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.
c. 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 rectangular
ducts must not be less than 3 in. (See Fig. 3.)
III.
SUPPLY-AIR
PLENUM
INSTALLATION
(DOWN-
FLOW)
A.
Installation On a Concrete Slab
1. Construct hole in floor per dimensions in Fig. 4 and Table
3.
2. Install plenum and furnace as shown in Fig. 5.
B.
Installation On a Combustible Floor
1. Cut and frame hole in floor per dimensions in Installation
Instructions packaged with downflow subbase.
2. When completed, downflow subbase, plenum, and furnace
(or coil casing when used) should be installed as shown in
Fig. 6.
Fig. 3—Air for Combustion and Ventilation
(Outside Air)
A93388
DUCTS TO
OUTDOORS
1 SQ IN.
PER 4000
BTUH
*
RETURN
AIR
VENT
THROUGH
ROOF
(CATEGORY I)
D
B
A
C
E
1 SQ IN.
PER 4000
BTUH
DUCT
TO
OUTDOORS
SUPPLY AIR
1 SQ IN.
PER 2000
BTUH*
DUCTS
TO
OUTSIDE
12
″
MAX
12
″
MAX
CONFINED
SPACE
12
″
MAX
12
″
MAX
1 SQ IN.
PER 4000
BTUH
*
OUTDOORS
1 SQ IN.
PER 4000
BTUH
*
1 SQ IN.
PER 2000
BTUH*
12
″
MAX
Use any of the following
combinations of openings:
A & B C & D D & E F & G
NOTE:
*Minimum dimensions of 3-In.
F
G
For Example:
373LAV FURNACE
INPUT BTUH
FREE AREA PER
OPENING (SQ IN.)
ROUND PIPE
(IN. DIA)
46,000
11.5
4
69,000
17.3
5
92,000
23.0
6
115,000
28.8
7
135,000
34.5
7
For Example:
373LAV FURNACE
INPUT BTUH
FREE AREA PER
OPENING (SQ IN.)
ROUND PIPE
(IN. DIA)
46,000
23.0
6
69,000
34.3
7
92,000
46.0
8
115,000
57.6
9
135,000
69.0
10
TABLE 3—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.
Fig. 4—Floor Opening for Concrete Slab
A73382
HOLE IN
FLOOR
A
B
Fig. 5—Furnace on a Concrete Slab (Non-Garage
Installation)
A73383
FURNACE
PLENUM
—5—
