speed.
3 - When there is a cooling demand, the indoor blower will
run on the cooling speed.
10% (Range: 108
5% (Range: 57 Hz to
Page 19
Page 20
ML180DF Schematic Wiring Diagram
FIGURE 20
Page 20
Page 21
Integrated Control
LED
PUSH BUTTON
TWIN
INTEGRATED CONTROL
(Automatic Hot Surface Ignition System)
TERMINAL DESIGNATIONS
HUM
LINE
XFMR
EAC
COOL
HEAT
PARK
CONT
NEUTRALS
TWIN
Humidifier (120VAC)
Input (120VAC)
Transformer (120VAC)
Indoor Air Qality Accessory Air Cleaner (120VAC)
Blower - Cooling Speed (120VAC)
Blower - Heating Speed (120VAC)
Dead terminals to park alternate spd taps
Continuous blower
Neutral terminals (120VAC)
Twinning Terminal (24VAC)
BLOWER OFF
DELAY JUMPER
Integrated Control Diagnostic Codes
DIAGNOSTIC CODES
LED Status DESCRIPTION
LED Off
LED On Normal operation.
1 Flash
2 Flashes
3 Flashes
4 Flashes Primary limit switch open.
5 Flashes Rollout switch open.
6 Flashes Pressure switch cycle lockout.
7 Flashes Lockout, burners fail to light.
8 Flashes
9 Flashes Line voltage polarity incorrect.
No power to control or control harware
fault detected.
Flame present with gas vavle
de-energized.
Pressure switch closed with combustion
air inducer de-energized.
Pressure switch open with combus
tion air inducer energized.
Lockout, buners lost flame too many
times.
FIGURE 21
Note - This control is equipped with a push button switch for
diagnostic code recall. The control stores the last 5 fault
codes in non-volatile memory. The most recent fault code
is flashed first, the oldest fault code is flashed last. There is
a 2 second pause between codes. When the push button
switch is pressed for less than 5 seconds, the control will
flash the stored fault codes when the switch is released.
The fault code history may be cleared by pressing the push
button switch for more than 5 seconds.
Page 21
Page 22
Unit Start-Up
FOR YOUR SAFETY READ BEFORE LIGHTING
WARNING
Do not use this furnace if any part has been underwa
ter. A flood-damaged furnace is extremely danger
ous. Attempts to use the furnace can result in fire or
explosion. Immediately call a qualified service tech
nician to inspect the furnace and to replace all gas
controls, control system parts, and electrical parts
that have been wet or to replace the furnace, if
deemed necessary.
WARNING
If overheating occurs or if gas supply fails to shut off,
shut off the manual gas valve to the appliance before
shutting off electrical supply.
CAUTION
Before attempting to perform any service or mainte
nance, turn the electrical power to unit OFF at dis
connect switch.
BEFORE LIGHTING smell all around the appliance area
for gas. Be sure to smell next to the floor because some gas
is heavier than air and will settle on the floor.
The gas valve on the ML180DF unit is equipped with a gas
control switch. Use only your hand to move the switch. Nev
er use tools. If the switch will not turn or if the control switch
will not move by hand, do not try to repair it.
Placing the furnace into operation:
ML180DF units are equipped with an automatic ignition
system. Do not attempt to manually light burners on these
furnaces. Each time the thermostat calls for heat, the burn
ers will automatically light. The ignitor does not get hot
when there is no call for heat on units with an automatic
ignition system.
WARNING
If you do not follow these instructions exactly, a fire
or explosion may result causing property damage,
personal injury or death.
Gas Valve Operation (Figure 22)
1 - STOP! Read the safety information at the beginning of
this section.
2 - Set the thermostat to the lowest setting.
3 - Turn off all electrical power to the unit.
4 - This furnace is equipped with an ignition device which
automatically lights the burners. Do not try to light the
burners by hand.
5 - Remove the access panel.
6 - Move switch on gas valve to OFF. Do not force. See
figure 22.
7 - Wait five minutes to clear out any gas. If you then smell
gas, STOP! Immediately call your gas supplier from a
neighbor's phone. Follow the gas supplier's instruc
tions. If you do not smell gas go to next step.
MANIFOLD
PRESSURE
ADJUSTMENT
SCREW
INLET
PRESSURE
PORT
8 - Move switch on gas valve to ON. Do not force. See fig
ure 22.
9 - Replace the access panel.
10- Turn on all electrical power to to the unit.
11- Set the thermostat to desired setting.
NOTE - When unit is initially started, steps 1 through 11
may need to be repeated to purge air from gas line.
12- If the appliance will not operate, follow the instructions
“Turning Off Gas to Unit” and call your service techni
cian or gas supplier.
Turning Off Gas to Unit
1 - Set the thermostat to the lowest setting.
2 - Turn off all electrical power to the unit if service is to be
performed.
3 - Remove the access panel.
4 - Move switch on gas valve to OFF. Do not force.
5 - Replace the upper access panel.
Failure To Operate
If the unit fails to operate, check the following:
1 - Is the thermostat calling for heat?
2 - Are access panels securely in place?
3 - Is the main disconnect switch closed?
4 - Is there a blown fuse or tripped circuit breaker?
5 - Is the filter dirty or plugged? Dirty or plugged filters will
cause the limit control to shut the unit off.
6 - Is gas turned on at the meter?
7 - Is the manual main shut‐off valve open?
8 - Is the internal manual shut‐off valve open?
9 - Is the unit ignition system in lock out? If the unit locks
out again, call the service technician to inspect the unit
for blockages.
10 - Is pressure switch closed? Obstructed flue will cause
unit to shut off at pressure switch. Check flue and outlet
for blockages.
GAS VALVE SHOWN IN ON POSITION
FIGURE 22
MANIFOLD
PRESSURE
OUTLET
PORT
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Page 23
11 - Are flame rollout switches tripped? If flame rollout
switches are tripped, call the service technician for in
spection.
Heating Sequence Of Operation
1 - When thermostat calls for heat, combustion air blower
starts.
2 - Combustion air pressure switch proves blower opera
tion. Switch is factory-set and requires no adjustment.
3 - After a 15-second prepurge, the hot surface ignitor en
ergizes.
4 - After a 20-second ignitor warm-up period, the gas
valve solenoid opens. A 4-second trial for ignition peri
od begins.
5 - Gas is ignited, flame sensor proves the flame, and the
combustion process continues.
6 - If flame is not detected after first ignition trial, the igni
tion control will repeat steps 3 and 4 four more times
before locking out the gas valve (”WATCHGUARD”
flame failure mode).The ignition control will then auto
matically repeat steps 1 through 6 after a 60 minute.
7 - To interrupt the 60-minute (“WATCHGUARD”) period,
move thermostat from “Heat” to “OFF” then back to
“Heat.” Heating sequence then restarts at step 1.
Gas Pressure Adjustment
Gas Flow (Approximate)
TABLE 9
GAS METER CLOCKING CHART
Seconds for One Revolution
ML180DF Unit
-045 80 160 200 400
-070 55 110 136 272
-090 41 82 102 204
-110 33 66 82 164
-135 27 54 68 136
Natural-1000 btu/cu ft LP-2500 btu/cu ft
Natural LP
1 cu ft
Dial
2 cu ft
Dial
1 cu ft
Dial
2 cu ft
DIAL
Supply Pressure Measurement
A threaded plug on the inlet side of the gas valve provides
access to the supply pressure tap. Remove the threaded
plug, install a field-provided barbed fitting and connect a
manometer to measure supply pressure. Furnace should
operate at least 5 minutes before checking supply pres
sure. See table 11 for proper supply pressure. Replace the
threaded plug after measurements have been taken.
Manifold Pressure Measurement
1 - Remove the threaded plug from the outlet side of the
gas valve and install a field-provided barbed fitting.
Connect to a manometer to measure manifold pres
sure.
2 - Start unit and allow 5 minutes for unit to reach steady
state.
3 - While waiting for the unit to stabilize, observe the
flame. Flame should be stable and should not lift from
burner. Natural gas should burn blue.
4 - After allowing unit to stabilize for 5 minutes, record
manifold pressure and compare to value given in table
11.
6 - Shut unit off and remove manometer as soon as an ac
curate reading has been obtained. Take care to re
place pressure tap plug.
7 - Start unit and perform leak check. Seal leaks if found.
Proper Combustion
Furnace should operate a minimum 15 minutes with cor
rect manifold pressure and gas flow rate before checking
combustion. Take combustion sample beyond the flue out
let and compare to the tables below. The maximum car
bon monoxide reading should not exceed 50 ppm.
TABLE 10
ML180DF Unit
-045
-070
-090
-110
CO2%
For Nat
6.0 - 7.5 6.9 - 8.5
CO2%
For
L.P.
High Altitude
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
9 below. If manifold pressure matches table 11 and rate is
incorrect, check gas orifices for proper size and restriction.
Remove temporary gas meter if installed.
NOTE - To obtain accurate reading, shut off all other gas
appliances connected to meter.
The manifold pressure may require adjustment and com
bustion air pressure switch may need replacing to ensure
proper combustion at higher altitudes. Refer to table 11 for
manifold pressure and table 12 for pressure switch change
and gas conversion kits.
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.
Page 23
Page 24
TABLE 11
Manifold Pressure Settings at all Altitudes
Model
Input Size
045
070
090
110
Model
Input Size
045
070 80W52
090 No Change
090(X)
110
High Altitude Pressure Switch Kit
0-4500 ft 4501-7500 ft 7501 - 10,000 ft 7501 - 10,000 ft 0 - 7500 ft 7501 - 10,000 ft 0 - 7500 ft
No Change
LP/propane 10.0 10.0 10.0 11.0 13.0
LP/propane 10.0 10.0 10.0 11.0 13.0
LP/propane 10.0 10.0 10.0 11.0 13.0
LP/propane 10.0 10.0 10.0 11.0 13.0
No Change
80W52
Supply Pressure
Gas 0-4500 ft 4501-7500 ft 7501 - 10,000 ft
Nat 3.5 3.5 3.5 4.5 13.0
Nat 3.5 3.3 3.5 4.5 13.0
Nat 3.5 3.3 3.5 4.5 13.0
Nat 3.5 3.3 3.5 4.5 13.0
in.wg.
Min Max
TABLE 12
Pressure Switch and Gas Conversion Kits at all Altitudes
High Altitude
Natural Gas Burner
Orifice Kit
80W51 73W37 11K49 11K44 73W81
Natural Gas to LP/Propane
Gas Change Over Kit
LP/Propane to
Natural Gas
Change Over Kit
NOTE - A natural to L.P. propane gas changeover kit is necessary to convert this unit. Refer to the changeover kit installation instruction for the conversion
procedure.
Other Unit Adjustments
Temperature Rise
After the furnace has been started, and supply and return
air temperatures have been allowed to stabilize, check the
NOTE - Do not secure the electrical conduit directly to the
air ducts or structure.
Primary and Secondary Limits
The primary limit is located on the heating compartment
vestibule panel. The secondary limits (if equipped) are lo
cated in the blower compartment, attached to the back side
of the blower. These auto reset limits are factory-set and
require no adjustment.
Flame Rollout Switch
The manually reset switch is located on the front of the
burner box.
Pressure Switch
The pressure switch is located in the heating compartment
adjacent to the combustion air inducer. The switch checks
for proper combustion air inducer operation before allow
ing ignition trial. The switch is factory-set and requires no
adjustment.
temperature rise. If necessary, adjust the blower speed to
maintain the temperature rise within the range shown on
the unit nameplate. Increase the blower speed to decrease
the temperature. Decrease the blower speed to increase
the temperature rise. Failure to adjust the temperature rise
may cause erratic limit operation.
Fan Control
The heat fan-on time of 30 seconds is not adjustable. The
heat 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
delay setting of 120 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-energized. Longer off delay settings provide lower re
turn air temperatures; shorter settings provide higher re
turn air temperatures. See figure 23.
Page 24
Page 25
HEAT FAN‐OFF TIME IN SECONDS
NO JUMPER
To adjust fan-off timing, reposition jumper across pins to
60 Second
off Time
60
90
120
180
achieve desired setting.
90 Second
off Time
60
90
120
180
120 Second
off Time
60
90
120
180
180 Second
off Time
60
90
120
180
FIGURE 23
Blower Speeds
Follow the steps below to change the blower speeds.
1 - Turn off electrical power to furnace.
2 - Remove blower access panel.
3 - Disconnect existing speed tap at integrated control
speed terminal.
See unit Product Specifications Manual for indoor blower
data.
NOTE - Termination of any unused motor leads must be in
sulated.
4 - Place unused blower speed tap on integrated control
“PARK” terminal or insulate.
5 - Refer to blower speed selection chart on unit wiring
diagram for desired heating or cooling speed. See
Blower performance data beginning on the next page.
6 - Connect selected speed tap at integrated control
speed terminal.
7 - Resecure blower access panel.
8 - Turn on electrical power to furnace.
9 - Recheck temperature rise.
Twinning 2 ML180DF Furnaces
The control board in this furnace is equipped with a provi
sion to ”twin” (interconnect) two(2) adjacent furnaces with a
common plenum such that they operate as one (1) large
unit.
When twinned, the circulating blower speeds are synchro
nized between the furnaces. If either furnace has a need to
run the blower, both furnaces will run the blower on the
same speed. The cooling speed has highest priority, fol
lowed by heating speed and fan speed.
Field installation of twinning consists of connecting wires
between the ”C” and ”Twin” terminals of the two controls.
The 24 VAC secondary of the two systems must be in
phase. All thermostat connections are made to one control
only. Figure 24 show wiring for two-stage and single stage
thermostats.
The twinned furnace without thermostat connections is to
have the call for heat supplied by an external 24VAC isola
tion relay to prevent its rollout switch from being bypassed
by the other twinned furnace. The coil of the isolation relay
connects from the thermostat ”W” to 24 VAC common. The
contacts of the relay connect ”R” to ”W” on the non-thermo
stat twin.
Page 25
Page 26
FIELD WIRING FOR TWINNING THE ML180DF
TWO-STAGE THERMOSTAT
R
Call For Cool
Call For Fan
Call For 1st
Stage Heat
Call For 2nd
StageHeat
Y
G
W1
W2
ISOLATION
RELAY
TWIN
TWIN
SINGLE STAGE THERMOSTAT
R
TWIN 1
Y
G
W
C
R
TWIN 2
Y
G
W
Call For Cool
Call For Fan
Call For Heat
R
Y
G
W
ISOLATION
RELAY
TWIN
TWIN
R
TWIN 1
Y
G
W
C
R
TWIN 2
Y
G
W
C
FIGURE 24
Service
WARNING
ELECTRICAL SHOCK, FIRE,
OR EXPLOSION HAZARD.
Failure to follow safety warnings exactly could result
in dangerous operation, serious injury, death or
property damage.
Improper servicing could result in dangerous opera
tion, serious injury, death, or property damage.
Before servicing, disconnect all electrical power to
furnace.
When servicing controls, label all wires prior to dis
connecting. Take care to reconnect wires correctly.
Verify proper operation after servicing.
WARNING
The blower access panel must be securely in place
when the blower and burners are operating. Gas
fumes, which could contain carbon monoxide, can
be drawn into living space resulting in personal inju
ry or death.
C
Annual Furnace Maintenance
At the beginning of each heating season, and to comply
with the Lennox Limited Warranty, your system should
be checked by a licensed professional technician (or equiv
alent) as follows:
1- Check wiring for loose connections, voltage at indoor
unit and amperage of indoor motor
.
2- Check the condition of the belt and shaft bearings if ap
plicable.
3- Inspect all gas pipe and connections for leaks
.
4- Check the cleanliness of filters and change if necessary
(monthly).
5- Check the condition and cleanliness of burners and
heat exchanger and clean if necessary.
6- Check the cleanliness of blower assembly and clean
the housing, blower wheel and blower motor if neces
sary
. The blower motors are prelubricated for extended
bearing life. No further lubrication is needed.
7- Inspect the combustion air inducer and clean if neces
sary.
8- Evaluate the heat exchanger integrity by inspecting the
heat exchanger per the AHRI heat exchanger inspec
Page 26
Page 27
tion procedure. This procedure can be viewed at
www.ahrinet.org
9- Ensure sufficient combustion air is available to the fur
nace. Fresh air grilles and louvers (on the unit and in the
room where the furnace is installed) must be properly
sized, open and unobstructed to provide combustion
air
.
10 Inspect the furnace venting system to make sure it is in
place, structurally sound, and without holes, corrosion,
or blockage. Vent system must be free and clear of ob
structions and must slope upward away from the fur
nace
. Vent system should be installed per the National
Fuel Gas Code
11 Inspect the furnace return air duct connection to ensure
the duct is sealed to the furnace. Check for air leaks on
supply and return ducts and seal where necessary
12 Check the condition of the furnace cabinet insulation
and repair if necessary.
13 Perform a complete combustion analysis during the fur
nace inspection to ensure proper combustion and oper
ation. Consult Service Literature for proper combustion
values.
.
manifold pressure adjustment is necessary, consult
the Service Literature for unit specific information on
adjusting gas pressure. Not all gas valves are ad
justable. Verify correct temperature rise.
Cleaning the Burners
NOTE - Use papers or protective covering in front of the fur
nace during cleaning.
1 - Turn off both electrical and gas power supplies to fur
nace.
3 - Label the wires from gas valve, rollout switches, prima
ry limit switch and make-up box then disconnect them.
4 - Disconnect gas supply piping. Remove the four screws
securing the burner manifold assembly to the vestibule
panel and remove the assembly from the unit.
5 - Remove 4 screws securing burner box and remove
burner box.
6- To clean burners, run a vacuum cleaner with a soft
brush attachment over the face of burners. Visually in
spect inside the burners and crossovers for any block
age caused by foreign matter. Remove any blockage.
Figure 25 shows burner detail.
7- Reinstall burner box, manifold assembly and burner
box cover.
8- Reconnect all wires.
9- Reconnect gas supply piping.
14 Verify operation of CO detectors and replace batteries
as required.
Perform a general system test. Turn on the furnace to
check operating functions such as the start-up and shut-off
operation.
1 - Check the operation of the ignition system, inspect
and clean flame sensor. Check microamps before and
after. Check controls and safety devices (gas valve,
flame sensor, temperature limits). Consult Service
Manual for proper operating range. Thermal Limits
should be checked by restricting airflow and not dis
connecting the indoor blower. For additional details,
please see Service and Application Note H049.
2 - Verify that system total static pressure and airflow set
tings are within specific operating parameters.
3 - Clock gas meter to ensure that the unit is operating at
the specified firing rate. Check the supply pressure
and the manifold pressure. On two-stage gas furnaces
check the manifold pressure on high fire and low fire. If
10- Turn on power and gas supply to unit.
11- Set thermostat and check for proper operation.
12- Check all piping connections, factory and field, for gas
leaks. Use a leak detecting solution or other preferred
means.
CAUTION
Some soaps used for leak detection are corrosive to
certain metals. Carefully rinse piping thoroughly af
ter leak test has been completed. Do not use
matches, candles, flame or other sources of ignition
to check for gas leaks.
13- If a leak is detected, shut gas and electricity off and re
pair leak.
14- Repeat steps 12 and 13 until no leaks are detected.
15- Replace access panel.
Page 27
Page 28
Ignitor
Rollout Switch
Sensor
ML180DF BURNER ASSEMBLY
Retention Rings
Cross Over
Gas Valve
Screws (4)
FIGURE 25
Repair Parts List
The following repair parts are available through independent Lennox dealers. When ordering parts, include the complete
furnace model number listed on the CSA International nameplate -- Example: ML180DF110P60C-01. All service must be
performed by a licensed professional installer (or equivalent), service agency, or gas supplier.
Cabinet Parts
Upper access panel
Blower panel
Top cap
Control Panel Parts
Transformer
Integrated control
Door interlock switch
Circuit breaker
Blower Parts
Blower wheel
Blower housing
Motor
Motor mounting frame
Motor capacitor
Blower housing cutoff plate
Heating Parts
Flame sensor
Heat exchanger assembly
Gas manifold
Combustion air inducer
Gas valve
Main burner cluster
Main burner orifices
Pressure switch
Ignitor
Primary limit control
Flame rollout switch (s)
Secondary limit
Page 28
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