Lennox ML180UH Merit Installation Instructions Manual

INSTALLATION

 2015 Lennox Industries Inc.

Dallas, Texas, USA

INSTRUCTIONS
ML180UH

MERIT® SERIES GAS FURNACE
UPFLOW / HORIZONTAL AIR DISCHARGE

507323-01
06/2015
Supersedes 507009-01

Litho U.S.A.

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.

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 HVAC in
staller (or equivalent), service agency or the gas sup
plier.

AIR FLOW

AIR FLOW

HORIZONTAL LEFT

UPFLOW

AIR FLOW

HORIZONTAL RIGHT

Table of Contents

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

ML180UH 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 8...............................

Filters 11..........................................

Duct System 12....................................

Venting 13........................................

CAUTION

As with any mechanical equipment, personal injury
can result from contact with sharp sheet metal
edges. Be careful when you handle this equipment.

Gas Piping 20.....................................

Electrical 22.......................................

Integrated Control 24...............................

Unit Start-Up 26...................................

Gas Pressure Adjustment 26........................

High Altitude 26....................................

Proper Combustion 26..............................

Other Unit Adjustments 28..........................

Twinning the ML180UH 29..........................

Service 30........................................

Repair Parts 32....................................

Page 1

Unit Dimensions - inches (mm)

1

NOTE - 60C and 60D units that require air volumes

over 1800 cfm (850 L/s) must have one of the following:

1. Single side return air with transition to accommodate
20 x 25 x 1 in. (508 x 635 x 25 mm) cleanable air
filter. (Required to maintain proper air velocity.)

2. Single side return with optional return air base.

3. Bottom return air.

4. Return air from both sides.

5. Bottom and one side return air.

See Blower Performance Tables for additional informa
tion.

2

Flue outlet may be horizontal but furnace must be

vented vertically

3

Optional external side return air filter kit cannot be used

with the optional RAB Return Air Base.

A

B

9/16 (14)

3

OPTIONAL

EXTERNAL

SIDE RETURN

AIR FILTER KIT

(Either Side)

33

(838)

D

FLUE OUTLET

(Top)

3

OPTIONAL

EXTERNAL

SIDE RETURN

AIR FILTER KIT

(Either Side)

3/4(19)

Front Panel

3 (76) Right

6−3/4 (171) Left

9−1/8 (232) Right

8−5/8 (219) Left

5−3/8 (137) Right

1−9/16 (40) Left

3−1/8 (79)

TOP VIEW

3−1/4 (83)

2

FLUE OUTLET

(Either Side)

ELECTRICAL INLET

(Either Side)

GAS PIPING INLET

(Either Side)

SUPPLY AIR

OPENING

23−3/4

(603)

25

(635)

27−3/4

(705)

19−7/16

(494)

9/16

(14)

3/4

(19)

FRONT VIEW

ML180UH Model No.

045P24A
045P36A
070P24A
070P36A

090P36B
090P48B

110P48C
110P60C

135P60D

C

1

Bottom Return

Air Opening

3/4

(19)

14−3/4

(375)

(16)

(406)

5/8

16

ELECTRICAL
INLET (Either Side)

(Either Side)

3−1/4

(83)

23

(584)

1

Side Return
Air Opening
(Either Side)

23−1/2

(597)

1

Bottom Return

Air Opening

14

(356)

1−15/16 (49)

1−1/2

(38)

SIDE VIEW

A B C D

in. mm in. mm in. mm in. mm

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

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

24-1/2 622 23-3/8 546 23 584 9-3/4 248

Page 2

ML180UH Gas Furnace

The ML180UH unit is shipped ready for installation in the
upflow or horizontal right position (for horizontal left posi
tion the combustion air pressure switch must be moved).
The furnace is shipped with the bottom panel in place. The
bottom panel must be removed if the unit is to be installed in
a horizontal application. The panel may also be removed in
upflow applications.

Shipping and Packing List

Package 1 of 1 contains

1 - Assembled ML180UH 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 - Suspension kit (for horizontal installations)
1 - Propane/LP changeover kit
1 - Return air base
1 - High altitude kit
1 - Side filter 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

ML180UH units are CSA International certified to ANSI
Z21.47.

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 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 figures 7 and 11. Accessibil
ity and service clearances must take precedence over fire
protection clearances.

NOTE - For installation on combustible floors, the furnace
shall not be installed directly on carpeting, tile, or other
combustible material other than wood flooring.

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.

Temperature Rise

NOTE - Furnace must be adjusted to obtain a temperature
rise within the range specified on the unit nameplate. Fail
ure to do so may cause erratic limit operation and may re
sult in premature heat exchanger failure.

This ML180UH furnace must be installed so that its electri
cal components are protected from water.

Page 3

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.
See figure 1. 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. If the damper
is manually operated, it must be equipped to prevent op
eration of either the heating or the cooling unit, unless it is in
the full HEAT or COOL setting. See figure 1.

Heating Unit Installed Parallel to Cooling Coil

Dampers

(open during heating operation only)

Gas Unit

Cooling Coil

Dampers

(open during cooling operation only)

Heating Unit Installed Upstream of Cooling Unit

The ML180UH furnace may be installed in alcoves, clos
ets, attics, basements, garages, crawl spaces and utility
rooms in the upflow or horizontal 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 ML180UH 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.

ML180UH 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.

Gas Unit

Cooling Coil

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.

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.

Page 4

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 ML180UH furnace:

 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.

 Do not install the furnace where drafts might blow direct

ly into it. This could cause improper combustion and un
safe 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 insu

lated space, keep insulation away from the furnace.

• Please consult the manufacturer of your evaporator coil

for their recommendations on distance required
between the heat exchanger and their drain pan. Ad
equate space must be provided between the drain pan
and the furnace heat exchanger.

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.

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.

will build to the point that a downdraft can occur in the fur
nace vent pipe or chimney. As a result, combustion gases
enter the living space creating a potentially dangerous situ
ation.

In the absence of local codes concerning air for combus
tion and ventilation, use the guidelines and procedures in
this section to install ML180UH furnaces to ensure efficient
and safe operation. You must consider combustion air
needs and requirements for exhaust vents and gas piping.
A portion of this information has been reprinted with per
mission from the National Fuel Gas Code (ANSI‐Z223.1).
This reprinted material is not the complete and official posi
tion of the ANSI on the referenced subject, which is repre
sented 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.

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 com
bustion 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

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 com
bustion. Take into account that exhaust fans, appliance
vents, chimneys, and fireplaces force additional air that
could be used for combustion out of the house. Unless out
side air is brought into the house for combustion, negative
pressure (outside pressure is greater than inside pressure)

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.

Page 5

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

3

) per 1,000 Btu (.29 kW) per hour of the com
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

3

) per 1,000 Btu (.29 kW) per hour of the
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 con
taining 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
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 condition. Air for combustion
and ventilation can be brought into the confined space ei
ther from inside the building or from outside.

EQUIPMENT IN CONFINED

SPACE ALL AIR FROM INSIDE

CHIMNEY

OR GAS

VENT

WATER

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

).

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.

Page 6

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,

(Inlet Air from Crawlspace and Outlet Air to Ventilated Attic)

CHIMNEY OR

GAS VENT

VENTILATION LOUVERS

(Each end of attic)

FURNACE

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.

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)

CHIMNEY

OR GAS

VENT

OUTLET

AIR

VENTILATION LOUVERS

(Each end of attic)

INLET AIR

(Ends 12 in.

above bottom)

WATER

HEATER

FURNACE

WATER

HEATER

EQUIPMENT IN

CONFINED SPACE

ALL AIR FROM

OUTSIDE

OUTLET AIR

INLET AIR

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

2

) per 4,000

Page 7

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 communi
cate directly with the outdoors, each opening shall have a free area
of at least one square inch (645 mm

2

) per 4,000 Btu (1.17 kW) per

hour of the total input rating of all other equipment in the enclosure.

FIGURE 5

Setting Equipment

WARNING

Do not install the furnace on its front or its back. Do
not connect the return air ducts to the back of the fur
nace. Doing so will adversely affect the operation of
the safety control devices, which could result in per
sonal injury or death.

NOTE - As an option, rubberlike pads defined as having an
elastic texture resembling rubber in flexibility or toughness,
may be placed under each corner of the furnace as ap
propriate to the application.

The ML180UH gas furnace can be installed as shipped in
either the upflow position or the horizontal position.

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 6. 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.

ML180UH09048B with 1/2 HP

BLOWER MOTOR

RIGID LEG

remove shipping bolt and washer

Upflow Applications

Allow for clearances to combustible materials as indicated
on the unit nameplate. Minimum clearances for closet or al
cove installations are shown in figure 7.

Upflow Application Installation Clearances

Top

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 0‡ 0‡

*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 single wall vent pipe is used.
‡For installation on a combustible floor, do not install the furnace
directly on carpeting, tile or other combustible materials other
than wood flooring.
†Left side requires 3 inches if a single wall vent is used on 14-1/2
in. cabinets, or 2inches if a single wall vent pipe is used on 17-1/2
in. cabinets.

Type C Type B1

Right Side

FIGURE 7

FIGURE 6

Page 8

Return Air -- Upflow Applications

Return air can be brought in through the bottom or either
side of the furnace installed in an upflow application. If the
furnace is installed on a platform with bottom return, make
an airtight seal between the bottom of the furnace and the
platform to ensure that the furnace operates properly and
safely. The furnace is equipped with a removable bottom
panel to facilitate installation.

Markings are provided on both sides of the furnace cabinet
for installations that require side return air. Cut the furnace
cabinet at the maximum dimensions shown on page 2.

Single Side Return Air

(with transition and filter)

20” X 25” X 1”

(508mm X 635mm X 25mm)

Cleanable Filter

NOTE -

60C and 60D units that require air volumes over 1800

cfm (850 L/s) must have one of the following:

1 - Single side return air with transition to accommodate 20

x 25 x 1 in. (508 x 635 x 25 mm) cleanable air filter. (Re
quired to maintain proper air velocity.) See figure 8.

2 - Single side return air with optional return airbase. See

figure 9.

3 - Bottom return air.

4 - Return air from both sides.

5 - Bottom and one side return air.

Refer to Engineering Handbook for additional information.

Optional Return Air Base

(Upflow Applications Only -- For use with A, B, C and D cabinets)

FURNACE

FRONT

IF BASE

IS USED

WITHOUT

IAQ CABINET,

A SINGLE

RETURN AIR

PLENUM

MUST

COVER BOTH

UNIT AND

RETURN

AIR BASE

OPENINGS

AIR FLOW

INDOOR AIR

QUALITY

CABINET

(PCO, Filter

Cabinet, etc.)

3−1/4

(83)

7−1/4

(184)

AIR FLOW

1

Unit side return air

5−5/8
(143)

Return Air

Transition

FIGURE 8

1

23 (584)

Overall

(Maximum)

Opening

SIDE RETURN

AIR OPENINGS

(Either Side)

Plenum

1

Minimum

11 (279)

2

Maximum

14 (356)

1

22−7/16

(570)

Overall

(Maximum)

14−1/2 (368) A Width (65W75)
17−1/2 (446) B Width (50W98)
21 (533) C Width (50W99)
24−1/2 (622) D Width (51W00)

NOTE- Optional Side Return Air Filter Kits are not for use with Optional Return Air Base.

1

Both the unit return air opening and the base return air opening must be covered by a single plenum or IAQ cabinet.

OPTIONAL
RETURN
AIR BASE

Minimum unit side return air opening dimensions for units requiring 1800 cfm or more of air (W x H): 23 x 11 in. (584 x 279 mm).
The opening can be cut as needed to accommodate plenum or IAQ cabinet while maintaining dimensions shown.
Side return air openings must be cut in the field. There are cutting guides stenciled on the cabinet for the side return air
opening. The size of the opening must not extend beyond the markings on the furnace cabinet.

2

To minimize pressure drop, the largest opening height possible (up to 14 inches 356 mm) is preferred.

FIGURE 9

Page 9

23

(584)

26−7/8

(683)

SIDE VIEW

3/4

(19)

Removing the Bottom Panel

Remove the two screws that secure the bottom cap to the
furnace. Pivot the bottom cap down to release the bottom
panel. Once the bottom panel has been removed, reinstall
the bottom cap. See figure 10.

Removing the Bottom Panel

Horizontal Applications

The ML180UH furnace can be installed in horizontal ap
plications. Order horizontal suspension kit (51W10) from
Lennox, or use equivalent suspension method.

Allow for clearances to combustible materials as indicated
on the unit nameplate. Minimum clearances for closet or al
cove installations are shown in figure 11.

This furnace may be installed in either an attic or a crawl
space. Either suspend the furnace from roof rafters or floor
joists, as shown in figure 12, or install the furnace on a plat
form, as shown in figure 13.

Typical Horizontal Application

Unit Suspended in Attic or Crawlspace

Metal

Internal

Strap

Brace

Screw

Bottom Cap

Bottom Panel

FIGURE 10

Horizontal Application

Installation Clearances

Left
End

Vent Connector

Type

Top 0 0

*Front 2-1/4 in. (57 mm)** 2-1/4 in. (57 mm)

Back 0 0

Ends 2 in. (51 mm) 2 in. (51 mm)

Vent 6 in. (152 mm) 1 in. (25 mm)

Floor 0‡ 0‡

*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 singlewall vent pipe is used.
‡For installations on a combustible floor, do not install the fur
nace directly on carpeting, tile or other combustible materials oth
er than wood flooring.

Top

Right

AIR FLOW

End

Bottom

Type C Type B1

Bracket

3/16

Air Flow

FIGURE 12

NOTE - Heavy-gauge perforated sheet metal straps may
be used to suspend the unit from roof rafters or ceiling
joists. When straps are used to suspend the unit in this way,
support must be provided for both the ends. The straps
must not interfere with the plenum or exhaust piping instal
lation. Cooling coils and supply and return air plenums

must be supported separately.
NOTE - When the furnace is installed on a platform in a

crawlspace, it must be elevated enough to avoid water
damage and to allow the evaporator coil to drain.

Return Air -- Horizontal Applications

Return air must be brought in through the end of a furnace
installed in a horizontal application. The furnace is
equipped with a removable bottom panel to facilitate instal
lation. See figure 10.

FIGURE 11

Page 10

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.

Horizontal Application

Unit Installed on Platform

Line contact is permissible but not preferred

See the unit nameplate for

clearances.

GAS

ENTRY

SERVICE PLATFORM

FIGURE 13

VENT

PIPE

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.

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.

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 1 lists recommended filter sizes.

IMPORTANT

If a highefficiency filter is being installed as part of
this system to ensure better indoor air quality, the fil
ter must be properly sized. Highefficiency filters
have a higher static pressure drop than standardef
ficiency glass/foam filters. If the pressure drop is too
great, system capacity and performance may be re
duced. The pressure drop may also cause the limit to
trip more frequently during the winter and the indoor
coil to freeze in the summer, resulting in an increase
in the number of service calls.

Before using any filter with this system, check the
specifications provided by the filter manufacturer
against the data given in the appropriate Lennox
Product Specifications bulletin. Additional informa
tion is provided in Service and Application Note
ACC002 (August 2000).

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

TABLE 1

Furnace

Cabinet Width

A - 14-1/2” 16 X 25 X 1 (1) 14 X 25 X 1 (1)
B - 17-1/2” 16 X 25 X 1 (1) 16 X 25 X 1 (1)

C - 21” 16 X 25 X 1 (1) 20 X 25 X 1 (1)

D - 24-1/2” 16 X 25 X 1 (2) 24 X 25 X 1 (1)

Side Return Bottom Return

Filter Size

Page 11

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.

Supply Air Plenum

If the furnace is installed without a cooling coil, a removable
access panel must be installed in the supply air duct. The
access panel should be large enough to permit inspection
(either by smoke or reflected light) of the heat exchanger
for leaks after the furnace is installed. The furnace access
panel must always be in place when the furnace is operat
ing and it must not allow leaks into the supply air duct sys
tem.

Return Air Plenum

NOTE - 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 de
vice (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 pressure, the flue products can be
pulled back down the vent pipe and into the room. This re
verse flow of the flue gas may result in incomplete combus
tion and the formation of carbon monoxide gas. This toxic
gas might then be distributed throughout the house by the
furnace duct system.

In upflow applications, the return air can be brought in
through the bottom or either side of the furnace. If a furnace
with bottom return air is installed on a platform, make an air
tight seal between the bottom of the furnace and the plat
form to ensure that the unit operates properly and safely.
Use fiberglass sealing strips, caulking, or equivalent seal
ing method between the plenum and the furnace cabinet to
ensure a tight seal. If a filter is installed, size the return air
duct to fit the filter frame.

Page 12

Venting

A 4-inch diameter flue transition is factory‐installed on the
combustion air inducer outlet of all models. Figure 15

shows the combustion air inducer as shipped from the
factory.

Mounting Screws Location

Flue Transition

Mounting Screws

1 - Remove the four mounting screws (figure 14) which

secure the combustion air inducer / pressure switch
assembly to the orifice plate. Lift the assembly and ro
tate it 90 degrees clockwise or counterclockwise to ei
ther the 3 o'clock position or to 9 o'clock position. Re­secure with four screws. Gasket should be left in place.

2 - Use tin snips to cut preferred opening on the cabinet

for repositioning the flue outlet. Use the cut-out piece
as a cover plate to patch unused opening on cabinet.

UPFLOW POSITION

Left Side Vent Discharge

Pressure

Switch

Vent Pipe

Collector Box

Make-Up

Box

FIGURE 14

Upflow Position

UPFLOW POSITION

Pressure

Switch

Make-Up

Box

Top Vent Discharge

Flue

Transition

AIR

Vent Pipe

Collector Box

FLOW

FIGURE 15

IMPORTANT

The unit will not vent properly with the flue transition
pointed down in the 6 o'clock position.
The combustion air inducer may be rotated clock
wise or counterclockwise by 90° to allow for top or
side vent discharge in all applications. When the unit
is installed, the flue transition must be in the 9
o'clock, 12 o'clock or 3 o'clock position.

If necessary, reposition the combustion air inducer, pres
sure switch and/or make-up box as needed per the follow
ing steps. See figures 15 through 21.

Flue

Transition

FLOW

AIR

 Gas supply piping must be brought into the unit from the right

side in order to accommodate the flue pipe.

 Cut combustion air inducer tubing from 9” to 8” to avoid

interference with inducer motor

 Remove make-up box assembly (2 screws) and cut wire tie to

free make-up box wires. Re-install make-up box on other side
of cabinet.

 Re-secure make-up box wires: Either pull excess wires

through the blower compartment and secure using supplied
wire tie, or coil excess wire and secure to the gas manifold.

FIGURE 16

UPFLOW POSITION

Right Side Vent Discharge

Collector Box

Pressure

Switch

Make-Up

Box

 Cut combustion air inducer tubing from 9” to 5” to avoid

interference with inducer motor

Cover Plate

AIR

Flue Transition

Vent
Pipe

FLOW

FIGURE 17

Page 13

Vent Pipe

Pressure

Switch

HORIZONTAL LEFT POSITION

Top Vent Discharge

Vent Pipe

Flue

Transitio

n

HORIZONTAL RIGHT POSITION

Top Vent Discharge

Pressure Switch

Cover Plate

Flue

Transition

Cover Plate

Make-Up Box

 Disconnect pressure switch hose from barbed fitting on the

pressure switch assembly. Remove pressure switch assembly
(1 screw) and cut wire tie to free pressure switch wires. Re­install pressure switch on the other side of orifice plate and re­connect pressure switch hose.

 Re-secure pressure switch wires: Either pull excess wires

through the blower compartment and secure using supplied
wire tie, or coil excess wire and secure to the gas manifold.

AIR

FLOW

Collector Box

FIGURE 18

HORIZONTAL LEFT POSITION

Side Vent Discharge

Pressure Switch

AIR

FLOW

 Gas supply piping must be brought into the unit from the bot

tom in order to accommodate the flue pipe.

 Cut combustion air inducer tubing from 9” to 8” to avoid

interference with inducer motor

 Remove make-up box assembly (2 screws) and cut wire tie to

free make-up box wires. Re-install make-up box on other side
of cabinet.

 Re-secure make-up box wires: Either pull excess wires through

the blower compartment and secure using supplied wire tie, or
coil excess wire and secure to the gas manifold.

Collector Box

Make-Up Box

FIGURE 20

AIR

FLOW

Vent Pipe

Make-Up Box

 Cut combustion air inducer tubing from 9” to 7” to avoid

interference with inducer motor

 Disconnect pressure switch hose from barbed fitting on the

pressure switch assembly. Remove pressure switch assembly
(1 screw) and cut wire tie to free pressure switch wires. Re­install pressure switch on the other side of orifice plate and re­connect pressure switch hose.

 Re-secure pressure switch wires: Either pull excess wires

through the blower compartment and secure using supplied
wire tie, or coil excess wire and secure to the gas manifold.

Collector Box

FIGURE 19

Make-Up Box

AIR

FLOW

HORIZONTAL RIGHT POSITION

Side Vent Discharge

Pressure Switch

Flue Transition

Collector Box

FIGURE 21

Page 14

The ML180UH 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) in the USA. A fan-assisted Category I fur
nace is an appliance equipped with an integral mechanical
means to either draw or force combustion products through
the combustion chamber and/or heat exchanger. The
ML180UH is not approved for use with horizontal venting.

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

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.

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 22.

Install the first vent connector elbow at a minimum of six
inches (152 mm) from the furnace vent outlet. See figure

22.

Venting Using a Masonry Chimney

The following additional requirements apply when a lined
masonry chimney is used to vent this furnace.

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 23 and 24 for com
mon venting.

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 23.

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. See figure 22. The
warning sticker is provided in the bag assembly. Or
der 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.

VENT CONNECTION UPFLOW AND HORIZONTAL POSITION

“DISCONNECTED VENT”

VENT PIPE

(min. 6” length)

FLUE TRANSITION

COLLAR

WARNING

STICKER

“DISCONNECTED VENT”

FIGURE 22

Page 15

WARNING

STICKER

VENT PIPE

(min. 6” length)

FLUE

TRANSITION

COLLAR

Common Venting Using Metal-Lined Masonry Chimney

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

EXTERIOR

CHIMNEY WITH

METAL

LINER

PERMANENTLY

SEALED FIREPLACE

OPENING

FIGURE 23
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:

 The chimney is currently serving at least one drafthood-

equipped appliance;

 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 ML180UH 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.

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 ML180UH 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.

Page 16

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

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

NOTE- Refer to provided venting
tables for installations.

OTHER

APPLIANCE

FIGURE 24

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 3) 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 (4 and 5) 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:

TABLE 2

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

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

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, attic or other areas which may be cold, that por
tion 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.

Page 17

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.

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.

21 - When connecting this appliance to an existing dedi

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.

TABLE 3

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 18

Vent

Height

H

(feet)

6

8

10

15

20

30

TABLE 4

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 5

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 19

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.

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 burner have operated for 5 minutes, test for

leaks of flue gases at the draft hood relief opening. Use
the flame of a match or candle.

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.

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.

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 (or top entry in horizontal applica
tions). Connect the gas supply to the piping 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 6
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 in 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 31.

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 20

TABLE 6

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)

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)

MANUAL

MAIN SHUT-OFF

VALVE

(With 1/8 in. NPT

Plugged Tap Shown)

GROUND

JOINT

UNION

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)

118

(3.34)

247

(7.00)

466

(13.19)

957

(27.09)

1430

(40.49)

2760

(78.15)

4400

(124.59)

95

(2.69)

199

(5.63)

374

(10.59)

768

(22.25)

1150

(32.56)

2220

(62.86)

3530

(99.95)

81

(2.29)

170

(4.81)

320

(9.06)

657

(18.60)

985

(27.89)

1900

(53.80)

3020

(85.51)

Left Side Piping

(Standard)

AUTOMAT

IC

GAS VALVE

3

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)

/hr (m3/hr)

65

(1.84)

137

(3.87)

257

(7.27)

528

(14.95)

791

(22.39)

1520

(43.04)

2480

(70.22)

AUTOMATIC

GAS VALVE

60

(1.69)

126

(3.56)

237

(6.71)

486

(13.76)

728

(20.61)

1400

(39.64)

2230

(63.14)

56

(1.58)

117

(3.31)

220

(6.23)

452

(12.79)

677

(19.17)

1300

(36.81)

2080

(58.89)

MAIN SHUT-OFF

52

(1.47)

110

(3.11)

207

(5.86)

424

(12.00)

635

(17.98)

1220

(34.55)

1950

(55.22)

MANUAL

VALVE

(With 1/8 in. NPT

Plugged Tap

Shown)

GROUND

JOINT

UNION

DRIP LEG

100

(30.480)

50

(1.42)

104

(2.94)

195

(5.52)

400

(11.33)

600

(17.00)

1160

(32.844)

1840

(52.10)

MANUAL

MAIN SHUT-OFF

VALVE

GROUND

JOINT

UNION

DRIP LEG

Horizontal Application

Right-Side Air Discharge

DRIP LEG

FIELD

PROVIDED

AND INSTALLED

NOTE - BLACK IRON PIPE ONLY TO BE ROUTED INSIDE OF CABINET

FIGURE 25

MANUAL

MAIN SHUT-OFF

VALVE

GROUND

JOINT

UNION

DRIP LEG

FIELD

PROVIDED

NOTE - BLACK IRON PIPE ONLY TO BE ROUTED INSIDE OF CABINET

AND INSTALLED

FIGURE 26

Right Side Piping

(Alternate)

Horizontal Application

Left-Side Air Discharge

MAIN SHUT-OFF

DRIP LEG

MANUAL

VALVE

GROUND

JOINT

UNION

Page 21

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.

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

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 by closing its individual
manual shut‐off valve and disconnecting from from the gas
supply system the during any pressure testing of the gas
supply system at pressures greater than 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 figure 27. 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

ISOLATE

GAS VALVE

FURNACE

Right Side

BOX

FIGURE 28

Refer to figure 30 for schematic wiring diagram and field
wiring.

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.

CAP

FIGURE 27

Electrical

ELECTROSTATIC DISCHARGE (ESD)

Precautions and Procedures

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.

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.

Complete the wiring connections to the equipment. Use the
provided unit and the field wiring diagram shown in figure

30. Use 18-gauge wire or larger that is suitable for Class II
rating for thermostat connections.

Page 22

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.

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

WARNING

Electric Shock Hazard. Can cause
injury or death. Unit must be properly
grounded in accordance with national
and local codes.

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 ML180UH furnace contains electronic compo
nents that are polarity sensitive. Make sure that the furnace
is wired correctly and is properly grounded.

Accessory Terminals

One line voltage “EAC” 1/4” spade terminal is provided on
the furnace integrated control. See figure 29 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 29 for integrated
control configuration. This terminal is energized in the
heating mode when the combustion air inducer is operat

The following requirements must be kept in mind when
specifying a generator for use with this equipment:

 The furnace requires 120 volts +

volts to 132 volts).

 The furnace operates at 60 Hz +

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 figure 30 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 ML180UH 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 23

Integrated Control

LED

PUSH BUTTON

TWIN

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

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 29

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 24

ML180UH Schematic Wiring Diagram and Sequence of Operation

FIGURE 30

Page 25

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 ML180UH unit will be 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:

ML180UH 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 31)

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 upper access panel.

6 - Move switch on gas valve to OFF. Do not force. See

figure 31.

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 31.

9 - Replace the upper 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 upper 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.
11 - Are flame rollout switches tripped? If flame rollout

switches are tripped, call the service technician for in

spection.

GAS VALVE SHOWN IN ON POSITION

FIGURE 31

MANIFOLD

PRESSURE

OUTLET

PORT

Page 26

Heating Sequence Of Operation
(follow steps below or see Figure 30 for more detail)

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 60 minutes.

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 7

GAS METER CLOCKING CHART

Seconds for One Revolution

ML180UH

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

access to the supply pressure tap. Remove the threaded
plug, install a field-provided barbed fitting and connect a
manometer to measure supply pressure. See table 9 for
proper line pressure. Replace the threaded plug after mea
surements 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

9.

5 - Shut unit off and remove manometer as soon as an ac

curate reading has been obtained. Take care to re
place 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 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 8

ML180UH

Unit

-045

-070

-090

-110

-135

CO2%

For Nat

7.2 - 7.8 7.5 - 9.0

CO2%

For L.P.

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
7 below. If manifold pressure matches table 9 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.

Supply Pressure Measurement

A threaded plug on the inlet side of the gas valve provides

High Altitude

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 9 for
manifold pressure and table 10 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 27

TABLE 9

Manifold Pressure Settings at all Altitudes

Model

Input Size

045

070

090

110

135

Gas 0-2000 ft 2001-4500 ft. 4501-7500 ft 7501 - 10,000 ft

Nat 3.5 3.2 3.0 3.5 4.5 13.0

LP/propane 10.0 10.0 10.0 10.0 11.0 13.0

Nat 3.5 3.2 2.8 3.5 4.5 13.0

LP/propane 10.0 10.0 10.0 10.0 11.0 13.0

Nat 3.5 3.2 2.7 3.5 4.5 13.0

LP/propane 10.0 10 9.6 10.0 11.0 13.0

Nat 3.5 3.5 3.0 3.5 4.5 13.0

LP/propane 10.0 10.0 9.6 10.0 11.0 13.0

Nat 3.5 3.5 2.9 3.5 4.5 13.0

LP/propane 10.0 10.0 9.6 10.0 11.0 13.0

Pressure Switch and Gas Conversion Kits at all Altitudes

Model

Input Size

045

070 80W52 80W51

090 80W52 80W51

110 80W57 80W52

135 80W52 80W51

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

80W52 80W51

No Change

TABLE 10

High Altitude

Natural Gas Burner

Orifice Kit

73W37 11K49 11K44 73W81

Natural Gas to LP/Propane

Burner Orifice Kit

Line Pressure in.wg.

Min Max

LP/Propane to

Natural Gas Burner

Orifice 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

air temperatures have been allowed to stabilize, check the
temperature rise. If necessary, adjust the blower speed to

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 Switches

This manually reset switches are 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

After the furnace has been started, and supply and return

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 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 in
tegrated 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 33.

Page 28

Twinning 2 ML180UH 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.

FIELD WIRING FOR TWINNING THE ML180UH

The 24 VAC secondary of the two systems must be in
phase. All thermostat connections are made to one control
only. Figure 32 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.

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

C

FIGURE 32

Page 29

HEAT FAN‐OFF TIME IN SECONDS

Service

WARNING

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

60

off Time

90

120

180

180 Second

off Time

60

90

120

180

FIGURE 33

NOTE - Do not secure the electrical conduit directly to the
air ducts or structure.

Electrical

1 - Check all wiring for loose connections.
2 - Check for the correct voltage at the furnace (furnace

operating). Correct voltage is 120VAC +

10%.

3 - Check amp-draw on the blower motor with inner blow

er panel in place.
Unit Nameplate__________Actual__________

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.

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.

Electronic Ignition

The integrated control has an added feature of an internal
Watchguard control. The feature serves as an automatic
reset device for integrated control lockout caused by igni
tion failure. This type of lockout is usually due to low gas
line pressure. After one hour of continuous thermostat de
mand for heat, the Watchguard will break and remake ther
mostat demand to the furnace and automatically reset the
integrated control to begin the ignition sequence.

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.

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
tion procedure. This procedure can be viewed at
www.ahrinet.org

.

.

Page 30

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.

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 H049.

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
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.

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

ry limit switch and make-up box then disconnect them.

3 - Disconnect gas supply piping. Remove the screw se

curing the burner box cover and remove cover. Re
move the four screws securing the burner manifold as
sembly to the vestibule panel and remove the
assembly from the unit.

4 - 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 34 shows burner detail.

5 - Reinstall burner box, manifold assembly and burner

box cover.

6 - Re-install gas supply and turn on electrical power to

furnace.

Page 31

Ignitor

ML180UH BURNER ASSEMBLY & REMOVAL

Burner Detail Front View

Rollout Switches

Sensor

Manifold And Gas Valve

FIGURE 34

Cross Over

Retention Rings

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: ML180UH045P24A-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 32