Duct systems and register sizes must be properly designed
for the CFM and external static pressure rating of the furnace.
Design the ductwork in accordance with the recommended
methods of "Air Conditioning Contractors of America"
Install the duct system in accordance with Standards of the
National Board of Fire Underwriters for the Installation of Air
Conditioning, Warm Air Heating and Ventilating Systems.
Pamphlets No. 90A and 90B.
A closed return duct system must be used, with the return
duct connected to the furnace.
NOTE: Ductwork must never be attached to the back of the
Flexible joints may be used for supply and return connections
to reduce noise transmission.
To prevent the blower from interfering with combustion air or
draft when a central return is used, a connecting duct must
be installed between the unit and the utility room wall.
Never use a room, closet or alcove as a return air chamber.
When the furnace is used in connection with a cooling unit, the
furnace should be installed in parallel with or on the upstream
side of the cooling unit to avoid condensation in the heating
With a parallel flow arrangement, the dampers or other means
used to control the flow of air must be adequate to prevent
chilled air from entering the furnace and, if manually operated,
must be equipped with means to prevent operation of either unit
unless the damper is in the full heat or cool position.
When the furnace is installed without a cooling coil, it is
recommended that a removable access panel be provided in the
outlet air duct. This opening shall be accessible when the furnace
is installed and shall be of such a size that the heat exchanger
can be viewed for visual light inspection or such that a sampling
probe can be inserted into the airstream. The access panel must
be made to prevent air leaks when the furnace is in operation.
When the furnace is heating, the temperature of the return air
entering the furnace must be between 55°F and 100°F (13ºC and
Bottom Return Air Opening—Upflow Models
The bottom return air opening on upflow models utilizes a "lance
and cut" method to remove sheet metal from the duct opening in
the base pan.
1. To remove, simply press out the lanced sections by hand to
expose the metal strips retaining the sheet metal over the
2. Using tin snips, cut the metal strips and remove the sheet
metal covering the duct opening.
CIRCULATING AIR AND FILTERS
3. In the corners of the opening, cut the sheet metal along the
scribe lines to free the duct flanges.
4. Using the scribe line along the duct flange as a guide, unfold
the duct flanges around the perimeter of the opening using a
pair of seamer pliers or seamer tongs.
NOTE: Airflow area will be reduced by approximately 18% if
duct flanges are not unfolded. This could cause performance
issues and noise issues.
Edges of sheet metal holes may be sharp. Use gloves as
a precaution when removing hole plugs.
Duct Flange Cutouts
A. Cut using tin snips.
C. Scribe lines outlining duct flanges.
B. Press out by hand.
D. Cut 4 corners after removing sheet metal.
Filters—Read This Section Before Installing the
Return Air Ductwork
Filters must be used with this furnace. Discuss filter
maintenance with the building owner.
Filters do not ship with this furnace, but must be provided,
sized and installed externally by the installer.
Filters must comply with UL900 or CAN/ULCS111 standards.
If the furnace is installed without filters, the warranty will be
On upflow units, guide dimples locate the side return cutout
locations. Use a straight edge to scribe lines connecting the
dimples. Cut out the opening on these lines.
NOTE: An undersized opening will cause reduced airflow.