507868-02
Page 12 of 48
Issue 2128
Figure 17.
Return Air Opening - Downflow Units
Return air may be brought in only through the top opening of
a furnace installed in the downflow position. The following
steps should be taken when installing plenum:
1. Bottom edge of plenum should be flanged with a
hemmed edge (see Figure 18 or Figure 19).
2. Sealing strips should be used to ensure an airtight
seal between the cabinet and the plenum.
3. In all cases, plenum should be secured to top of
furnace using sheet metal screws.
4. Make certain that an adequate seal is made.
Figure 18.
Figure 19.
Filters
This unit is not equipped with a filter or rack. A field provided
filter is required for the unit to operate properly. Table 3 lists
recommended filter sizes.
A filter must be in place whenever the unit is operating.
Table 3.
Furnace Cabinet Width
Filter Size
B (17.5”)
16 x 25 x 1 (1)
C (21”)
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:
This furnace is not certified for operation in heating
mode (indoor blower operating at selected heating speed)
with an external static pressure which exceeds 0.5 inches
w.c. Operation at these conditions may result in improper
limit operation.
Supply Air Plenum
If the furnace is installed without a cooling coil, a removable
access panel should be installed in the supply air duct. The
access panel should be large enough to permit inspection
(by 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 operating and it must not
allow leaks into the supply air duct system.
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 device (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
reverse flow of the flue gas may result in incomplete
combustion and the formation of carbon monoxide gas.
This toxic gas might then be distributed throughout the
house by the furnace duct system.
Use fiberglass sealing strips, caulking, or equivalent
sealing 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.
