b. Not less than the sum of the areas of all vent connectors in
the space.
The opening shall commence within 12
″
(300 mm) of the
ceiling. Appliances in the space shall have clearances of at
least 1
″
(25 mm) from the sides and back and 6
″
(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.
Indoor Combustion Air© NFPA & AGA
Standard and Known-Air-Infiltration Rate Methods
Indoor combustion air is permitted for combustion, ventilation,
and dilution, if the Standard or Known-Air-Infiltration Rate
Method is used.
CARBON MONOXIDE POISONING HAZARD
Failure to supply outdoor air via grilles or ducts could result
in death and/or personal injury.
Many homes require air to be supplied from outdoors for
furnace combustion, ventilation, and dilution of flue gases.
The furnace combustion air supply must be provided in
accordance with this instruction manual.
The Standard Method:
1. The space has no less volume than 50 cubic feet per 1,000
Btuh of the maximum input ratings for all gas appliances
installed in the space and
2. The air infiltration rate is not known to be less than 0.40 air
changes per hour (ACH).
The Known Air Infiltration Rate Method shall be used, if the
infiltration rate is known to be:
1. Less than 0.40 ACH and
2. Equal to or greater than 0.10 ACH
Infiltration rates greater than 0.60 ACH shall not be used. The
minimum required volume of the space varies with the number of
ACH and shall be determined per Table 2 or Equations 1 and 2.
Determine the minimum required volume for each appliance in the
space and add the volumes together to get the total minimum
required volume for the space.
Table 2-Minimum Space Volumes were determined by using the
following equations from the National Fuel Gas Code ANSI
Z223.1-2002/NFPA 54-2002,8.3.2.2:
1. For other than fan-assisted appliances, such as a draft
hood-equipped water heater:
2. For fan-assisted appliances such as this furnace:
Table 1—Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
FURNACE
INPUT
(BTUH)
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH) (1,100 SQ. MM/KW)
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH) (734 SQ. MM/KW)
TWO OPENINGS OR VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH) (550 SQ. MM/KW)
Free Area of
Opening and Duct
(Sq. In.)
Round Duct
(in. Dia)
Free Area of
Opening and Duct
(sq In.)
Round Duct
(in. Dia)
Free Area of
Opening and Duct
(Sq In.)
Round Duct
(In. Dia)
44,000
22
6
14.7
5
11
4
66,000
33
7
22
6
16.5
5
88,000
44
8
29.3
7
22
6
110,000
55
9
36.7
7
27.5
6
132,000
66
10
44
8
33
7
154,000
77
10
51.3
9
38.5
8
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
→
Table 2—Minimum Space Volumes for 100% Combustion, Ventilation and Dilution Air from Outdoors
ACH
OTHER THAN FAN-ASSISTED TOTAL
(1,000’S BTUH GAS INPUT RATE)
FAN-ASSISTED TOTAL
(1,000’S BTUH GAS INPUT RATE)
30
40
50
44
66
88
110
132
154
Space Volume (ft
3
)
0.60
1,050
1,400
1,750
1,100
1,650
2,200
2,750
3,300
3,850
0.50
1,260
1,680
2,100
1,320
1,980
2,640
3,300
3,960
4,620
0.40
1,575
2,100
2,625
1,650
2,475
3,300
4,125
4,950
5,775
0.30
2,100
2,800
3,500
2,200
3,300
4,400
5,500
6,600
7,700
0.20
3,150
4,200
5,250
3,300
4,950
6,600
8,250
9,900
11,550
0.10
6,300
8,400
10,500
6,600
9,900
13,200
16,500
19,800
23,100
0.00
NP
NP
NP
NP
NP
NP
NP
NP
NP
A04002
Volume
Other
= 21ft
3
ACH
I
other
1000 Btu/hr
A04003
Volume
Fan
= 15ft
3
ACH
I
fan
1000 Btu/hr
17
→
→
