Filter
Top Products
035-19992-001 Rev. A (0404)
Unitary Products Group 5
If, during the inspection of your furnace, you find any of the following
conditions:
• Excessive amounts of dust and lint on components.
• Damaged or deteriorated components or surfaces.
• Leaks or blockage in the vent pipe passages.
• Water on any surface inside or outside of the furnace.
Do not operate the furnace, call a certified dealer / servicing contractor
to check and / or clean your furnace, or for more information if you have
questions about the operation of your furnace.
If all components appear to be in good operating condition, replace the
front panels. Turn ON the gas and electrical power supplies to the fur-
nace, and set thermostat to the desired temperature.
Air Filters
Dirty filters greatly restrict the flow of air and may cause damage to the
moving parts of the furnace. If the filters become clogged the heat
exchangers and blower motor could overheat resulting in a potentially
dangerous situation.
The filters should be checked every 3 months. On new construction,
check the filters every week for the first four weeks and every three
weeks after that, especially if the indoor fan is running continuously.
When replacing the filter(s) you must use filters that are the same size
as those recommended in Table 1. Use the following procedure to
determine the filter size. Never operate your furnace without a suitable
air filter.
1. Measure the furnace width. Use the following measurements to
determine the cabinet size.
• A 17-1/2” wide cabinet is a “B” cabinet.
• A 21” wide cabinet is a “C” cabinet.
• A 24-1/2” wide cabinet is a “D” cabinet.
2. Locate the cabinet size on Table 1, then determine whether you
have a bottom, side or top return using the following method.
a. If the return air filters are on the left or right side of the fur-
nace it is a side return. Refer to Figure 11.
b. If the air filter is on the bottom of the furnace then you have a
bottom return. Refer to Figure 12 or for horizontal applica-
tions refer to Figure 9.
c. If the air filters are on the bottom and the side of the furnace
then you have a bottom and side return. You must replace
both air filters. Brackets with the number two (2) before the fil-
ter size requires 2 filters. Refer to Figures 11 & 12.
d. If the air filters are on both sides of the furnace then you have
a (2) sided return. You must replace both air filters.
e. If the air filters are in the return air plenum, above the furnace
then you have a top return (Downflow Furnace). Refer to Fig-
ure 10.
3. After you determine the cabinet size and what return configuration
you have, look up the recommended filter size from Table 1. You
must replace the air filters with the same size as recommended.
Removing Filters
Externally Mounted Air Filters
The air filter is in a rack that is attached to the casing of the furnace or
placed in the return air duct. Figures 9, 10, 11 & 12 indicate possible
external air filter box locations.
FIGURE 9: External Air Filter Rack - Horizontal Left Position
FIGURE 10: Downflow Filter
FIGURE 11: External Air Filter Rack - Left & Right Side Position
FIGURE 12: External Air Filter Rack - Bottom Position
AIR FILTER RACK
ACCESS DOOR
FILTER
RACK
18”
MIN
FURNACE
AIR FLOW
FILTER RACK
(factory supplied with some furnaces)
RACKAND FILTERS SECURED
INSIDE BLOWER SECTION
FOR SHIPMENT
DUCTWORK
FILTERS
BRANCH
DUCTS
FH
CROSS SECTION A-A
(with Plenum and filters)
FILTER
RACK
NOTE: FILTER ACCESS THROUGH
DUCTWORK MUST BE PROVIDED
FOR REMOVALAND CLEANING
CASING SIZE
DIMENSION FH
16-1/4”
22-1/4”
26-1/4”
12-3/4”
11”
8-1/4”
AIR FLOW
LEFT SIDE
FILTER RACK
RIGHT SIDE
FILTER RACK
DUCT
AIR FILTER
RACK
ACCESS
DOOR
AIR FILTER
RACK
ACCESS
DOOR
FURNACE
DUCT
AIR FLOW
AIR FLOW
AIR FILTER RACK
ACCESS DOOR
DUCT
BOTTOMFILTER RACK
FURNACE
AIR FLOW