Step 2—Ductwork acoustical treatment
Metal duct systems that do not have a 90 degree elbow and 10 ft
of main duct to the first branch take-off may require internal
acoustical lining. As an alternative, fibrous ductwork may be used
if constructed and installed in accordance with the latest edition of
SMACNA construction standard on fibrous glass ducts. Both
acoustical lining and fibrous ductwork shall comply with NFPA
90B as tested by UL Standard 181 for Class 1 Rigid air ducts.
Step 3—Supply Air Connections
DOWNFLOW FURNACES
Connect supply-air duct to supply-air opening (outlet) on furnace.
(See Fig. 1.) The supply-air duct attachment must ONLY be
connected to furnace supply/outlet or air conditioning coil casing
(when used), when installed on non-combustible material. When
installed on combustible material, supply-air duct attachment must
ONLY be connected to an accessory subbase or factory approved
air conditioning coil casing. 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.
HORIZONTAL FURNACES
Connect supply-air duct to supply-air (outlet) opening on furnace.
(See Fig. 1.) The supply-air duct attachment must ONLY be
connected to furnace 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
DOWNFLOW AND HORIZONTAL FURNACES
The return-air duct must be connected to return-air opening (inlet)
provided as shown in Fig. 1. 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.
Fig. 3—Confined Space: Air for Combustion and
Ventilation from an Unconfined Indoor Space
A93387
RETURN
AIR
6
″
MIN
(FRONT)
†
SUPPLY AIR
VENT THROUGH ROOF
(CATEGORY I)
1 SQ IN.
PER 1000
BTUH* IN DOOR
OR WALL
12
″
MAX
1 SQ IN.
PER 1000
BTUH* IN DOOR
OR WALL
12
″
MAX
INTERIOR
HEATED
SPACE
* Minimum opening size is 100 sq in. with
minimum dimensions of 3 in.
†
Minimum of 3 in. when type B-1 vent is used.
UNCONFINED
SPACE
Fig. 4—Confined Space: Air for Combustion and
Ventilation from Outdoors
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
→
Table 2—Minimum Free Area Of Combustion Air Opening
58RAV
FURNACE
INPUT
(BTUH)
AIR FROM INDOOR
UNCONFINED SPACE
OUTDOOR AIR THROUGH
VERTICAL DUCTS
OUTDOOR AIR THROUGH
HORIZONTAL DUCTS
OUTDOOR AIR THROUGH
SINGLE DUCT
Free Area
of Opening
(Sq In.)
Free Area of
Opening and Duct
(Sq In.)
Round
Pipe
(In. Dia)
Free Area of
Opening and Duct
(Sq In.)
Round
Pipe
(In. Dia)
Free Area of
Opening and Duct
(Sq In.)
Round
Pipe
(In. Dia)
46,000
100
11.50
4
23.0
6
15.33
5
69,000
100
17.25
5
34.5
7
23.00
6
92,000
100
23.00
6
46.0
8
30.67
7
115,000
115
28.75
7
57.5
9
38.33
7
135,000
135
33.75
7
67.5
10
45.00
8
6
