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DOWNFLOW RETURN PLENUM CONNECTION
The return air duct and the return air plenum are not required by the fur-
nace manufacturer provided the combustion air and vent system have
been installed and maintained as a Two Pipe Sealed Combustion Air
System and provided a return air duct and plenum are not required by
state, local or regional codes.
• If a return air duct system and return plenum are required by
state, local or regional codes then the return plenum may be con-
nected to the furnace inlet and must terminate outside the space
containing the furnace. The external air filter accessory shown in
Figure 4 is designed and recommended for use inside the return
plenum. Attach the return plenum to the furnace inlet duct
flanges. This is typically through the use of S cleat material when
a metal plenum is used. Installations where a return air duct sys-
tem is utilized, the use of a listed flexible duct connector is recom-
mended. The connection of the plenum to the furnace and all the
ducts connecting to the plenum must be sealed to prevent air
leakage. The sheet metal must be crosshatched to eliminate any
popping of the sheet metal when the indoor fan is energized. The
duct system is a very important part of the installation. If the duct
system is improperly sized, the furnace does not operate prop-
erly. The ducts attached to the furnace must be of sufficient size
so that the furnace operates at the specified external static pres-
sure and within the air temperature rise specified on the furnace
nameplate and the air temperature rise is also specified in Table
5.
• If the return air duct and plenum are not required by state, local or
regional codes then the filter can be attached to the top of the fur-
nace as shown in Figure 6.
IMPORTANT: When replacing an existing furnace, if a return air
plenum is used and the existing plenum is not the same size as the
new furnace, then the existing plenum must be removed and a new
plenum installed that is the proper size for the new furnace. If a
return duct mounted filter rack or filter grille is being used, see the
instructions provided with that accessory for proper opening size.
Downflow Filters
Downflow furnaces typically are installed with the filters located above
the furnace, extending into the return air plenum or duct. Any branch
duct (rectangular or round duct) attached to the plenum must attach to
the vertical plenum above the filter height.
Filter(s) may be located in the duct system external to the furnace using
an external duct filter box attached to the furnace plenum or at the end
of the duct in a return filter grille(s). The use of straps and/or supports is
required to support the weight of the external filter box.
FIGURE 4: Mobile Home Filter Frame Accessories
Johnson Controls Ducted Systems
BLEND AIR INSTALLATIONS
Upflow
This bracket is to be used only for Alcove application on the 90% Gas
Furnaces. It helps in installation of Blend Air Damper Tube close to the
furnace. It is recommended that the Blend Air be installed with the help
of this bracket as close to the furnace as possible. The Blend Air
Bracket Assembly must be installed no higher than 18 in. above the fur-
nace. See Figure 5.
FIGURE 5: Blend Air Installation (Optional)
Downflow
This bracket is to be used only for Alcove application on the 95% Gas
Furnaces. It helps in installation of Blend Air Damper Tube close to the
furnace. It is recommended that the Blend Air be installed with the help
of this bracket as close to the furnace as possible. The Blend Air
Bracket Assembly must be installed as close to the return air grille as
possible. See Figure 6.
FIGURE 6: Blend Air Bracket
5715184-BIM-A-0519
7
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