11
WARNING:
Do not allow combustion products to enter the
circulating air supply. Failure to prevent the
circulation of combustion products into the
living space can create potentially hazardous
conditions including carbon monoxide
poisoning that could result in personal injury
or death.
All return ductwork must be secured to
the furnace with sheet metal screws. For
installations in confi ned spaces, all return
ductwork must be adequately sealed. When
return air is provided through the bottom of the
furnace, the joint between the furnace and the
return air plenum must be air tight.
The surface that the furnace is mounted on must
provide sound physical support of the furnace
with no gaps, cracks or sagging between the
furnace and the fl oor or platform.
Return air and circulating air ductwork must
not be connected to any other heat producing
device such as a fi replace insert, stove, etc. This
may result in fi re, explosion, carbon monoxide
poisoning, personal injury, or property damage.
Plenums and Air Ducts
• Plenums and air ducts must be installed in accordance
with the Standard for the Installation of Air Conditioning
and Ventilating Systems (NFPA No. 90A) or the
Standard for the Installation of Warm Air Heating and
Air Conditioning Systems (NFPA No. 90B).
• Tables 5 - 7 (pages 29 - 32) contain the maximum airfl ow
and temperature rise data for fi xed and variable speed
motors. If the maximum airfl ow is 1,600 CFM or more,
it is recommended that two openings be used for return
air on upfl ow furnaces. Downfl ow furnaces can only use
one return opening.
• It is recommended that the outlet duct contain a
removable access panel. The opening should be
accessible when the furnace is installed in service and
shall be of a size that smoke or refl ected light may be
observed inside the casing to indicate the presence of
leaks in the heat exchanger. The cover for the opening
shall be attached in such a manner as to prevent leaks.
• If outside air is used as return air to the furnace for
ventilation or to improve indoor air quality, the system
must be designed so that the return air is not less than
60° F (15° C) during operation. If a combination of indoor
and outdoor air is used, the ducts and damper system
must be designed so that the return air supply to the
furnace is equal to the return air supply under normal,
indoor return air applications.
• When a cooling system is installed which uses the
furnace blower to provide airfl ow over the indoor coil,
the coil must be installed downstream (on the outlet
side) of the furnace or in parallel with the furnace.
• If a cooling system is installed in parallel with the
furnace, a damper must be installed to prevent chilled
air from entering the furnace and condensing on the
heat exchanger. If a manually operated damper is
installed, it must be designed so that operation of
the furnace is prevented when the damper is in the
cooling position and operation of the cooling system is
prevented when the damper is in the heating position.
• It is good practice to seal all connections and joints
with industrial grade sealing tape or liquid sealant.
Requirements for sealing ductwork vary from region
to region. Consult with local codes for requirements
specifi c to your area.
Supply Air Connections
The supply air must be delivered to the heated space by
duct(s) secured to the furnace casing, running full size
and without interruption.
Upfl ow and Horizontal Furnaces
To attach the supply air duct to the furnace, bend the
furnace fl anges (Figure 20, page 28) upward 90° with a
pair of wide duct pliers. Position the duct on top of the
furnace and secure together with sheet metal screws. The
screws must penetrate the sheet metal casing and furnace
fl ange. Tape or seal all seams if required by local code.
Downfl ow Furnaces
To attach the supply air duct to the downfl ow furnace,
position the furnace over the duct and secure together
with sheet metal screws. The screws must penetrate the
duct and furnace casing.
Return Air Connections
In applications where the supply ducts carry heated air
to areas outside the space where the furnace is installed,
the return air must be delivered to the furnace by duct(s)
secured to the furnace casing, running full size and
without interruption.
Upfl ow and Horizontal Furnaces
For upfl ow installations, the return air ductwork may
be connected to the left side, right side, or bottom. The
bottom panel (Figure 20) must be installed for left or right
return air.
NOTE: Do not use the back of the furnace
for return air.
Side Return Installations
To attach the return air duct to the left or right side of the
furnace, punch out the four knockouts (Figure 20) from
the preferred side of the furnace. Using sharp metal
CIRCULATING AIR REQUIREMENTS
