Lennox ML180DF045P24A, ML180DF090P36B, ML180DF045P36A, ML180DF110P60C, ML180DF090P48B Installation Instructions Manual

© 2017 Lennox Industries Inc.

DOWNFLOW

Dallas, Texas USA

INSTALLATION
INSTRUCTIONS
ML180DF

MERIT® SERIES GAS FURNACE
DOWNFLOW AIRDISCHARGE

507326-02
11/2017
Supersedes 05/2016

THIS MANUAL MUST BE LEFT WITH THE

HOMEOWNER FOR FUTURE REFERENCE

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

CAUTION

As with any mechanical equipment, contact with sharp
sheet metal edges can result in personal injury. Take
care while handling this equipment and wear gloves and
protective clothing.

AIR FLOW

Contents

Unit Dimensions .............................................................2

ML180DF Gas Furnace .................................................. 3

Shipping and Packing List ..............................................3

Safety Information ..........................................................3

Use of Furnace as Construction Heater .........................4

General ........................................................................... 4

Combustion, Dilution & Ventilation Air ............................5

Setting Equipment .........................................................9

Filters ............................................................................ 10

Duct System ................................................................. 11

Venting..........................................................................11

WARNING

Improper installation, adjustment, alteration, service
or maintenance can cause property damage, personal
injury or loss of life. Installation and service must be
performed by a licensed professional HVAC installer or
equivalent, service agency, or the gas supplier.

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

Repait Parts .................................................................. 28

Page 1

Unit Dimensions - inches (mm)

A

B

AIR FLOW

9/16

(14)

FLUE OUTLET

(Top)

3/4 (19)

Front Panel

3-1/8

(79)

D

9/16

(14)

RETURN AIR

OPENING

B

19-7/16

(494)

9/16

(14)

TOP VIEW

27-3/4

(705)

19-7/16

(494)

9/16

(14)

9/16
(14)

ELECTRICAL INLET

(Either Side)

GAS PIPING INLET

(Either Side)

19-1/4

Supply

(489)

Air

3/4

(19)

C

Supply

Air

33

(838)

5-3/8 (137) Right
2-3/16 (56) Left

6-5/8 (168) Right
7-1/8 (181) Left

3/4

(19)

FRONT VIEW SIDE VIEW

Model No.

A B C D

in mm in mm in mm in mm

ML180DF045P24A

ML180DF045P36A

14-1/2 368 13-3/8 340 13 330 4-3/4 121

ML180DF070P36A

ML180DF090P36B

ML180DF090P48B

ML180DF110P60C 21 533 19-7/8 504 19-1/2 495 8 203

17-1/2 446 16-3/8 416 16 406 6-1/4

3/4

(19)

159

Page 2

ML180DF Gas Furnace

The ML180DF gas furnace is shipped ready for installa­tion in the downow position fueled by natural gas. A con­version kit (ordered separately) is required for use in LP/
Propane 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 ooring base

1 - High altitude kit

Check equipment for shipping damage. If you nd 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 oor and may be
difcult 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, personal
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, contact with sharp
sheet metal edges can result in personal injury. Take
care while handling this equipment and wear gloves
and protective clothing.

Certications

ML180DF units are CSA International certied.

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 open­ings 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 certied for installation

clearances to combustible material as listed on the unit

nameplate and in the tables in gure 13. Accessibility and
service clearances must take precedence over re 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

oor. 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 atmosphere, the furnace must be installed ac­cording to recommended good practice requirements and
current National Fuel Gas Code.

NOTE - Furnace must be adjusted to obtain a temperature

rise within the range(s) specied 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 electrical
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 ow arrangement, a damper (or other
means to control the ow of air) must adequately prevent
chilled air from entering the furnace (see gure 1). If the

damper is manually operated, it must be equipped to pre­vent operation of either the heating or the cooling.

Page 3

• The return air duct must be provided and sealed

to the furnace.

• Return air temperature range between 60°F (16°C)

and 80°F (27°C) must be maintained.

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 ad­dress:

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
operation 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ow position.

This furnace design has not been CSA International

certied for installation in mobile homes, recreational

vehicles, or outdoors.

Safety Information

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

operation of the unit with clogged or misplaced lters will

damage the unit.
ML180DF units may be used for heating of buildings or

structures under construction, if the following conditions
are met:

• The vent system must be permanently installed per

these installation instructions.

• A room thermostat must control the furnace. The

use of xed jumpers that will provide continuous
heating is not allowed.

• Air lters must be installed in the system and must

be maintained during construction.

• Air lters must be replaced upon construction com-

pletion.

• The input rate and temperature rise must be set per

the furnace rating plate.

• One hundred percent (100%) outdoor air must be

provided for combustion air requirements during
construction. 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 conned space with air
from outside.

• The furnace heat exchanger, components, duct

system, air lters and evaporator coils must be
thoroughly cleaned following nal construction
clean-up.

• All furnace operating conditions (including ignition,

input rate, temperature rise and venting) must be
veried 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
following general recommendations must be considered

when installing a ML180DF furnace:

• Place the furnace as close to the center of the air
distribution system as possible. The furnace should
also be located close to the chimney or vent termi­nation point.

• Do not install the furnace where drafts might blow
directly into it. This could cause improper combus­tion and unsafe operation.

• Do not block the furnace combustion air openings
with clothing, boxes, doors, etc. Air is needed for
proper combustion and safe unit operation.

• When the furnace is installed in an attic or other
insulated space, keep insulation away from the fur-

nace.

NOTE - The Commonwealth of Massachusetts
stipulates these additional requirements:

• Gas furnaces shall be installed by a licensed

plumber or tter only.

• The gas cock must be “T handle” type.

• When a furnace is installed in an attic, the pas-

sageway to and service area surrounding the
equipment shall be oored.

Page 4

WARNING

This prodcut contains a chemical known to the State
of California to cause cancer, birth defects, or other
reproductive harm.

Combustion, Dilution & Ventilation Air

In the past, there was no problem in bringing in sufcient
outdoor air for combustion. Inltration provided all the air

that was needed. In today’s homes, tight construction
practices make it necessary to bring in air from outside for
combustion. Take into account that exhaust fans, appli-

ance vents, chimneys, and replaces force additional air

that could be used for combustion out of the house. Un­less outside air is brought into the house for combustion,
negative pressure (outside pressure is greater than inside
pressure) 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
dangerous situation.

In the absence of local codes concerning air for combus­tion and ventilation, use the guidelines and procedures in
this section to install ML180DF furnaces to ensure ef­cient 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 reprint-

ed material is not the complete and ofcial position of the

ANSI on the referenced subject,

CAUTION

Do not install the furnace in a corrosive or contaminated
atmosphere. Meet all combustion and ventilation air
requirements, as well as all local codes.

All gas-red appliances require air for the combustion
process. If sufcient combustion air is not available, the
furnace or other appliances will operate inefciently and

unsafely. Enough air must be provided to meet the needs
of all fuel-burning appliances and appliances such as ex­haust fans which force air out of the house. When re­places, 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.

Insufcient air causes incomplete combustion which can

result in carbon monoxide.

CAUTION

Insufcient combustion air can cause headaches,

nausea, dizziness or asphyxiation. It will also cause
excess water in the heat exchanger resulting in rusting
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
dilutes contaminants in the indoor air. These contaminants
may include bleaches, adhesives, detergents, solvents
and other contaminants which can corrode furnace com­ponents.

The requirements for providing air for combustion and
ventilation depend largely on whether the furnace is in-

stalled in an unconned or a conned space.

Unconned Space

An unconned space is an area such as a basement or

large equipment room with a volume greater than 50 cubic
feet (1.42 m3) per 1,000 Btu (.29 kW) per hour of the com­bined 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

unconned, it might be necessary to bring in outdoor air

for combustion if the structure does not provide enough air

by inltration. 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.

Conned Space

A conned space is an area with a volume less than 50

cubic feet (1.42 m3) per 1,000 Btu (.29 kW) per hour of
the combined input rating of all appliances installed in that

space. This denition includes furnace closets or small

equipment rooms.

Page 5

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
terminate outside the space containing the furnace. This
is especially important when the furnace is mounted on

a platform in a conned 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 con­dition. Air for combustion and ventilation can be brought

into the conned space either from inside the building or

from outside.

EQUIPMENT IN CONFINED

CHIMNEY

OR GAS

VENT

FURNACE

AIR FLOW

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

2

).

SPACE ALL AIR FROM INSIDE

WATER

HEATER

OPENINGS

(To Adjacent

Room)

FIGURE 2

Air from Inside

If the conned space that houses the furnace adjoins a
space categorized as unconned, 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 mm2) per 1,000 Btu (.29 kW) per hour of

total input rating of all gas-red equipment in the conned

space. Each opening must be at least 100 square inches
(64516 mm2). 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 gure 2.

Air from Outside

If air from outside is brought in for combustion and ventila­tion, the conned space must have two permanent open­ings. 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 directly or by ducts with the outdoors or spaces (crawl
or attic) that freely communicate with the outdoors or in­directly through vertical ducts. Each opening shall have a
minimum free area of 1 square inch (645 mm2) per 4,000
Btu (1.17 kW) per hour of total input rating of all equipment
in the enclosure. See gures 3 and 4. When communi­cating with the outdoors through horizontal ducts, each
opening shall have a minimum free area of 1 square inch
(645 mm2) per 2,000 Btu (.56 kW) per total input rating of

all equipment in the enclosure. See gure 5.

When ducts are used, they shall be of the same cross-sec­tional 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 covering is not known for calculating the size opening
required, it may be assumed that wood louvers will have
20 to 25 percent free area and metal louvers and grilles
will have 60 to 75 percent free area. Louvers and grilles

must be xed in the open position or interlocked with the

equipment so that they are opened automatically during
equipment operation.

Page 6

CHIMNEY OR

EQUIPMENT IN CONFINED SPACE

ment in the enclosure.

S

other equipment in the enclosure.

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)

AIR FLOW

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

ALL AIR FROM OUTSIDE

(All Air Through Ventilated Attic)

VENTILATION LOUVER

(Each end of attic)

OUTLET

AIR

INLET AIR

(Ends 12 in.

above bottom)

FURNACE

CHIMNEY

OR GAS

VENT

CONFINED SPACE

WATER

HEATER

OUTLET AIR

EQUIPMENT IN

ALL AIR FROM

OUTSIDE

AIR FLOW

NOTE-The inlet and outlet air openings shall each have a
free area of at least one square inch (645 mm

FIGURE 4

WATER

HEATER

2

) per 4,000

Page 7

AIR FLOW

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

INLET AIR

the total input rating of all equipment in the enclosure. If the
equipment room is located against an outside wall and the air
openings
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

FIGURE 5

2

)

Downow Installation

Downow unit installs in three ways: on non-combustible
ooring, on combustible ooring using a downow com-

bustible ooring base, or on a reverse-ow cooling cabi­net. Do not drag the unit across the oor.

Installation on Non-Combustible Flooring (Figure 6)

1 - Cut oor opening keeping in mind clearances listed

on unit rating plate. Also keep in mind gas supply

connections, electrical supply, ue and air intake
connections and sufcient installation and servicing
clearances. See table 1 for correct oor 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.

SUPPLY AIR

OPENING

PLENUM

FIGURE 6

TABLE 1

NON-COMBUSTIBLE FLOOR OPENING SIZE

Cabinet

Width

A (14.5”)

B (17.5”) 16-3/4 413

Front to Rear Side to Side

in mm in mm

13-1/4 337

19-3/4 502

C (21”) 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.

Installation on Combustible Flooring (Figure 7)

1 - When unit is installed on a combustible oor,

a downow combustible ooring base must be
installed between the furnace and the oor. The

base must be ordered separately.

See table 2 for opening size to cut in oor.

CAUTION

The furnace and downow combustible ooring base

shall not be installed directly on carpeting, tile, or other

combustible material other than wood ooring.

SUPPLY AIR PLENUM

PROPERLY

SIZED FLOOR

OPENING

FIGURE 7

TABLE 2

COMBUSTIBLE FLOOR OPENING SIZE

Cabinet

Width

Front to Rear Side to Side

in mm in mm

A (14.5”)

B (17.5”) 18-3/4 476

22 569

C (21”) 22-3/4

2 - After opening is cut, set combustible ooring base

into opening.

3 - Check sealing strips on combustible ooring base to

make sure they are properly glued and positioned.

4 - Lower supply air plenum into downow combustible

ooring base until plenum anges 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 s adequate.

Installation on Cooling Cabinet (Figure 8)

1 - Refer to reverse-ow coil installation instructions for

correctly sized opening in oor and installation of

cabinet.

NOTE - Downow combustible ooring 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.

DOWNFLOW

COMBUSTIBLE

FLOORING BASE

15-3/4 400

578

Page 8

PROPERLY

Blower motor with three exible legs and one

SIZED FLOOR

OPENING

FIGURE 8

Return Air Opening -- Downow Units

COOLING COIL

PLENUM

Setting Equipment

WARNING

Do not install the furnace on its front, back or in the

horizontal position. See gure 12. Do not connect the

return air ducts to the back of the furnace. Doing so
will adversely affect the operation of the safety control
devices, which could result in personal injury or death.

Install the ML180DF gas furnace as shipped in the down-

ow position only. Do not install the furnace horizon­tally.

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.

The following steps should be taken when installing ple­num:

1 - Bottom edge of plenum should be anged with a

hemmed edge (See gure 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

anges of furnace with sheet metal screws.

PLENUM

SIDE CABINET

HEMMED EDGE

Side View

NOTE - Units with 1/3HP and 1/2HP blower motors are
equipped with three exible legs and one shipping leg. See
gure 11. In some units with 1/3HP blower motor, the ship­ping leg is exible with rubber mounting grommets similar
to the other three mounting legs and require no modica­tion. The rest of the units with 1/3HP and 1/2HP blower
motors (these blower motor housings will be tagged) have
a rigid shipping leg equipped with a shipping bolt and at
white plastic washer. The bolt and washer must be re-
moved before the furnace is placed into operation. Af­ter the bolt and washer have been removed, the rigid leg
will not touch the blower housing.

rigid shipping leg. Blower motor housings
will be tagged for shipping bolt removal

RIGID LEG

(Remove shipping bolt

and washer)

FIGURE 11

FIBERGLASS
SEALING STRIP
(Field Provided)

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 gure 10).

5 - Make certain that an adequate seal is made.

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 injury or death.

Page 9

Front

Back

Figure 12

Downow Application

Allow for clearances to combustible materials as indicated
on the unit nameplate. Minimum clearances for closet or

alcove installations are shown in gure 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 inlet 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.

inets, 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 personal

injury or death. Combustion and ue 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 plenum. 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. Allow 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

replace or stove, etc. Fire, explosion, carbon monoxide

poisoning, personal injury and/or property damage could
result.

Filters

This unit is not equipped with a lter or rack. A eld-pro­vided high-velocity lter is required for the unit to operate
properly. Table 3 lists recommended lter sizes.

A lter must be in place any time the unit is operating.

Page 10

TABLE 3

V

Cabinet Width inches Air Filter Size inches

A - 14-1/2 14 x 25 x 1

B - 17-1/2 16 x 25 x 1

C - 21 20 x 25 x 1

Duct System

Use industry-approved standards (such as those pub­lished by Air Conditioning Contractors of America or Amer­ican 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
limit 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

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 replace) 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 ue products can be pulled back down the vent
pipe and into the room. This reverse ow of the ue gas

may result in incomplete combustion and the formation of
carbon monoxide gas. This toxic gas might then be dis­tributed throughout the house by the furnace duct system

.

Venting

A 4-inch diameter ue transition is factory-installed on all

models. Modifying or removing the ue transition will

cause the unit to operate unsafely and will void the

unit certication. The vent connector does not require
insulation.

The ML180DF series units are classied 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
appliance equipped with an integral mechanical means
to either draw or force combustion products through the
combustion chamber and/or heat exchanger.

NOTE - Use these instructions as a guide. They do not
supersede local codes. This furnace must be vented ac­cording to all local codes, these installation instructions,
and the provided venting tables in these instructions.

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
application, 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 dening

document.
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
“Disconnected 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 furnace

must be securely connected to the furnace ue transition

at all times.

ENT CONNECTION

VENT PIPE

(min. 6” length)

FLUE TRANSITION

COLLAR

FIGURE 14

Use self-drilling sheet metal screws or a mechanical fas-

tener to rmly secure the vent pipe to the round collar of
the ue 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 gure 14.
Install the rst vent connector elbow at a minimum of six

inches (152 mm) from the furnace vent outlet. Masonry
chimneys used to vent Category I central furnaces must
be either tile-lined or lined with a listed metal lining sys­tem or dedicated gas vent. Unlined masonry chimneys are

prohibited. See gures 15 and 16 for common venting.

“DISCONNECTED VENT”

WARNING

STICKER

Page 11

Common Venting Using Tile-Lined Interior Masonry Chimney and Combined Vent Connector

Common Venting Using Metal-Lined Masonry Chimney

MINIMUM LENGTH = AS SHORT AS PRACTICAL.
FOR MAXIMUM LENGTH SEE NOTE TO LEFT

NOTE- Refer to provided venting tables
for installations.

OTHER

APPLIANCE

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 exible metal vent.

An exterior tile-lined chimney that is sealed and capped

may be lined with a listed uninsulated exible 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
method must be found.

Insulation for the exible vent pipe must be an encapsu­lated berglass sleeve recommended by the exible vent
pipe manufacturer. See gure 16.

SEALED

MAX. LENGTH

-- SEE NOTE 1

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. Refer
to the capacity requirements shown in the provided venting tables.

BELOW.

MIN. LENGTH -- AS

SHORT AS PRACTICAL

FURNACE

AIR FLOW

CHIMNEY WITH

PERMANENTLY

SEALED FIREPLACE

OPENING

FIGURE 16

AIR FLOW

EXTERIOR

METAL

LINER

VENT

CONNECTOR

FURNACE

Figure 15

DO NOT insulate the space between the liner and
the chimney wall with puffed mica or any other loose
granular insulating material

SINGLE appliance venting of a fan-assisted furnace into
a tile-lined masonry chimney (interior or outside wall)

is PROHIBITED. The chimney must rst be lined with
either type B1 vent or an insulated single wall exible

vent lining system which has been sized according 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:

If type B1 double-wall vent is used inside a chimney, no
other appliance can be vented into the chimney. The out-

er wall of type B1 vent pipe must not be exposed to ue

products.
A type B1 vent or masonry chimney liner shall terminate

above the roof surface with a listed cap or a listed roof as­sembly 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 ue restrictor between the furnace and the chimney.

INTERIOR TILE-LINED
MASONRY CHIMNEY

NOTE - the chimney must be properly
sized per provided venting tables or
lined with listed metal lining system.

PERMANENTLY
SEALED FIREPLACE
OPENING

IMPORTANT

• The chimney is currently serving at least one draft­hood equipped appliance

• The vent connectors and chimney are sized accord­ing to the provided venting tables.

Page 12

Never connect a Category I appliance to a chimney that is

servicing a solid-fuel appliance. If a replace chimney ue
is used to vent this appliance, the replace opening must

be permanently sealed.
A type B or listed chimney lining system that passes

through an unused masonry chimney ue is not consid­ered to be exposed to the outdoors.

General Venting Requirements

Vent all ML180DF furnaces according to these instruc­tions:

1 - Vent diameter recommendations and maximum

allowable 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 specied in the

provided venting tables.

3 - The minimum vent capacity determined by the

sizing tables must be less than the low re input

rating and the maximum vent capacity must be

greater than the high re input rating.

4 - Single appliance vents - If the vertical vent or tile-

lined chimney has a larger diameter or ow area than

the vent connector, use the vertical vent diameter to
determine the minimum vent capacity and the vent
connector diameter to determine the maximum vent

capacity. The ow area of the vertical vent, however,
shall not exceed 7 times the ow area of the listed

appliance categorized vent area, drafthood outlet

area or ue collar area unless designed according

to approved engineering methods.

5 - Multiple appliance vents - The ow 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 ue

collar area unless designed according to approved
engineering methods.

6 - The entire length of single wall metal vent connector

shall be readily accessible for inspection, cleaning,
and replacement.

7 - Single appliance venting congurations with zero

lateral lengths (table 5) are assumed to have
no elbows in the vent system. For all other vent

congurations, 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
requirements 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)

Maximum Horizontal

Connector Length feet (m)

3 (76) 4-1/2 (1.37)

4 (102) 6 (1.83)

5 (152) 7-1/2 (2.29)

6 (152) 9 (2.74)

7 (178)

10-1/2 (3.20)

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
complete the job. Route the vent connector to the
vent using 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 rmly attached to the

furnace ue 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

appliances must be located in or pass through a
crawlspace 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 oors, walls, and

ceilings must be installed with the listed clearance

to combustible materials and be re stopped

according to local 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 maximum common vent capacity).

18 - The common vent diameter must always be at least

as large as the largest vent connector diameter.

Page 13

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 ue collar outlet.

20 - Do not install a manual damper, barometric draft

regulator or ue restrictor between the furnace and

the chimney.

21 - When connecting this appliance to an existing

dedicated 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 requirements, it must
be resized.

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

NOTE - Single appliance venting congurations with zero lateral lengths are assumed to have no elbows in the vent system. For
all other vent congurations, 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 maximum listed capacity).

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

Vent and Connector Diameter - D (inches)

Appliance Input Rating in Thousands of Btu Per Hour

Page 14

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

Lateral

L

(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 2 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 11 2 195 164 283 228 427 352

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 7 inch 7 inch

Vent and Connector Diameter - D (inches)

Appliance Input Rating in Thousands of Btu Per Hour

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 applianc­es, the venting system is likely to be too large to properly
vent the remaining attached appliances.

Conduct the following test while each appliance is oper­ating and the other appliances (which are not operating)
remain connected to the common venting system. If the
venting system has been installed improperly, you must
correct the system as indicated in the general venting re-

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 monoxide poisoning
or death. The following steps shall be followed for each
appliance connected to the venting system being placed
into operation, while all other appliances connected to
the venting system are not in operation:

WARNING

quirements section.

Page 15

1 - Seal any unused openings in the common venting

system.

2 - Inspect the venting system for proper size and

horizontal pitch. Determine that there is no blockage,

restriction, leakage, corrosion, or other deciencies

which could cause an unsafe condition.

3 - Close all building doors and windows and all

doors between the space in which the appliances
remaining connected to the common venting system
are located and other spaces of the building. Turn on
clothes dryers 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 operate at maximum speed. Do not

operate a summer exhaust fan. Close replace

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 ue gases at the draft hood relief
opening. Use the ame 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,

replace dampers, and any other gas-burning

appliances to their previous mode of operation.

7 - If a venting problem is found during any of the

preceding tests, the common venting system must

be modied to correct the problem.

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 standards 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.

CAUTION

If a exible 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 exible 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

installation of gas piping (or top entry in horizontal
applications). Connect the gas supply piping into the
gas valve. The maximum torque is 800 in lbs and
minimum torque is 350 in lbs when when attaching
the gas piping to the gas valve.

WARNING

Do not over torque (800 in-lbs) or under torque (350 in­lbs) when attaching the gas piping to the gas valve.

2 - When connecting the gas supply piping, consider

factors such as length of run, number of ttings, and

furnace 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
furnace. The piping must be supported at proper
intervals [every 8 to 10 feet (2.44 to 3.01 m)] with
suitable hangers 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 gure 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.

CAUTION

Compounds used on threaded joints of gas piping must

be resistant to the actions of liquied petroleum 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

TABLE 8

Gas Pipe Capacity - ft3/hr (m3/hr)

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)

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)

172

(4.87)

360

(10.19)

678

(19.19)

1350

(38.22)

2090

(59.18)

4020

(113.83)

6400

(181.22)

11300

(319.98)

20

(6.096)

118

(3.34)

247

(7.000)

466

(13.19)

957

(27.09)

1430

(40.49)

2760

(78.15)

4400

(124.59)

7780

(220.30)

30

(9,144)

95

(2.69)

199

(5.63)

374

(10.59)

768

(22.25)

1150

(32.56)

2220

(62.86)

3530

(99.95)

6250

(176.98)

40

(12,192)

81

(2.29)

170

(4.81)

320

(9.06)

657

(18.60)

985

(27.89)

1900

(53.80)

3020

(85.51)

5350

(151.49)

Length of Pipe - feet (m)

50

(15.240)

72

(2.03)

151

(4.23)

284

(8.04)

583

(16.50)

873

(24.72)

1680

(47.57)

2680

(75.88)

4740

(134.22)

60

(18.288)

65

(1.84)

137

(3.87)

257

(7.27)

528

(14.95)

791

(22.39)

1520

(43.04)

2480

(70.22)

4290

(121.47)

70

(21.336)

60

(1.69)

126

(3.56)

237

(6.71)

486

(13.76)

728

(20.61)

1400

(39.64)

2230

(63.14)

3950

(111.85)

(24.384)

(1.58)

(3.31)

(6.23)

(12.79)

(19.17)

1300

(36.81)

2080

(58.89)

3670

(103.92)

NOTE - Capacity given in cubic feet (m3 ) of gas per hour and based on 0.60 specic gravity gas.

.

80

56

117

220

452

677

90

(27.432)

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

VALV E

GROUND

JOINT

UNION

Left Side Piping

(Standard)

DRIP LEG

AUTOMATIC

GAS VALV E

AUTOMATIC

GAS VALV E

(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

VALV E

GROUND

JOINT

UNION

DRIP LEG

Right Side Piping

(Alternate)

Page 17

Leak Check

INTERIOR MAKE-UP BOX INSTALLATION

Cut the two wire ties to extend power wires for right side onl

y

After gas piping is completed, carefully check all piping

connections (factory- and eld-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
devices.

CAUTION

Some soaps used for leak detection are corrosive to
certain metals. Carefully rinse piping thoroughly after
leak test has been completed. Do not use matches,

candles, ame 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.)

.

IMPORTANT

When testing pressure of gas lines, gas valve must be

disconnected and isolated. See gure 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 VALV E

WILL NOT HOLD

NORMAL TEST

PRESSURE

1/8 NPT PLUG

CAP

FIGURE 18

Electrical

ELECTROSTATIC DISCHARGE (ESD)

Precautions and Procedures

ISOLATE

GAS VALV E

FURNACE

CAUTION

Electrostatic discharge can affect electronic
components. Take precautions to neutralize
electrostatic charge by touching your hand
and tools to metal prior to handling the
control.

The unit is equipped with a eld 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.

(Right Side)

MAKE-UP

BOX

FIGURE 19

Refer to gure 20 for schematic wiring diagram and trou­bleshooting.

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 Specications 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.

WARNING

Fire Hazard. Use of aluminum wire with this product may

result in a re, causing property damage, severe injury

or death. Use copper wire only with this product.

Page 18

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 con­nections.

Electrically ground the unit according to local codes or, in
the absence of local codes, according to the current Na­tional Electric Code (ANSI/NFPA No. 70). A green ground

wire is provided in the eld make-up box.

NOTE - The ML180DF furnace contains electronic com­ponents 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 gure 21 for integrated
control conguration. 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 external relay.

One line voltage “HUM” 1/4” spade terminal is provided

on the furnace integrated control. See gure 21 for inte­grated control conguration. This terminal is energized in

the heating mode when the combustion air inducer is op­erating.

Any humidier 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 a hu-

midier rated at greater than one amp is connected to this

terminal, 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:

• The furnace requires 120 volts + 10% (Range: 108
volts to 132 volts).

• The furnace operates at 60 Hz + 5% (Range: 57 Hz
to 63 Hz).

• 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.

• 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 gure 20 for thermostat

designations. If the furnace is being matched with a heat
pump, refer to the FM21 installation instruction or appro­priate 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.

Page 19

ML180DF Schematic Wiring Diagram

Figure 20

Page 20

Integrated Control

LED

TWIN

PUSH BUTTON

INTEGRATED CONTROL

(Automatic Hot Surface Ignition System)

TERMINAL DESIGNATIONS

HUM
LINE
XFMR
EAC
COOL
HEAT
PARK
CONT
NEUTRALS

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)

BLOWER OFF

DELAY JUMPER

Integrated Control Diagnostic Codes

LED Status Description

LED Off

LED Off 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
combustion air inducer ener­gized.

Lockout, buners lost ame 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 ashed rst, the oldest fault code is ashed last.
There is a 2 second pause between codes. When the
push button switch is pressed for less than 5 seconds, the
control will ash 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

Unit Start-Up

FOR YOUR SAFETY READ BEFORE LIGHTING

WARNING

Electric shock hazard. Can cause injury
or death. Do not use this unit if any part
has been under water. Immediately call a

qualied service technician to inspect the

unit and to replace any part of the control
system and any gas control which has been
under water.

7 - Wait ve 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
instructions. If you do not smell gas go to next step.

8 - Move switch on gas valve to ON. Do not force. See

gure 22.

MANIFOLD
PRESSURE

ADJUSTMENT

SCREW

MANIFOLD

PRESSURE

OUTLET

PORT

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 maintenance,
turn the electrical power to unit OFF at disconnect switch.

BEFORE LIGHTING smell all around the appliance area

for gas. Be sure to smell next to the oor because some
gas is heavier than air and will settle on the oor.

The gas valve on the ML180DF unit is equipped with a
gas control switch. Use only your hand to move the switch.
Never 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
burners will automatically light. The ignitor does not get
hot when there is no call for heat on units with an automat­ic ignition system.

WARNING

Danger of explosion and re. Can cause

injury or product or property damage. You
must follow these instructions exactly.

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

gure 22.

INLET

PRESSURE

PORT

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 technician 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 lter dirty or plugged? Dirty or plugged lters

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 ue will

cause unit to shut off at pressure switch. Check ue

and outlet for blockages.

GAS VALVE SHOWN IN ON POSITION

FIGURE 22

Page 22

11 - Are ame rollout switches tripped? If ame rollout

switches are tripped, call the service technician for
inspection.

Heating Sequence Of Operation

1 - When thermostat calls for heat, combustion air

blower starts.

2 - Combustion air pressure switch proves blower

operation. Switch is factory-set and requires no
adjustment.

3 - After a 15-second prepurge, the hot surface ignitor

energizes.

4 - After a 20-second ignitor warm-up period, the gas

valve solenoid opens. A 4-second trial for ignition
period begins.

5 - Gas is ignited, ame sensor proves the ame, and

the combustion process continues.

6 - If ame is not detected after rst ignition trial,

the ignition control will repeat steps 3 and 4 four
more times before locking out the gas valve

(”WATCHGUARD” ame failure mode).The ignition

control will then automatically 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

ML180

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

Furnace should operate at least 5 minutes before check­ing gas ow. 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 orices 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.

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 eld-provided barbed tting and connect a

manometer to measure supply pressure. Furnace should
operate at least 5 minutes before checking supply pres­sure. 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 eld-provided barbed
tting. Connect to a manometer to measure manifold

pressure.

2 - Start unit and allow 5 minutes for unit to reach

steady state.
3 - While waiting for the unit to stabilize, observe the

ame. 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.
5 - Shut unit off and remove manometer as soon as an

accurate reading has been obtained. Take care to

replace pressure tap plug.
6 - 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 ow rate before checking
combustion. Take combustion sample beyond the ue out-

let and compare to the tables below. The maximum car­bon monoxide reading should not exceed 100 ppm.

TABLE 10

ML180UH Unit CO2% For Nat CO2% For L.P.

-045

-070

-090

-110

6.0 - 7.5 6.9 - 8.5

High Altitude

The manifold pressure may require adjustment and com­bustion 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

TABLE 11

Manifold Pressure Settings at all Altitudes

Model

Input Size

045

070

090

110

Gas 0 - 4500 ft. 4501 - 7500 ft . 7501 - 10,000 ft.

Nat 3.5 3.5 3.5 4.5 13.0

LP/Propane 10.0 10.0 10.0 11.0 13.0

Nat 3.5 3.3 3.5 4.5 13.0

LP/Propane 10.0 10.0 10.0 11.0 13.0

Nat 3.5 3.3 3.5 4.5 13.0

LP/Propane 10.0 10.0 10.0 11.0 13.0

Nat 3.5 3.3 3.5 4.5 13.0

LP/Propane 10.0 10.0 10.0 11.0 13.0

Line Pressure in. wg.

Min Max

TABLE 12

Pressure Switch and Gas Conversion Kits at all Altitudes

Model Input

Size

045 0-4500 ft 4501-7500 ft 7501-10,000 ft 0 - 7500 ft 7501 - 10,000 ft 0 - 7500 ft 7501 - 10,000 ft

070

110 80W52

090 No Change

090 (X)

110

High Altitude Pressure Switch Kit Natuarl Gas Orice Kit LP/Propane Orice Kit

No Change

No Change

80W52

80W51 73W81 73W37 11K49 11K44

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

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 lo­cated 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 allow­ing ignition trial. The switch is factory-set and requires no
adjustment.

Temperature Rise

After the furnace has been started, and supply and return
air temperatures have been allowed to stabilize, check the
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 oper-

ates after the heat demand has been satised) may be
adjusted by changing the jumper position across the ve

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 return air temperatures; shorter settings

provide higher return air temperatures. See gure 23.

Page 24

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 Specications Manual for indoor blower

data.

NOTE - Termination of any unused motor leads must be
insulated.

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 provi­sion 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 synchro­nized 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, fol­lowed 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
isolation 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-thermostat twin.

Page 25

TWO-STAGE THERMOSTAT

TWIN

FIELD WIRING FOR TWINNING

SINGLE STAGE THERMOSTAT

TWIN

Call For Cool

Call For Fan

Call For 1st
Stage Heat

Call For 2nd

StageHeat

R

Y

G

W1

W2

ISOLATION
RELAY

TWIN

R

TWIN 1

Y

G

W

C

R

TWIN 2

Y

G

W

C

Call For Cool

Call For Fan

Call For Heat

R

Y

G

W

ISOLATION
RELAY

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 operation,
serious injury, death, or property damage.

Before servicing, disconnect all electrical power to
furnace.

When servicing controls, label all wires prior to
disconnecting. 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 injury or death.

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 equiva­lent) 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 applicable.
3 - Inspect all gas pipe and connections for leaks.
4 - Check the cleanliness of lters 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 necessary . The blower motors are prelubricated

for extended bearing life. No further lubrication is

needed.
7 - Inspect the combustion air inducer and clean if

necessary.
8 - Evaluate the heat exchanger integrity by inspecting

the heat exchanger per the AHRI heat exchanger

inspection procedure. This procedure can be

viewed at www.ahrinet.org.

Page 26

9 - Ensure sufcient combustion air is available to the

furnace. 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 obstructions and must slope upward
away from the furnace . 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

furnace inspection to ensure proper combustion
and operation. Consult Service Literature for proper
combustion values.

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 ame sensor. Check microamps before

and after. Check controls and safety devices (gas

valve, ame sensor, temperature limits). Consult

Service Manual for proper operating range. Thermal

Limits should be checked by restricting airow and

not disconnecting the indoor blower. For additional
details, please see Service and Application Note
H049.

2 - Verify that system total static pressure and airow

settings are within specic operating parameters.

3 - Clock gas meter to ensure that the unit is operating

at the specied ring rate. Check the supply

pressure and the manifold pressure. On two-stage
gas furnaces check the manifold pressure on high

re and low re. If manifold pressure adjustment is

necessary, consult the Service Literature for unit

specic information on adjusting gas pressure.

Not all gas valves are adjustable. Verify correct
temperature rise.

Cleaning the Burners

NOTE - Use papers or protective covering in front of the
furnace

during cleaning.

1 - Turn off both electrical and gas power supplies to

furnace.

2 - Label the wires from gas valve, rollout switches,

primary limit switch and make-up box then

disconnect them.
3 - Disconnect gas supply piping. Remove the four

screws securing the burner manifold assembly to

the vestibule panel and remove the assembly from

the unit.
4 - Remove 4 screws securing burner box and remove

burner box.
5 - To clean burners, run a vacuum cleaner with a soft

brush attachment over the face of burners. Visually

inspect inside the burners and crossovers for any

blockage caused by foreign matter. Remove any

blockage. Figure 25 shows burner detail.
6 - Reinstall burner box, manifold assembly and burner

box cover.
7 - Reconnect all wires.
8 - Reconnect gas supply piping.
9 - Turn on power and gas supply to unit.
10 - Set thermostat and check for proper operation.
11 - Check all piping connections, factory and eld, 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 after
leak test has been completed. Do not use matches,

candles, ame or other sources of ignition to check for

gas leaks.

12 - If a leak is detected, shut gas and electricity off and

repair leak.
13 - Repeat steps 12 and 13 until no leaks are detected.
14 - Replace access panel.

Page 27

Ignitor

BURNER ASSEMBLY & REMOVAL

Burner Detail Front View

Rollout Switches

Sensor

Manifold And Gas Valve

Cross Over

Retention Rings

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

Heating Parts

Flame sensor
Heat exchanger assembly
Gas manifold
Combustion air inducer
Gas valve
Main burner cluster

Main burner orices

Pressure switch
Ignitor
Primary limit control
Flame rollout switch (s)
Secondary limit

Motor mounting frame
Motor capacitor
Blower housing cutoff plate

Page 28