supply/outlet or air conditioning coil casing (when used). DO NOT
cut main furnace casing to attach supply side air duct, humidifier,
or other accessories. All accessories MUST be connected external
to furnace main casing.
Step 4—Return Air Connections
The furnace and its return air system shall be designed and
installed so that negative pressure created by the air circulating fan
cannot affect another appliance’s combustion air supply or act to
mix products of combustion with circulating air and the air
circulating fan of the furnace, if installed in an enclosure commu-
nicating with another fuel-burning appliance not of the direct-vent
type, shall be operable only when any door or panel covering an
opening in the furnace fan compartment or in return air plenum on
ducts is in the closed position.
Never connect return-air ducts to the back of the furnace.
Return-air duct connections on furnace side(s) permitted in
upflow applications only. A failure to follow this warning
could result in fire, personal injury, or death.
UPFLOW FURNACES
The return-air duct must be connected to end, sides (left or right),
or a combination of end and side(s) of main furnace casing as
shown in Fig. 2. Bypass humidifier may be attached into unused
side return air portion of the furnace casing. DO NOT connect any
portion of return-air duct to back of furnace casing.
DOWNFLOW AND HORIZONTAL FURNACES
The return-air duct must be connected to end inlet opening
provided as shown in Fig. 2. DO NOT cut into casing sides or back
to attach any portion of return-air duct. Bypass humidifier connec-
tions should be made at ductwork or coil casing sides exterior to
furnace.
Step 5—Filter Arrangement
Never operate a unit without a filter or with filter access door
removed. Failure to follow this warning can cause fire,
personal injury, or death.
Factory-supplied washable framed filters are shipped in blower
compartment. Determine location for filter and relocate filter
retaining wire if necessary. See Table 2 to determine correct filter
size for desired filter location. Table 2 indicates filter size,
location, and quantity shipped with this furnace. See Fig. 2 for
location and size of bottom and side return-air openings.
Use care when cutting support rods in filters to protect against
flying pieces and sharp rod ends. Wear safety glasses, gloves,
and appropriate protective clothing. Failure to follow this
caution could result in personal injury.
For airflow requirements above 1800 CFM, see Air Delivery
table in Product Data literature for specific use of single side
inlets. The use of both side inlets, a combination of 1 side and
the bottom, or the bottom only will ensure adequate return air
openings for airflow requirements above 1800 CFM.
NOTE: Side return-air openings can ONLY be used in UPFLOW
configurations. Install filter(s) as shown in Fig. 23.
Bottom return-air opening may be used with all 4 orientations.
Filter may need to be cut to fit some furnace widths. Install filter
as shown in Fig. 24.
Table 2—Filter Information
FURNACE
CASING
WIDTH (IN.)
FILTER SIZE (IN.)*
FILTER TYPE
FRAMED
Side Return Bottom Return
17-1/2 (1)16X25X1† (1) 16 X 25 X 1 Cleanable
21 (1)16X25X1 (1)20X25X1† Cleanable
24-1/2 (1or2)16X25X1 (1)24X25X1† Cleanable
* Filters can be field modified by cutting to the desired size. Alternate sizes
can be ordered from your distributor or dealer.
† Factory-provided with furnace.
Fig. 23—Filter Installed for Side Inlet
A00217
Fig. 24—Bottom Filter Arrangement
A00213
WASHABLE
FILTER
FILTER
SUPPORT
FILTER
RETAINER
17
1
⁄2-IN. WIDE
CASINGS ONLY:
INSTALL FIELD-SUPPLIED
FILTER FILLER STRIP
UNDER FILTER.
1″
24
1
/
2
″
3″
17
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