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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 11 or 12).
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.
506552-01
Figure 10
Figure 11
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 size.
A filter must be in place whenever the unit is operating.
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: Operation of this furnace in heating mode (indoor
blower operating at selected heating speed) with an external
static pressure which exceeds 0.8 inches w.c. may result in
erratic limit operation.
Return Air Plenum
Return air must not be drawn from a room where this furnace,
or any other gas fueled appliance (Le., water heater), or
carbon monoxide producing device (Le., 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.
Issue 1034
Figure 12
Table 3
Page 9 of 52
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