8
441 01 2101 05
Specifications are subject to change without notice.
d. TWO OPENINGS OR VERTICAL DUCTS require 1
−
in
2
of free area per 4,000 Btuh (550 mm
2
/kW) for
combined input of all gas appliances in the space per
3. ONE OUTDOOR OPENING requires:
a. One square inch of free area per 3,000 Btuh (734
mm
2
/kW) for combined input of all gas appliances in
b. Not less than the sum of the areas of all vent
connectors in the space.
Table 2 – Minimum Free Area Required for Each Combustion Air opening of Duct to Outdoors
FURNACE
INPUT
(BTUH)
TWO HORIZONTAL DUCTS
SINGLE DUCT OR OPENING
TWO OPENINGS OR VERTICAL DUCTS
(1 SQ. IN./2,000 BTUH) (1,100 SQ. MM/KW)
(1 SQ. IN./3,000 BTUH) (734 SQ. MM/KW)
(1 SQ. IN./4,000 BTUH) (550 SQ. MM/KW)
Free Area of Opening
and Duct
Sq. In. (Sq. mm)
Round Duct
Dia.
In. (mm)
Free Area of Opening
and Duct
Sq. In. (Sq. mm)
Round Duct
Dia.
In. (mm)
Free Area of Opening
and Duct
Sq. In. (Sq. mm)
Round Duct
Dia.
In. (mm)
44,000
22 (14194)
6 (152)
14.7 (9484)
5 (127)
11 (7096)
4 (102)
66,000
33 (21290)
7 (178)
22 (14193)
6 (152)
16.5 (10645)
5 (127)
88,000
44 (28387)
8 (203)
29.3 (18903)
7 (178)
22 (14193)
6 (152)
110,000
55 (35484)
9 (229)
36.7 (23677)
7 (178)
27.5 (17742)
6 (152)
EXAMPLES: Determining Free Area
FURNACE
WATER HEATER
TOTAL INPUT
110,000
+
30,000
=
(140,000 divided by 4,000)
=
35.0 Sq. In. for each two Vertical Ducts or Openings
66,000
+
40,000
=
(106,000 divided by 3,000)
=
35.3 Sq. In. for a Single Duct or Opening
88,000
+
30,000
=
(118,000 divided by 2,000)
=
59.0 Sq. In. for each of two Horizontal Ducts
Figure 7
−
Air for Combustion, Ventilation, and Dilution
for Outdoors
1 SQ IN .
PER
4000
BTUH*
DUCTS
TO
O UTDOORS
1 SQ IN.
PER 4000
BTUH*
C
IRCULA
TING
AIR DUCTS
VENT
THR OUGH
R OOF
D
B
A
C
E
1 SQ IN.
PER 4000
BTUH*
DUCT
TO
OUTDOORS
CIRCULA TING AIR DUCT S
1 SQ IN.
PER 2000
BTUH*
1 SQ IN.
PER 2000
BTUH*
DUCT S
TO
OUTDOORS
12
″
MAX
12
″
MAX
12
″
MAX
12
″
MAX
12
″
MAX
OUTDOORS
1 SQ IN .
PER
4000
BTUH*
F
G
CLEARANCE IN FRONT
OF COMB
USTION AIR
OPENINGS SHALL BE
AT LEAST 3 IN
.
(305mm)
(305mm)
(305mm)
(305mm)
(305mm)
(76mm)
*Minimum dimensions of 3
−
in. (76 mm).
Note: Use any of the following combinations of openings:
A & B C & D D & E F & G
A03174
The opening shall commence within 12 in. (300 mm) of the
ceiling. Appliances in the space shall have clearances of at
least 1 in. (25 mm) from the sides and back and 6 in. (150
mm) from the front. The opening shall directly communicate
with the outdoors or shall communicate through a vertical or
horizontal duct to the outdoors or spaces (crawl or attic) that
freely communicate with the outdoors.
Figure 8
−
Air for Combustion, Ventilation, and Dilution
from Indoors
CIRCULATING AIR
DUCTS
6" MIN
(FRONT)
Ü
CIRCULATING AIR DUCTS
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" MA X
UNCONFINED
SPACE
INTERIOR
HEATED
SPACE
CLEARANCE IN FRONT OF COMBUSTION AIR
OPENINGS SHALL
BE A
T LEAST 3 IN.
(305mm)
(152mm)
(305mm)
* Minimum opening size is 100 sq in. (64516 sq. mm) with minimum dimensions of
3 in. (76 mm)
† Minimum of 3 in. (76 mm) when type‐B1 vent is used.
A03175
Indoor Combustion Air
E
NFPA & AGA Standard and
Known
−
Air
−
Infiltration Rate Methods
Indoor air
is permitted for combustion, ventilation, and
dilution, if the
Standard
or
Known
−
Air
−
Infiltration
Method is
used.
