NOTE: In determining the free area of an opening, the blocking
effect of the louvers, grilles, and screens must be considered. If the
free area of a louver or grille design is unknown, assume that wood
louvers have a 20 percent free area and metal louvers or grilles
have a 60 percent free area. Screens, when used, must not be
smaller than 1/4-in. mesh. Louvers and grilles must be constructed
so they cannot be closed.
The size of the openings depends upon whether the air comes from
outside of the structure or an unconfined space inside the structure.
1. All air from inside the structure:
a. Each opening MUST have at least 1 sq in. of free area per
1000 Btuh of the total input for all equipment within the
confined space, but not less than 100 sq in. per opening.
(See Fig. 2.) The minimum dimension of air openings shall
not be less than 3 in.
For Example:
COMBUSTION AIR FROM UNCONFINED SPACE
58TMA Furnace
High-Fire Input Btuh
Free Area per Opening
(Sq In.)
63,000 100
84,000 100
105,000 105
123,000 123
b. If the building is constructed unusually tight, a permanent
opening directly communicating with the outdoors should
be provided. This opening should have a minimum free
area of 1 sq in. per 5000 Btuh of total input rating for all
equipment in the enclosure. (See Fig. 3.)
c. If the furnace is installed on a raised platform to provide a
return-air plenum, and return air is taken directly from the
hallway or space adjacent to the furnace, all air for
combustion must come from outdoors. (See Fig. 3.)
2. All air from outside the structure:
a. If combustion air is taken from outdoors through vertical
ducts, the openings and ducts MUST have at least 1 sq in.
of free area per 4000 Btuh of the total input for all
equipment within the confined space.
For Example:
COMBUSTION AIR FROM OUTDOORS THROUGH
VERTICAL DUCTS
58TMA Furnace
High-Fire
Input Btuh
Free Area per Opening
(Sq In.)
Round Pipe
(In. Dia)
63,000 15.8 5
84,000 21.0 6
105,000 26.3 6
123,000 30.8 7
b. If combustion air is taken from the outdoors through
horizontal ducts, the openings and ducts MUST have at
least 1 sq in. of free area per 2000 Btuh of the total input
for all equipment within the confined space. (See Fig. 3.)
For Example:
COMBUSTION AIR FROM OUTDOORS THROUGH
HORIZONTAL DUCTS
58TMA Furnace
High-Fire
Input Btuh
Free Area per Opening
(Sq In.)
Round Pipe
(In. Dia)
63,000 31.5 7
84,000 42.0 8
105,000 52.5 9
123,000 61.5 9
When ducts are used, they must be of the same cross-sectional area
as the free area of the openings to which they connect. The
minimum dimension of ducts must not be less than 3 in. (See Fig.
3.)
Step 3—Supply-Air Plenum Installation (Downflow)
INSTALLATION ON A CONCRETE SLAB
1. Construct hole in floor per dimensions in Fig. 4.
2. Place plenum and furnace as shown in Fig. 5.
INSTALLATION ON A COMBUSTIBLE FLOOR
1. Cut and frame hole in floor per dimensions in Installation
Instructions packaged with downflow subbase.
Fig. 4—Floor Opening for Concrete Slab
Opening Dimensions (In.)
FURNACE CASING
WIDTH
A
B
Heat-Only Heat/Cool*
14-3/16 13-1/8 19-5/8 19-7/16
17-1/2 16-7/16 19-5/8 19-7/16
21 19-7/8 19-5/8 19-7/16
24-1/2 23-7/16 19-5/8 19-7/16
* These dimensions apply when a model CB or CD Evaporator Coil casing is
to be installed.
A73382
HOLE IN
FLOOR
A
B
Fig. 5—Furnace on a Concrete Slab (Non-Garage
Installation)
A73383
FURNACE
PLENUM
5
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