Lennox ML180UHT Installation Instructions Manual

INSTALLATION

 2015 Lennox Industries Inc.

Dallas, Texas, USA

AIR FLOW

UPFLOW

INSTRUCTIONS
ML180UHT

MERIT® SERIES GAS FURNACE
UPFLOW / HORIZONTAL AIR DISCHARGE

507408-01
06/2015 Supersedes 507061-01

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 installer (or
equivalent), service agency or the gas supplier.

Litho U.S.A.

AIR FLOW

AIR FLOW

HORIZONTAL LEFT

HORIZONTAL RIGHT

Los calefactores están
aprobados por el Instituto
del Gas Argentino (IGA)
para funcionar con gas
natural y no se pueden
convertir para funcionar
con gas licuado.

Table of Contents

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

ML180UHT Gas Furnace 3.........................

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

Requirements 3...............................

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

Combustion, Dilution & Ventilation Air 4..............

Setting Equipment 7...............................

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

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

Venting 12........................................

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 19.....................................

Electrical 21.......................................

Integrated Control 22...............................

Unit Start-Up 24...................................

Gas Pressure Adjustment 25........................

Proper Combustion 25..............................

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

Other Unit Adjustments 26..........................

Service 27........................................

Repair Parts 28...................................

Page 1

Unit Dimensions - 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
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.

D

FLUE OUTLET

(Top)

3

OPTIONAL

EXTERNAL

SIDE RETURN

AIR FILTER KIT

(Either Side)

79

SUPPLY AIR

OPENING

603

635

TOP VIEW

19

A

B

C

1

Bottom Return

Air Opening

FRONT VIEW

14

3

OPTIONAL

EXTERNAL

SIDE RETURN

AIR FILTER KIT

(Either Side)

375

406

16

19

838

19

Front Panel

76 Right

171 Left

232 Right

219 Left

137 Right

40 Left

ELECTRICAL
INLET (Either Side)

(Either Side)

83

38

83

2

FLUE OUTLET

(Either Side)

GAS PIPING INLET

(Either Side)

1

Side Return
Air Opening
(Either Side)

1

Bottom Return

Air Opening

SIDE VIEW

584

597

705

494

14

38

356

49

ML180UHT Model No.

070TP36A

090TP48B

110TP36C
110TP60C

135TP60D

A B C D

mm mm mm mm

368 340 330 121

446 416 406 159

533 504 495 203

622 546 584 248

Page 2

ML180UHT Gas Furnace

The ML180UHT 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 with bottom return air.

Shipping and Packing List

Package 1 of 1 contains

1 - Assembled ML180UHT 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

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.

Requirements

The ML180UHT furnace is approved for installation for in
side of building only, with clearances as listed in figure 6.
(Accessibility and service clearances must take prece
dence over fire protection clearances). Installation of Len
nox forced air central furnaces must be in accordance with
these instructions, local gas fitting regulations, municipal
building codes and electrical wiring regulations

The acceptable natural gas Wobbe number values for use
with Lennox furnaces are shown in Table 1.

TABLE 1

Wobbe

Number*

Gas Properties 101.3 kPa, 15° C Dry bulb

* Wobbe number = heating value  square root of specific
gravity

Min Normal Max

46.95 mj / m

(1260 Btu / ft3)

(14.73 psia, 60° F Dry Bulb)

3

50.15 mj / m

(1346 Btu / ft3)

3

53.28 mj / m

(1430 Btu / ft3)

3

If you need to know whether a gas is acceptable, request
the Wobbe number from your gas supplier and compare to
the chart. If the number is within the ranges listed for Natu
ral gas, the gas is acceptable to use. If the Wobbe number
is outside the acceptable ranges, the gas is not acceptable
to use.
As LP (Propane) is a regulated/manufactured gas, a
Wobbe number is not required to determine acceptability.
For propane installations, the acceptable heating value
range is 93.2 mj / m

3

(2500 btu/ft3) +/- 5%.

NOTE - Lennox does not recommend using unrefined or
”well head” gas on Lennox gas-fired furnaces. The quality
of gas is not controlled and may have contaminants which
may lead to corrosion, premature heat exchanger failure or
may void the equipment warranty. The information pro
vided above is intended for evaluating fuel gases available
commercially through gas utilities or LP (propane) suppli
ers.

Adequate clearance must be made around the air open
ings into the vestibule area. Provisions shall be made for
proper operation and for combustion air and ventilation air
supply according to local codes or these instructions.

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

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 457 mm (18 in) 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 local
codes.

Unit must be adjusted to obtain a temperature rise within
the range specified on appliance rating plate.

This ML180UHT furnace must be installed so that its elec
trical components are protected from water.

When this furnace is used with cooling units, it shall be
installed in parallel with, or on the upstream side of, cooling
units to avoid condensation in the heating compartment.
With a parallel flow arrangement, a damper (or other
means to control the flow of air) must adequately prevent
chilled air from entering the furnace. If the damper is manu
ally operated, it must be equipped to prevent operation of
either the heating or the cooling unit, unless it is in the full

HEAT or COOL setting.

When installed, this furnace must be electrically
grounded according to local codes.

Page 3

All field wiring must comply with local codes or what is
listed in these instructions.

When furnace is installed so that supply ducts carry air cir
culated by furnace to areas outside of space containing
ufurnace, return air shall be handled by a duct(s) sealed to
a furnace casing and terminating outside space containing
furnace.

NOTE - ML180UHT series units must not be used as a
“construction heater” during any phase of construction.
Very low temperatures, harmful vapors and misplacement
of the filters will damage the unit and its efficiency.

The ML180UHT furnace may be installed in alcoves, clos
ets, attics, basements, garages, crawl spaces and utility
rooms in any of its approved installation positions.

This furnace design has not been approved for instal
lation in mobile homes, recreational vehicles, or out
doors.

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.

General

These instructions are intended as a general guide and
do not supersede local codes in any way. Consult au
thorities having jurisdiction before installation.

In addition to the requirements outlined previously, the fol
lowing general recommendations must be considered
when installing a ML180UHT 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.

WARNING

The State of California has determined that this prod
uct may contain or produce a chemical or chemicals,
in very low doses, which may cause serious illness
or death. It may also cause cancer, birth defects or
reproductive harm.

Combustion, Dilution & Ventilation Air

In the past, there was no problem in bringing in sufficient
outdoor air for combustion. Infiltration provided all the air
that was needed. In today's homes, tight construction prac
tices make it necessary to bring in air from outside for 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)
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.

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.

Page 4

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

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.

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 1.42 m
(50 cubic feet) per .29 kW (1,000 Btu) 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 1.42

3

m

(50 cubic feet) per .29 kW (1,000 Btu) 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 645 mm
square inch) per .29 kW (1,000 Btu) per hour of the total input rating
of all equipment in the enclosure, but not less than 64516 mm
square inches).

HEATER

OPENINGS

(To Adjacent

FIGURE 1

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

3

spaces. Each opening must have a minimum free area of
645 mm

2

(1 square inch) per .29 kW (1,000 Btu) per hour of
total input rating of all gas-fired equipment in the confined
space. Each opening must be at least 64516 mm
square inches). One opening shall be within 305 mm (12
inches) of the top of the enclosure and one opening within
305 mm (12 inches) of the bottom. See figure 1.

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 305 mm (12 inches) of the

Room)

2 (

one

2

1(00

2

(100

Page 5

top of the enclosure and one opening within 305 mm (12
inches) 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 indirect
ly through vertical ducts. Each opening shall have a mini
mum free area of 645 mm

2

(1 square inch) per 1.17 kW

(4,000 Btu) per hour of total input rating of all equipment in
the enclosure. See figures 2 and 3. When communicating
with the outdoors through horizontal ducts, each opening
shall have a minimum free area of 645 mm

2

(1 square inch)

per .56 kW (2,000 Btu) per total input rating of all equipment
in the enclosure. See figure 4.

When ducts are used, they shall be of the same cross-sec

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

CHIMNEY OR

GAS VENT

VENTILATION LOUVERS

(Each end of attic)

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 75 mm (3 inches). In calculating free
area, the blocking effect of louvers, grilles, or screens must
be considered. If the design and free area of protective cov
ering is not known for calculating the size opening required,
it may be assumed that wood louvers will have 20 to 25 per
cent free area and metal louvers and grilles will have 60 to
75 percent free area. Louvers and grilles must be fixed in
the open position or interlocked with the equipment so that
they are opened automatically during equipment opera
tion.

EQUIPMENT IN CONFINED SPACE

ALL AIR FROM OUTSIDE

OUTLET

FURNACE

VENTILATION

LOUVERS

(For unheated crawl space)

NOTE-The inlet and outlet air openings shall each have a free area of at least 645 mm
square inch) per 1.17 kW (4,000 Btu) per hour of the total input rating of all equipment in the enclo
sure.

AIR

WATER

HEATER

INLET

AIR

2

(one

FIGURE 2

Page 6

CHIMNEY

OR GAS

VENT

FURNACE

EQUIPMENT IN CONFINED SPACE

ALL AIR FROM OUTSIDE

(All Air Through Ventilated Attic)

VENTILATION LOUVERS

(Each end of attic)

OUTLET

AIR

INLET AIR

(Ends 12 in.

above bottom)

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 ML180UHT 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 [610 mm (24 inches)] at
unit front. The unit must be level.

WATER

HEATER

NOTE-The inlet and outlet air openings shall each have a
free area of at least 645 mm
kW (4,000 Btu) per hour of the total input rating of all
equipment in the enclosure.

2 (

one square inch) per 1.17

FIGURE 3

EQUIPMENT IN

CONFINED SPACE

ALL AIR FROM

OUTSIDE

OUTLET AIR

INLET AIR

FURNACE

CHIMNEY

OR GAS

VENT

WATER

HEATER

NOTE - Units with 1/2 hp blower motors
(ML180UH110TP36C and ML180UH090TP48B)are
equipped with three flexible legs and one rigid leg. See fig
ure 5. 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 operation. After the bolt and washer have been re

moved, the rigid leg will not touch the blower housing.

NOTE - ML180UH090TP48B, & ML180UH110TP36C
are also equipped with a shipping pad under the blower
housing. Remove the shipping pad prior to operation.

ML180UH110TP36C and

ML180UH090TP48B with 1/2 HP

BLOWER MOTOR

RIGID LEG

remove shipping bolt and washer

NOTE - Each air duct opening shall have a free area of at least 645

2 (

mm

one square inch) per .59 kW (2,000 Btu) 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 645 mm
hour of the total input rating of all other equipment in the enclosure.

2

(one square inch) per 1.17 kW (4,000 Btu) per

FIGURE 4

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.

FIGURE 5

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.

Page 7

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 6.

Upflow Application Installation Clearances

Top

Left Side

Bottom

Type of Vent

Connector

Top 25 mm (in.) 25 mm (1 in.)

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

Back 0 0

Sides 0† 0

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

Floor 0‡ 0‡

*Front clearance in alcove installation must be 610 mm (24 in.).
Maintain a minimum of 610 (24 in.) mm for front service access.
** 114 mm (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 76 mm (3 in.) if a single wall vent is used on 368
mm (14-1/2 in.) cabinets, or 51 mm (2 in.) if a single wall vent pipe
is used on 445 mm (17-1/2 in.) cabinets.

Type C Type B1

Right Side

FIGURE 6

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.

NOTE -

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

1 - Single side return air with transition to accommodate

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

3 - Bottom return air.

4 - Return air from both sides.

5 - Bottom and one side return air.

60C and 60D units that require air volumes over 850

508 x 635 x 25 mm (20 x 25 x 1 in.) cleanable air filter. (Re
quired to maintain proper air velocity.) See figure 7.

figure 8.

Refer to Product Specifications bulletin for additional infor
mation.

Single Side Return Air

(with transition and filter)

508mm X 635mm X 25mm
(20” X 25” X 1”)

Cleanable Filter

Return Air

Plenum

AIR FLOW

Transition

FIGURE 7

Page 8

Optional Return Air Base

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

FURNACE

FRONT

IF BASE
IS USED

INDOOR AIR
QUALITY
CABINET

AIR FLOW

368 (14−1/2) A WIDTH (65W75)
446 (17−1/2) B WIDTH (50W89)
533 (21) C WIDTH (50W99)
622 (24−1/2) 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.

(PCO, FILTER
CABINET, ETC

OPTIONAL
RETURN
AIR BASE

WITHOUT

IAQ CABINET,

A SINGLE

RETURN AIR

PLENUM

MUST

COVER BOTH

UNIT AND

RETURN

AIR BASE

OPENINGS

83 (3−1/4)

184 (7−1/4)

1

584 (26)

Overall

(Maximum)

1

Unit side return air

Opening

143

(5−5/8)

SIDE RETURN

AIR OPENINGS

(Either Side)

683

(26−7/8)

SIDE VIEW

584
(23)

1

Minimum

279 (11)

2

Maximum

356 (14)

Minimum unit side return air opening dimensions for units requiring 850 L/s (1800 cfm) or more of air (W x H): 584 x 279

mm. (23 X 11 in.)

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 356 mm (14 in.)] is preferred.

FIGURE 8

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 9.

1

570
(22)

Overall

(Maximum)

19
(3/4)

Page 9

Removing the Bottom Panel

)

Screw

Allow for clearances to combustible materials as indicated
on the unit nameplate. Minimum clearances for closet or al
cove installations are shown in figure 10.

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 11, or install the furnace on a plat
form, as shown in figure 12.

Typical Horizontal Application

Unit Suspended in Attic or Crawlspace

Metal

Internal

Bracket

Strap

Brace

4.8 mm
(3/16 in

Bottom Cap

Bottom Panel

FIGURE 9

Horizontal Application

Installation Clearances

Left
End

Vent Connector

Type

Top 0 0

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

Back 0 0

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

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

Floor 0‡ 0‡

*Front clearance in alcove installation must be 610 mm (24 in)
Maintain a minimum of 610 mm (24 in.) for front service access.
** 114 mm (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

Air Flow

FIGURE 11

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 9.

FIGURE 10

Horizontal Applications

The ML180UHT furnace can be installed in horizontal ap
plications. Order horizontal suspension kit (51W10) from
Lennox, or use equivalent suspension method.

Page 10

Horizontal Application

Unit Installed on Platform

Line contact is permissible but not preferred

See the unit nameplate for

clearances.

GAS

ENTRY

SERVICE PLATFORM

VENT

PIPE

FIGURE 12

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 2 lists recommended filter sizes.
A filter must be in place any time the unit is operating.

TABLE 2

Furnace

Cabinet

Width

A - 338

(14-1/2”)

B- 446

( 17-1/2”)

C - 533

( 21”)

D - 622

(24-1/2”)

Filter Size mm (in) and Quantity

Side Return Bottom Return

406 X 635 X 25 1

(16 X 25 X 1)

406 X 635 X 25 1

(16 X 25 X 1)

405 X 635 X 25 1

(16 X 25 X 1)

405 X 635 X 25 2

(16 X 25 X 1)

356 X 635 X 25 1

(14 X 25 X 1)

406 X 635 X 25 1

(16 X 25 X 1)

508 X 635 X 25 1

(20 X 25 X 1)

610 X 635 X 25 1

(24 X 25 X 1)

Duct System

Use industry‐approved standards 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.

Page 11

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.

Venting

The ML180UHT is a fan assisted furnace equipped with an
integral mechanical means to either draw or force products
of combustion through the combustion chamber and/or
heat exchanger. When installed in the vertical position and
vented vertically, the furnace operates with a non-positive
vent stack pressure. When installed in the horizontal posi
tion and vented vertically, the furnace operates with a posi
tive stack pressure.

IMPORTANT

The ML180UHT is certified for horizontal installation
but DO NOT vent flue horizontally. The ML180UHT
furnace is not certified for horizontal venting. See
figure 21.

General Venting Requirements

A 102 mm (4 in.) diameter flue transition is factory‐installed
on the combustion air inducer outlet of all models. Figure

14 shows the combustion air inducer as shipped from
the factory.

Mounting Screws Location

Flue Transition

Mounting Screws

Upflow Position

UPFLOW POSITION

Pressure

Switch

Top Vent Discharge

Flue

Transition

AIR

Vent Pipe

Collector Box

FLOW

FIGURE 14

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.

FIGURE 13

If necessary, reposition the combustion air inducer, pres
sure switch and/or make-up box as needed per the follow
ing steps. See figures 14 through 20.

1 - Remove the four mounting screws (figure 13) which

secure the combustion air inducer / pressure switch
assembly to the orifice plate. Lift the inducer assembly
and rotate it 90 degrees clockwise or counterclockwise
to either the 3 o'clock position or to 9 o'clock position.
Re-secure 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

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 229 mm (9”) to 203 mm

(8”) to avoid interference with inducer motor

Collector Box

FIGURE 15

Page 12

Collector Box

Pressure

Switch

UPFLOW POSITION

Right Side Vent Discharge

Cover Plate

Flue Transition

HORIZONTAL RIGHT POSITION

Side Vent Discharge

Pressure Switch

Vent
Pipe

AIR

FLOW

Flue Transition

FLOW

AIR

 Cut combustion air inducer tubing from 229 mm ( 9” ) to 127 mm

(5”) to avoid interference with inducer motor

FIGURE 16

HORIZONTAL LEFT POSITION

Vent Pipe

Pressure

Switch

Flue

Transition

Cover Plate

 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.

Top Vent Discharge

AIR

FLOW

Collector Box

Collector Box

FIGURE 18

HORIZONTAL LEFT POSITION

Side Vent Discharge

Pressure Switch

AIR

FLOW

Vent Pipe

Make-Up Box

 Cut combustion air inducer tubing from 227 mm (9”) to

178 mm (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 17

FIGURE 19

Page 13

HORIZONTAL RIGHT POSITION

Top Vent Discharge

Vent Pipe

Pressure Switch

Flue

Transitio

n

AIR

FLOW

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

bottom in order to accommodate the flue pipe.

 Cut combustion air inducer tubing from 227 mm (9”) to 203

mm (8”) to avoid interference with inducer motor

Cover Plate

Collector Box

FIGURE 20

The ML180UHT 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
ML180UHT 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 21.

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

21.

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 22 and 23 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 22.

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 21. 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

VENT PIPE

(min. 6” length)

FLUE

TRANSITION

COLLAR

“DISCONNECTED

VENT” WARNING

STICKER

FIGURE 21

Page 14

VENT PIPE

(min. 6” length)

“DISCONNECTED

VENT” WARNING

STICKER

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 22
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 ML180UHT 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 ML180UHT 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 15

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 23

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 4) 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 (5 and 6) 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 3

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 16

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 4

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 17

Vent

Height

H

(feet)

6

8

10

15

20

30

TABLE 5

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 6

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 18

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.

Using local codes, resize the common venting system
to the minimum vent pipe size.

Gas Piping

See local codes for gas piping requirements and gas
pipe sizing tables.

Gas supply piping should not allow more than 125 Pa
(0.5”W.C.) drop in pressure between gas meter and unit.
Supply gas pipe must not be smaller than unit gas connec
tion.

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.

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.

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 6.4 mm (1/4 in) per 4.57 m

(15 ft.) upward toward the meter from the furnace. The
piping must be supported at proper intervals every

2.44 to 3.01 m (8 to 10 ft) with suitable hangers or
straps. Install a drip leg in vertical pipe runs to the unit.

5 - A 3 mm (1/8”) N.P.T. plugged tap or pressure post is

located on the gas valve to facilitate test gauge con
nection. See figure 29.

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 19

MANUAL

MAIN SHUT-OFF

VALVE

(With 1/8 in. NPT

Plugged Tap Shown)

GROUND

JOINT

UNION

Left Side Piping

(Standard)

AUTOMAT

IC

GAS VALVE

AUTOMATIC

GAS VALVE

MANUAL

MAIN SHUT-OFF

VALVE

(With 1/8 in. NPT

Plugged Tap

Shown)

GROUND

JOINT

UNION

DRIP LEG

MANUAL

MAIN SHUT-OFF

VALVE

GROUND

JOINT

UNION

DRIP LEG

DRIP LEG

FIELD

PROVIDED

AND INSTALLED

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

FIGURE 24

MANUAL

MAIN SHUT-OFF

VALVE

GROUND

JOINT

UNION

DRIP LEG

Right Side Piping

(Alternate)

Horizontal Application

Left-Side Air Discharge

MAIN SHUT-OFF

MANUAL

VALVE

Horizontal Application

Right-Side Air Discharge

FIELD

PROVIDED

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

AND INSTALLED

FIGURE 25

GROUND

JOINT

UNION

DRIP LEG

Page 20

Leak Check

After gas piping is completed, carefully check all piping
connections (factory- and field-installed) for gas leaks. Use
a leak detecting solution or other preferred means.

NOTE - If emergency shutoff is necessary, shut off the
main manual gas valve and disconnect the main power to
the furnace. The installer should properly label these de
vices.

CAUTION

Some soaps used for leak detection are corrosive to
certain metals. Carefully rinse piping thoroughly af
ter leak test has been completed. Do not use
matches, candles, flame or other sources of ignition
to check for gas leaks.

The furnace must be isolated 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 26. 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

FIGURE 26

ISOLATE

GAS VALVE

FURNACE

CAP

Electrical

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.

Refer to figure 28 schematic wiring diagram.

1 - Ensure that power connection point is located with 2m

of furnace.

2 - Holes are on both sides of the furnace cabinet to facili

tate thermostat wiring.

3 - Before connecting the thermostat wiring, check to

make sure the wires will be long enough for servicing at
a later date. Remove the blower access panel to check
the length of the wire.

4 - Complete the wiring connections to the equipment.

Use the provided unit wiring diagram shown in figure

28. 18-gauge wire or larger that is suitable for Class II
rating for thermostat connections.

5 - Electrically ground the unit according to local codes.

NOTE - The ML180UHT furnace contains electronic
components that are polarity sensitive. Make sure that
the furnace is wired correctly and is properly grounded.

6 - One line voltage “EAC” spade terminal is provided on

the furnace control board. Any electronic air cleaner
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. See figure 27 for
control board configuration. This terminal is energized
when the indoor blower is operating.

7 - One line voltage “HUM” spade terminal is provided on

the furnace control board. Any humidifier rated up to
one amp ca`n be connected to this terminal with the
neutral leg of the circuit being connected to one of the
provided neutral terminals. See figure 27 for control
board configuration. This terminal is energized in the
heating mode when the combustion air inducer is oper
ating.

8 - Install the room thermostat according to the instruc

tions provided with the thermostat. See figure 28 for
thermostat designations. If the furnace is being
matched with a heat pump, refer to the instruction
packaged with the dual fuel thermostat.

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 ML180UHT 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.

Generator Use - Voltage Requirements

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

 The furnace requires 230 volts +
 The furnace operates at 50 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)

10%.
5% .

Page 21

Integrated Control

INTEGRATED CONTROL

(Automatic Hot Surface Ignition System)

TERMINAL DESIGNATIONS

HUM - Humidifier 240VAC
LINE - Input 240VAC
XFMR - Transformer 240VAC
EAC - Electronic Air Cleaner 240VAC
COOL - Cool Speed 240VAC
PARK1, PARK2 - Dead terminals for alternate speed
taps
NEUTRALS - Neutral terminals 240VAC
HEAT - Heating Speed 240VAC
FS - Flame Sense

FIGURE 27

Page 22

ML180UHT Schematic Wiring Diagram

FIGURE 28

Page 23

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 ML180UHT 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:

ML.180UHT 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

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 lever on gas valve to OFF. See figure 29.

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

OUTLET

PORT

INLET

PRESSURE

PORT

8 - Move lever on gas valve to ON. See figure 29.

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.

White Rodgers 3600 Gas Valve

MANIFOLD

PRESSURE

ADJUSTMENT

SCREW

GAS VALVE SHOWN IN OFF POSITION

FIGURE 29

Page 24

4 - Move lever on gas valve to OFF. See figure 29.
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.

Heating Sequence Of Operation
(follow steps below or see Figure 28 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

To check for proper flow to combustion chamber, deter
mine Btu input from appliance rating plate. Divide this input
rating by the Btu per cubic foot of available gas. Result is
the required number of cubic ft. per hour. Determine the
flow of gas through the gas meter for two minutes and multi
ply by 30 to get the hourly flow of gas.

IMPORTANT

The White Rodgers 36G24 gas valve (figure 29) is
equipped with pressure posts for measuring supply
and manifold pressures. The posts provide built-in

7.9mm (5/16”) hose connections and have an inte
gral 2.4mm (3/32”) Allen-head screw. Rotate the
screw counterclockwise one full turn to permit pres
sure measurement. Reseat the screw (rotate one full
turn clockwise) after measurements have been taken
to prevent gas leakage.

Use pressure measurement kit 69M1701 to connect

7.9mm (5/16”) pressure post on gas valve to 6.4mm (1/4”)
hose off the manometer.

Line Pressure Measurement

1 - Check the gas line pressure with the unit firing at maxi

mum rate. See table 8 for maximum and minimum line
pressures.

After the line pressure has been checked and adjusted,
check the manifold pressure per table 8. See figure 29 for
the location of the manifold pressure adjustment screw.

Manifold Pressure Measurement

1 - Connect the outlet side of the gas 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

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

8.

NOTE - Shut unit off and remove manometer as soon as an
accurate reading has been obtained.

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 100 ppm.

TABLE 7

Unit

-070

-090

-110

-135

CO2%

For Nat

6.3 - 7.8 7.0 - 9.0

CO2%

For L.P.

Page 25

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 8 for
manifold pressure and table 9 for pressure switch change
and gas conversion kits.

TABLE 8

Manifold Pressure Settings at all Altitudes

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.

Model

Input Size

All Models

Gas

Nat 0.87 0.72 0.67 0.62 0.87 1.12 3.20

LP/propane 2.49 2.27 2.19 2.12 2.49 2.74 3.20

0-610 m*

(0-2000 ft)

611-914 m*

(2001-3000 ft.)

915-1219 m*

(3001-4000 ft)

1220-1524 m*

(4001-5000 ft.)

1525-1981 m*
(5001-6500 ft)

Line Pressure

kPa

Min Max

* See table 9 for proper high altitude gas conversion kit.

TABLE 9

Pressure Switch and Gas Conversion Kits at all Altitudes

High Altitude

Model

Input Size

070

090 80W51 80W56

110 80W51 80W56

135 No Change 80W51

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.

High Altitude Pressure Switch Kit

0-610 m

(0-2000 ft)

No Change

611-1219 m

(2001-4000 ft)

80W51 80W56

1220-1981 m

(4001-6500 ft)

Other Unit Adjustments

Natural Gas Burner

Orifice Kit

1525-1981 m

(5001-6500 ft)

73W37 11K15 97W04 96W95

air temperatures have been allowed to stabilize, check the

Natural Gas to LP/Propane

Gas Change over Kit

0-1524 m

(0-5000 ft)

1525-1981 m

(5001-6500 ft)

LP/Propane to

Natural Gas

Change over Kit

1-1524 m

(1 5000 ft)

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 top 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 60 seconds. The fan-off delay affects com
fort and is adjustable to satisfy individual applications. Ad
just the fan-off delay to achieve a supply air temperature
between 32°C and 43°C 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 30.

Page 26

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

off Time

60

90

120

180

180 Second

off Time

60

90

120

180

FIGURE 30

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 230VAC +

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 bulletin for indoor blower
data.

NOTE - Termination of any unused motor leads must be in
sulated.

4 - Place unused blower speed tap on integrated control

“PARK” terminal or insulate.

5 - Refer to blower speed selection chart on unit wiring

diagram for desired heating or cooling speed. See
Blower performance data beginning on the next page.

6 - Connect selected speed tap at integrated control

speed terminal.
7 - Resecure blower access panel.
8 - Turn on electrical power to furnace.
9 - Recheck temperature rise.

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.

At the beginning of each heating season, a qualified techni
cian should check the system as follows:

Blower

Check the blower wheel for debris and clean if necessary.
The blower motors are prelubricated for extended bearing
life. No further lubrication is needed.

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

All ML180UHT filters are installed external to the unit. Fil
ters should be inspected monthly. Clean or replace the fil
ters when necessary to ensure that the furnace operates
properly. Replacement filters must be rated for high veloc
ity airflow. Table 2 lists recommended filter sizes.

Flue And Chimney

1 - Check flue pipe, chimney and all connections for tight

ness and to make sure there is no blockage.

2 - Check unit for proper draft.

Electrical

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

operating). Correct voltage is 230VAC +

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

er panel in place.
Unit Nameplate__________Actual__________

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.

10%.

Page 27

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

ML180UHT BURNER ASSEMBLY & REMOVAL

age caused by foreign matter. Remove any blockage.
Figure 31 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.

Retention Rings

Rollout Switches

Sensor

Ignitor

Cross Over

Manifold And Gas Valve

FIGURE 31

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: ML180UH070TP36A-01. All service must

be performed by a licensed professional installer (or equivalent), service agency, or gas supplier.

Cabinet Parts

Upper access panel
Blower panel
Top cap

Control Panel Parts

Transformer
Integrated control
Door interlock switch
Circuit breaker

Blower Parts

Blower wheel
Blower housing
Motor
Motor mounting frame
Motor capacitor
Blower housing cutoff plate

Heating Parts

Flame sensor
Heat exchanger assembly
Gas manifold
Combustion air inducer
Gas valve
Main burner cluster
Main burner orifices
Pressure switch
Ignitor
Primary limit control
Flame rollout switch (s)
Secondary limit

Page 28