Flexible connections should be used between ductwork and
furnace to prevent transmission of vibration. Ductwork passing
through unconditioned space should be insulated to enhance
system performance. When air conditioning is used, a vapor
barrier is recommended.
Maintain a 1-in. clearance from combustible materials to supply air
ductwork for a distance of 36 in. horizontally from the furnace. See
NFPA 90B or local code for further requirements.
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
Connect supply-air duct to 3/4-in. flange on furnace supply-air
outlet. The supply-air duct attachment must ONLY be connected
to furnace supply-/outlet-air duct flanges or air conditioning coil
casing (when used). DO NOT cut main furnace casing to attach
supply side air duct, humidifier, or other accessories. All accesso-
ries MUST be connected external to furnace main casing.
Fig. 3—Confined Space: Air For Combustion and
Ventilation from an Indoor Unconfined Space
A89012
SUPPLY
AIR
6
″
MIN
(FRONT)
†
RETURN AIR
VENT THROUGH ROOF
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-B1 vent is used.
UNCONFINED
SPACE
CONFINED
SPACE
Fig. 4—Confined Space: Air For Combustion and
Ventilation from Outdoors
A89013
1 SQ IN.
PER
4000
BTUH*
DUCTS
TO
OUTDOORS
1 SQ IN.
PER 4000
BTUH*
SUPPLY
AIR
VENT
THROUGH
ROOF
D
B
A
C
E
1 SQ IN.
PER 4000
BTUH*
DUCT
TO
OUTDOORS
RETURN AIR
1 SQ IN.
PER 2000
BTUH*
1 SQ IN.
PER 2000
BTUH*
DUCTS
TO
OUTDOORS
12
″
MAX
12
″
MAX
12
″
MAX
Use any of the following
combinations of openings:
A & B C & D D & E F & G
NOTE:
*Minimum dimensions of 3 in.
CONFINED
SPACE
12
″
MAX
12
″
MAX
OUTDOORS
1 SQ IN.
PER
4000
BTUH*
F
G
Table 2—Minimum Free Area Of Combustion Air Opening
58PAV
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)
44,000
100
11.0
4
22.0
6
14.67
5
66,000
100
16.5
5
33.0
7
22.00
6
88,000
100
22.0
6
44.0
8
29.33
7
110,000
110
27.5
6
55.0
9
36.67
7
132,000
132
33.0
7
66.0
10
44.00
8
154,000
154
38.5
8
77.0
10
51.33
9
* Free area shall be equal to or greater than the sum of the areas of all vent connectors in the confined space. Opening area must be increased if other gas appliances
in the space require combustion air.
6
