Unitary products group GM9T-UP User manual

272317-UIM-A-0407
HIGH EFFICIENCY
TUBULAR HEAT EXCHANGER SERIES
MODELS: GM9T-UP
(Two Stage Upflow)
40 - 120 MBH INPUT
(11.7 - 35.1 KW) INPUT
INSTALLATION MANUAL
EFFICIENCY
RATING
CERTIFIED
ISO 9001
Certified Quality
Management System
SECTION I: SAFETY
This is a safety alert symbol. When you see this symbol on
labels or in manuals, be alert to the potential for personal
injury.
Understand and pay particular attention to the signal words DANGER,
WARNING, or CAUTION.
DANGER indicates an imminently hazardous situation, which, if not
avoided, will result in death or serious injury.
WARNING indicates a potentially hazardous situation, which, if not
avoided, could result in death or serious injury.
CAUTION indicates a potentially hazardous situation, which, if not
avoided may result in minor or moderate injury. It is also used to
alert against unsafe practices and hazards involving only property dam-
age.
Improper installation may create a condition where the operation of
the product could cause personal injury or property damage.
Improper installation, adjustment, alteration, service or mainte-
nance can cause injury or property damage. Refer to this manual
for assistance or for additional information, consult a qualified con-
tractor, installer or service agency.
This product must be installed in strict compliance with the installa-
tion instructions and any applicable local, state, and national codes
including, but not limited to building, electrical, and mechanical
codes.
LIST OF SECTIONS
SAFETY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
DUCTWORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
FILTERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
GAS PIPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
ELECTRICAL POWER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
COMBUSTION AIR AND VENT SYSTEM . . . . . . . . . . . . . . . . . . . . .13
CONDENSATE PIPING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
SAFETY CONTROLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
START-UP AND ADJUSTMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . .21
WIRING DIAGRAM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
LIST OF FIGURES
Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Gas Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Gas Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Electrical Wiring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Thermostat Chart - AC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Thermostat Chart - HP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Accessory Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Twinning Wiring Diagram - 1H/1C Thermostat . . . . . . . . . . . . . . . . .13
Twinning Wiring Diagram - 2H/1C Thermostat . . . . . . . . . . . . . . . . .13
Twinning Wiring Diagram - 2H/2C Thermostat . . . . . . . . . . . . . . . . .13
Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Home Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Termination Configuration - 1 Pipe . . . . . . . . . . . . . . . . . . . . . . . . . .17
Termination Configuration - 2 Pipe . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Termination Configuration - 2 Pipe Basement . . . . . . . . . . . . . . . . . .17
Double Horizontal Sealed Combustion Air and Vent Termination . . .18
Double Vertical Sealed Combustion Air and Vent Termination . . . . .18
Sealed Combustion Air Intake Connection and Vent Connection . . .18
Combustion Airflow Path Through The Furnace Casing . . . . . . . . . .18
Outside and Ambient Combustion Air . . . . . . . . . . . . . . . . . . . . . . . . .19
Attic and Crawl Space Combustion Air Termination . . . . . . . . . . . . . .20
Condensate Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Pressure Switch Tube Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Gas Valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
Reading Gas Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
Furnace Control Board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
Wiring Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
LIST OF TABLES
Unit Clearances to Combustibles . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Minimum Duct Sizing For Proper Airflow . . . . . . . . . . . . . . . . . . . . . . .5
Round Duct Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Recommended Filter Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Nominal Manifold Pressure - High Fire . . . . . . . . . . . . . . . . . . . . . . . . 8
Nominal Manifold Pressure - Low Fire . . . . . . . . . . . . . . . . . . . . . . . . .8
Electrical and Performance Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Maximum Equivalent Pipe Length . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Equivalent Length of Fittings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Elbow Dimensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Combustion Air Intake and Vent Connection Size at Furnace
(All Models) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15
Estimated Free Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Free Area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Unconfined Space Minimum Area in Square Inch . . . . . . . . . . . . . . .19
Inlet Gas Pressure Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
Nominal Manifold Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
Blower Performance CFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Filter Performance - Pressure Drop Inches W.C. and (kPa) . . . . . . . .27
Field Installed Accessories - Non Electrical . . . . . . . . . . . . . . . . . . . .28

272317-UIM-A-0407
2Unitary Products Group
SPECIFIC SAFETY RULES AND PRECAUTIONS
1. Only Natural gas or Propane (LP) gas are approved for use with
this furnace. Refer to the furnace rating plate or Section IV of
these instructions.
2. Install this furnace only in a location and position as specified in
SECTION I of these instructions.
3. A gas-fired furnace for installation in a residential garage must be
installed as specified in SECTION I of these instructions.
4. Provide adequate combustion and ventilation air to the furnace
space as specified in SECTION VI of these instructions.
5. Combustion products must be discharged outdoors. Connect this
furnace to an approved vent system only, as specified in SEC-
TION VI of these instructions.
6. Tests for gas leaks as specified in SECTION XI of these instruc-
tions.
7. Always install the furnace to operate within the furnace’s intended
temperature rise range. Only connect the furnace to a duct system
which has an external static pressure within the allowable range,
as specified on the furnace rating plate.
8. When a furnace is installed so that supply ducts carry air circulated
by the furnace to areas outside the space containing the furnace,
the return air shall also be handled by duct(s) sealed to the fur-
nace casing and terminating outside the space containing the fur-
nace.
9. It is permitted to use the furnace for heating of buildings or struc-
tures under construction. Installation must comply with all manu-
facturer’s installation instructions including:
• Proper vent installation;
• Furnace operating under thermostatic control;
• Return air duct sealed to the furnace;
• Air filters in place;
• Set furnace input rate and temperature rise per rating plate
marking;
• Means for providing outdoor air required for combustion;
• Return air temperature maintained between 55ºF (13ºC) and
80ºF (27ºC);
• The air filter must be replaced upon substantial completion of
the construction process;
• Clean furnace, duct work and components upon substantial
completion of the construction process, and verify furnace-
operating conditions including ignition, input rate, temperature
rise and venting, according to the manufacturer’s instructions.
10. When installed in a Non-HUD-Approved Modular Home or building
constructed on-site, combustion air shall not be supplied from
occupied spaces.
11. The size of the unit should be based on an acceptable heat loss
calculation for the structure. ACCA, Manual J or other approved
methods may be used.
SAFETY REQUIREMENTS
• This furnace should be installed in accordance with all national
and local building/safety codes and requirements, local plumbing
or wastewater codes, and other applicable codes. In the absence
of local codes, install in accordance with the National Fuel Gas
Code ANSI Z223.1/NFPA 54, National Fuel Gas Code, and/or
CAN/CGA B149.1 Natural Gas and Propane Installation Code
(latest editions). Furnaces have been certified to the latest edition
of standard ANSI Z21-47 • CSA 2.3.
• Refer to the unit rating plate for the furnace model number, and
then see the dimensions page of this instruction for return air ple-
num dimensions in Figure 1. The plenum must be installed
according to the instructions.
• Provide clearances from combustible materials as listed under
Clearances to Combustibles in Table 1.
• Provide clearances for servicing ensuring that service access is
allowed for both the burners and blower.
• These models ARE NOT CSA listed or approved for installation
into a HUD Approved Modular Home or a Manufactured
(Mobile) Home.
• This furnace is not approved for installation in trailers or recre-
ational vehicles.
• Failure to carefully read and follow all instructions in this
manual can result in furnace malfunction, death, personal
injury and/or property damage.
• Furnaces for installation on combustible flooring shall not be
installed directly on carpeting, tile or other combustible material
other than wood flooring.
• Check the rating plate and power supply to be sure that the elec-
trical characteristics match. All models use nominal 115 VAC, 1
Phase, 60-Hertz power supply. DO NOT CONNECT THIS APPLI-
ANCE TO A 50 HZ POWER SUPPLY OR A VOLTAGE ABOVE
130 VOLTS.
• Furnace shall be installed so the electrical components are pro-
tected from water.
• Installing and servicing heating equipment can be hazardous due
to the electrical components and the gas fired components. Only
trained and qualified personnel should install, repair, or service
gas heating equipment. Untrained service personnel can perform
basic maintenance functions such as cleaning and replacing the
air filters. When working on heating equipment, observe precau-
tions in the manuals and on the labels attached to the unit and
other safety precautions that may apply.
• These instructions cover minimum requirements and conform to
existing national standards and safety codes. In some instances
these instructions exceed certain local codes and ordinances,
especially those who have not kept up with changing residential
and non-HUD modular home construction practices. These
instructions are required as a minimum for a safe installation.
COMBUSTION AIR QUALITY
(LIST OF CONTAMINANTS)
The furnace will require OUTDOOR AIR for combustion when the fur-
nace is located in any of the following environments.
• Restricted Environments
• Commercial buildings
• Buildings with indoor pools
• Furnaces installed in laundry rooms
• Furnaces installed in hobby or craft rooms
• Furnaces installed near chemical storage areas
• Chemical Exposure
The furnace will require OUTDOOR AIR for combustion when the fur-
nace is located in an area where the furnace is being exposed to the fol-
lowing substances and / or chemicals.
• Permanent wave solutions
• Chlorinated waxes and cleaners
• Chlorine based 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
When outdoor air is used for combustion, the combustion air intake pipe
termination must be located external to the building and in an area
where there will be no exposure to the substances listed above.
FIRE OR EXPLOSION HAZARD
Failure to follow the safety warnings exactly could result in serious
injury, death or property damage.
Never test for gas leaks with an open flame. Use a commercially
available soap solution made specifically for detection of leaks to
check all connections. A fire or explosion may result causing prop-
erty damage, personal injury or loss of life.

272317-UIM-A-0407
Unitary Products Group 3
INSPECTION
As soon as a unit is received, it should be inspected for possible dam-
age during transit. If damage is evident, the extent of the damage
should be noted on the carrier’s freight bill. A separate request for
inspection by the carrier’s agent should be made in writing. Also, before
installation the unit should be checked for screws or bolts, which may
have loosened in transit. There are no shipping or spacer brackets
which need to be removed.
FURNACE LOCATION AND CLEARANCES
The furnace shall be located using the following guidelines:
1. Where a minimum amount of air intake/vent piping and elbows will
be required.
2. As centralized with the air distribution as possible.
3. Where adequate combustion air will be available (particularly
when the appliance is not using outdoor combustion air).
4. Where it will not interfere with proper air circulation in the confined
space.
5. Where the outdoor combustion air/vent terminal will not be blocked
or restricted. Refer to “COMBUSTION AIR / VENT CLEAR-
ANCES” located in SECTION VII of these instructions. These min-
imum clearances must be maintained in the installation.
6. Where the unit will be installed in a level position with no more
than 1/4” (6.4 mm) slope side-to-side and front-to-back to provide
proper condensate drainage.
Installation in freezing temperatures:
1. Furnace shall be installed in an area where ventilation facilities
provide for safe limits of ambient temperature under normal oper-
ating conditions. Ambient temperatures must not fall below 32°F
(0°C) unless the condensate system is protected from freezing.
2. Do not allow return air temperature to be below 55º F (13° C) for
extended periods. To do so may cause condensation to occur in
the main heat exchanger, leading to premature heat exchanger
failure.
3. If this furnace is installed in any area where the ambient tempera-
ture may drop below 32° F (0° C), a UL listed self-regulated heat
tape must be installed on any condensate drain lines. It is required
that self regulating heat tape rated at 3 watts per foot be used.
This must be installed around the condensate drain lines in the
unconditioned space. Always install the heat tape per the manu-
facturer's instructions. Cover the self-regulating heat tape with
fiberglass, Armaflex or other heat resistant insulating material.
4. If this unit is installed in an unconditioned space and an extended
power failure occurs, there will be potential damage to the conden-
sate trap, drain lines and internal unit components. Following a
power failure situation, do not operate the unit until inspection and
repairs are performed.
Clearances for access:
Ample clearances should be provided to permit easy access to the unit.
The following minimum clearances are recommended:
1. Twenty-four (24) inches (61 cm) between the front of the furnace
and an adjacent wall or another appliance, when access is
required for servicing and cleaning.
2. Eighteen (18) inches (46 cm) at the side where access is required
for passage to the front when servicing or for inspection or
replacement of flue/vent connections.
In all cases, accessibility clearances shall take precedence over clear-
ances for combustible materials where accessibility clearances are
greater.
Installation in a residential garage:
A gas-fired furnace for installation in a residential garage must be
installed so the burner(s) and the ignition source are located not less
than 18 inches (46 cm) above the floor, and the furnace must be located
or protected to avoid physical damage by vehicles.
The furnace area must not be used as a broom closet or for any
other storage purposes, as a fire hazard may be created. Never
store items such as the following on, near or in contact with the fur-
nace.
1. Spray or aerosol cans, rags, brooms, dust mops, vacuum
cleaners or other cleaning tools.
2. Soap powders, bleaches, waxes or other cleaning com-
pounds; plastic items or containers; gasoline, kerosene, ciga-
rette lighter fluid, dry cleaning fluids or other volatile fluid.
3. Paint thinners and other painting compounds.
4. Paper bags, boxes or other paper products
Never operate the furnace with the blower door removed. To
do so could result in serious personal injury and/or equipment
damage.
FOR FURNACES INSTALLED IN THE COMMON-
WEALTH OF MASSACHUSETTS ONLY
For all side wall horizontally vented gas fueled equipment installed in
every dwelling, building or structure used in whole or in part for resi-
dential purposes, including those owned or operated by the Com-
monwealth and where the side wall exhaust vent termination is less
than seven (7) feet above finished grade in the area of the venting,
including but not limited to decks and porches, the following require-
ments shall be satisfied:
1. INSTALLATION OF CARBON MONOXIDE DETECTORS. At
the time of installation of the side wall horizontal vented gas
fueled equipment, the installing plumber or gasfitter shall
observe that a hard wired carbon monoxide detector with an
alarm and battery back-up is installed on the floor level where
the gas equipment is to be installed. In addition, the installing
plumber or gasfitter shall observe that a battery operated or
hard wired carbon monoxide detector with an alarm is installed
on each additional level of the dwelling, building or structure
served by the side wall horizontal vented gas fueled equipment.
It shall be the responsibility of the property owner to secure the
services of qualified licensed professionals for the installation of
hard wired carbon monoxide detectors
a. In the event that the side wall horizontally vented gas
fueled equipment is installed in a crawl space or an attic,
the hard wired carbon monoxide detector with alarm and
battery back-up may be installed on the next adjacent floor
level.
b. In the event that the requirements of this subdivision can
not be met at the time of completion of installation, the
owner shall have a period of thirty (30) days to comply with
the above requirements; provided, however, that during
said thirty (30) day period, a battery operated carbon mon-
oxide detector with an alarm shall be installed.
2. APPROVED CARBON MONOXIDE DETECTORS. Each car-
bon monoxide detector as required in accordance with the
above provisions shall comply with NFPA 720 and be ANSI/UL
2034 listed and IAS certified.
3. SIGNAGE. A metal or plastic identification plate shall be perma-
nently mounted to the exterior of the building at a minimum
height of eight (8) feet above grade directly in line with the
exhaust vent terminal for the horizontally vented gas fueled
heating appliance or equipment. The sign shall read, in print
size no less than one-half (1/2) inch in size, "GAS VENT
DIRECTLY BELOW. KEEP CLEAR OF ALL OBSTRUC-
TIONS".
4. INSPECTION. The state or local gas inspector of the side wall
horizontally vented gas fueled equipment shall not approve the
installation unless, upon inspection, the inspector observes car-
bon monoxide detectors and signage installed in accordance
with the provisions of 248 CMR 5.08(2)(a)1 through 4.
Improper installation in an ambient below 32ºF (0.0° C) could create
a hazard, resulting in damage, injury or death.

272317-UIM-A-0407
4Unitary Products Group
SECTION II: DUCTWORK
DUCTWORK GENERAL INFORMATION
The duct system’s design and installation must:
1. Handle an air volume appropriate for the served space and within
the operating parameters of the furnace specifications.
2. Be installed in accordance with standards of NFPA (National Fire
Protection Association) as outlined in NFPA pamphlets 90A and
90B (latest editions) or applicable national, provincial, or state, and
local fire and safety codes.
3. Create a closed duct system. For residential and Non-HUD Modu-
lar Home installations, when a furnace is installed so that the sup-
ply ducts carry air circulated by the furnace to areas outside the
space containing the furnace, the return air shall also be handled
by a duct(s) sealed to the furnace casing and terminating outside
the space containing the furnace.
4. Complete a path for heated or cooled air to circulate through the
air conditioning and heating equipment and to and from the condi-
tioned space.
When the furnace is used in conjunction with a cooling coil, the coil
must be installed parallel with, or in the supply air side of the furnace to
avoid condensation in the primary heat exchanger. When a parallel flow
arrangement is used, dampers or other means used to control airflow
must be adequate to prevent chilled air from entering the furnace. If
manually operated, the damper must be equipped with means to pre-
vent the furnace or the air conditioner from operating unless the damper
is in full heat or cool position.
DUCTWORK INSTALLATION AND SUPPLY PLENUM
CONNECTION
A proper Heat Loss/Gain Calculation should be done on all
installations for proper application of equipment. From this
the ductwork sizing can be calculated, ACCA Manual J and
D and industry standards are helpful.
Attach the supply plenum to the furnace or coil outlet duct
connection flanges. This is typically through the use of S
cleat material when a metal plenum is used. The use of an
approved flexible duct connector is recommended on all installations to
prevent noise transmission. All connections should be sealed to prevent
air leakage. Sheet metal should be crosshatched to eliminate any pop-
ping when the indoor fan is energized.
When replacing an existing furnace, if the existing supply plenum is not
the same size as the new furnace then the existing plenum must be
removed and a new plenum installed that is of the proper size for the
new furnace. The minimum plenum height is 12” (30 cm). If the plenum
is shorter than 12” (30 cm) the turbulent air flow may cause the limit
controls not to operate as designed if at all.
NOTE: When attaching duct flange, do not shoot the screw down into
the casing. Use the formed flange intended for duct flange attachment.
The duct system is a very important part of the installation. If the duct
system is improperly sized the furnace will not operate properly.
The ducts attached to the furnace plenum should be of sufficient size so
that the furnace operates at the specified external static pressure and
within the air temperature rise specified on the nameplate.
Table 2 is a guide for determining whether the rectangular duct system
that the furnace is being connected to be of sufficient size for proper fur-
nace operation.
Use the Example below to help you in calculating the duct area to deter-
mine whether the ducts have sufficient area so that the furnace oper-
ates at the specified external static pressure and within the air
temperature rise specified on the nameplate.
The following are general duct sizing guidelines that may not serve to
requirements of every application.
Example: The furnace input is 80,000 BTUH, 1,200 CFM blower
requirement. The recommended duct area is 216 sq.in, there are two 8
x 12 rectangular ducts attached to the plenum and there are two 7 inch
round ducts attached to the furnace.
1. Take 8 x 12, which equals 96 square inch x 2 = 192 square inches
then go to round duct size located in Table 3.
2. The square inch area for 7 inch round ducts, 38.4 square inch x 2
= 76.8 square inches,
3. Then take the 192 square inch from the rectangular duct and add it
to the 76.8 square inch of round duct. The total square inch of duct
attached to the furnace supply plenum is 268.8 square inches.
This exceeds the recommended 216 square inch of duct.
In this example, the duct system attached to the plenum has a sufficient
area so that the furnace operates at the specified external static pres-
sure and within the air temperature rise specified on the nameplate.
Providing the return duct is properly sized as well.
TABLE 1:
Unit Clearances to Combustibles
Application Top Front Rear Left Side Right Side Flue Floor/
Bottom Closet Alcove Attic
In. (cm) In. (cm) In. (cm) In. (cm) In. (cm) In. (cm)
Upflow 1 (2.5) 3 (7.6) 0 (0) 0 (0) 0 (0) 0 (0) Combustible Yes Yes
The cooling coil must be installed in the supply air duct, down-
stream of the furnace. Cooled air may not be passed over the heat
exchanger.
The duct system must be properly sized to obtain the correct airflow
for the furnace size that is being installed.
Refer to Table 7 and the furnace rating plate for the correct rise
range and static pressures
If the ducts are undersized, the result will be high duct static pres-
sures and/or high temperature rises which can result in a heat
exchanger OVERHEATING CONDITION. This condition can result
in premature heat exchanger failure, which can result in personal
injury, property damage, or death.

272317-UIM-A-0407
Unitary Products Group 5
1. The Air Temperature Rise is determined by subtracting the Return
Air Temperature Reading from the Supply Air Temperature Read-
ing.
2. The External Static Pressure is determined by adding the Supply
Duct Static Pressure reading to the Return Duct Static Pressure
reading and adding the pressure drop across any applied coil.
TABLES 2 AND 3 are to be used as a guide only to help the installer
determine if the duct sizes are large enough to obtain the proper air flow
(CFM) through the furnace. TABLES 2 and 3 ARE NOT to be used to
design ductwork for the building where the furnace is being installed.
There are several variables associated with proper duct sizing that are
not included in the tables. To properly design the ductwork for the build-
ing, Refer to the ASHRAE Fundamentals Handbook, Chapter on
“DUCT DESIGN” or a company that specializes in Residential and Mod-
ular Home duct designs.
IMPORTANT: The minimum plenum height is 12” (30 cm). The furnace
will not operate properly on a shorter plenum height. The minimum rec-
ommended rectangular duct height is 4” (10 cm) attached to the ple-
num.
IMPORTANT: The air temperature rise should be taken only after the
furnace has been operating for at least 15 minutes. Temperatures and
external static pressures should be taken 6” (15 cm) past the first bend
from the furnace in the supply duct and the return duct. If an external fil-
ter box or an electronic air cleaner is installed, take the return air read-
ings before the filter box or air cleaner.
If a matching cooling coil is used, it may be placed directly on the fur-
nace outlet and sealed to prevent leakage. Follow the coil instructions
for installing the supply plenum. On all installations without a coil, a
removable access panel is recommended in the outlet duct such that
smoke or reflected light would be observable inside the casing to indi-
cate the presence of leaks in the heat exchanger. This access cover
shall be attached in such a manner as to prevent leaks.
TABLE 2:
Minimum Duct Sizing For Proper Airflow
Input Airflow Return1Rectangular2Round2Supply3Rectangular2Round2
BTU/H (kW) CFM (m³/min) In² (cm²) in. x in. (cm x cm) in. (cm) dia. In² (cm²) in. x in. (cm x cm) in. (cm) dia.
40,000(17.6) 1,200 (34.0) 280 (1806) 14 x 20 (35.6 x 50.8) 18 (45.7) 216 (1393) 12 x 18 (30.5 x 45.7) 16 (40.6)
60,000 (17.6) 1,200 (34.0) 280 (1806) 14 x 20 (35.6 x 50.8) 18 (45.7) 216 (1393) 12 x 18 (30.5 x 45.7) 16 (40.6)
80,000 (23.4) 1,200 (34.0) 280 (1806) 14 x 20 (35.6 x 50.8) 18 (45.7) 216 (1393) 12 x 18 (30.5 x 45.7) 16 (40.6)
80,000 (23.4) 1,600 (45.3) 360 (2322) 18 x 20 (45.7 x 50.8) 22 (55.8) 280 (1806) 14 x 20 (35.6 x 50.8) 18 (45.7)
100,000 (29.3) 1,600 (45.3) 360 (2322) 18 x 20 (45.7 x 50.8) 22 (55.8) 280 (1806) 14 x 20 (35.6 x 50.8) 18 (45.7)
100,000 (29.3) 2,000 (56.6) 440 (2838) 20 x 22 (50.8 x 55.8) 24 (60.9) 390 (2516) 16 x 22 (40.6 x 55.8) 22 (55.8)
120,000 (35.2) 2,000 (56.6) 440 (2838) 20 x 22 (50.8 x 55.8) 24 (60.9) 390 (2516) 16 x 22 (40.6 x 55.8) 22 (55.8)
NOTE: This chart does not replace proper duct sizing calculations or take into account static pressure drop for run length and fittings. Watch out for the temperature rise
and static pressures.
1. Maximum return air velocity in rigid duct @ 700 feet per minute (213 m/min).
2. Example return main trunk duct minimum dimensions.
3. Maximum supply air velocity in rigid duct @ 900 feet per minute (274m/min)..
TABLE 3: Round Duct Size
Round Duct Size Calculated Area For Each Round Duct Size
inches (cm) Sq.in (cm2)
5 (13) 19.6 (126)
6 (15) 28.2 (182)
7 (18) 38.4 (248)
8 (20) 50.2 (324)
9 (23) 63.6 (410)
10 (25) 78.5 (506)
11 (28) 95 (613)
12 (30) 113.1 (730)
13 (33) 132.7 (856)
14 (36) 153.9 (993) The supply air temperature MUST NEVER exceed the Maximum
Supply Air Temperature, specified on the nameplate.
Operating the furnace above the maximum supply air temperature
will cause the heat exchanger to overheat, causing premature heat
exchanger failure. Improper duct sizing, dirty air filters, incorrect
manifold pressure, incorrect gas orifice and/or a faulty limit switch
can cause the furnace to operate above the maximum supply air
temperature. Refer to sections II and III for additional information on
correcting the problem.