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S Spaces having at least 50 cubic feet per 1,000 Btuh may use
the INDOOR COMBUSTION AIR, STANDARD or
KNOWN AIR INFILTRATION METHOD.
Outdoor Combustion Air Method
1. Provide the space with sufficient air for proper
combustion, ventilation, and dilution of flue gases using
permanent horizontal or vertical duct(s) or opening(s)
directly communicating with the outdoors or spaces that
freely communicate with the outdoors.
2. Fig. 18 illustrates how to provide TWO OUTDOOR
OPENINGS, one inlet and one outlet combustion and
ventilation air openings to the outdoors.
a. One opening MUST commence within 12" (300 mm)
of the ceiling and the second opening MUST
commence within 12" (300 mm) of the floor.
b. Size openings and ducts per Fig. 18 and Table 1.
c. TWO HORIZONTAL DUCTS require 1 square inch of
free area per 2,000 Btuh (1,100 mm
input for all gas appliances in the space per Fig. 18 and
Table 1.
d. TWO OPENINGS OR VERTICAL DUCTS require 1
square inch of free area per 4,000 Btuh (550 mm
Table 1 – Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH)
FURNACE
INPUT
Free Area of
(BTUH)
Opening and
Duct ---
Sq. In (Sq. mm)
40,000
20 (12903)
60,000
30 (19354)
80,000
40 (25806)
100,000
50 (32258)
120,000
60 (38709)
EXAMPLES: Determining Free Area
FURNACE
WATER HEATER
100,000
+
40,000
60,000
+
40,000
80,000
+
30,000
Table 2 – Minimum Space Volumes for 100% Combustion, Ventilation and Dilution Air from Outdoors
OTHER THAN FAN-ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE)
30
ACH
1,050
0.60
(29.7)
1,260
0.50
(35.6)
1,575
0.40
(44.5)
2,100
0.30
(59.4)
3,150
0.20
(89.1)
(118.9)
6,300
0.10
(178.3)
(237.8)
0.00
NP
/kW) of combined
2
/kW)
2
(1,100 SQ.
MM/KW)
Round Duct
In. (mm) Dia
Sq. In (Sq. mm)
6 (152)
7 (178)
8 (203)
8 (203)
9 (229)
TOTAL INPUT
=
(140,000 divided by 4,000)
=
(100,000 divided by 3,000)
=
(110,000 divided by 2,000)
40
50
1,400
1,750
1,100
(39.6)
(49.5)
(31.1)
1,680
2,100
1,320
(47.5)
(59.4)
(37.3)
2,100
2,625
1,650
(59.4)
(74.3)
(46.7)
2,800
3,500
2,200
(79.2)
(99.1)
(62.2)
4,200
5,250
3,300
(148.6)
(93.4)
8,400
10,500
6,600
(297.3)
(186.8)
NP
NP
for combined input of all gas appliances in the space
per Fig. 18 and Table 1.
3. ONE OUTDOOR OPENING requires:
a. 1 square inch of free area per 3,000 Btuh
(734 mm
2
appliances in the space per Table 1 and
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
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.
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH)
(734 SQ. MM/KW)
Free Area of
Opening and
Round Duct ---
Duct ---
In. (mm) Dia
13.4 (8645)
5 (127)
20 (12903)
6 (152)
26.7 (17225)
6 (152)
33.4 (21548)
7 (178)
40 (25806)
8 (203)
= 35.0 Sq. In. for each two Vertical Ducts or Openings
= 33.4 Sq. In. for a Single Duct or Opening
= 55.0 Sq. In. for each of two Horizontal Ducts
FAN-ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE)
44
66
88
3
3
Space Volume Ft
(M
)
1,650
2,200
(46.7)
(62.2)
1,980
2,640
(56.0)
(74.7)
2,475
3,300
(70.0)
(93.4)
3,300
4,400
(93.4)
(124.5)
4,950
6,600
(140.1)
(186.8)
9,900
13,200
(280.3)
(373.7)
NP
NP
NP
- - 19 - -
/kW) for combined input of all gas
©
NFPA & AGA
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of
Opening and
Round Duct ---
Duct ---
In. (mm) Dia
Sq. In (mm)
10 (6451)
15 (9677)
20 (12903)
25 (16129)
30 (19354)
110
132
2,750
3,300
(77.8)
(93.4)
3,300
3,960
(93.4)
(112.1)
4,125
4,950
(116.8)
(140.1)
5,500
6,600
(155.7)
(186.8)
8,250
9,900
(233.6)
(280.3)
16,500
19,800
(467.2)
(560.6)
NP
NP
4 (102)
5 (127)
6 (152)
6 (152)
7 (178)
154
3,850
(109.0)
4,620
(130.8)
5,775
(163.5)
7,700
(218.0)
11,550
(327.0)
23,100
(654.1)
NP
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