10 440 01 4300 00
Specifications subject to change without notice.
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. Figure 6 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-in.
(300 mm) of the ceiling and the second opening
MUST commence within 12-in. (300 mm) of the floor.
b. Size openings and ducts per Figure 6 and Table 3.
c. TWO HORIZONTAL DUCTS require 1 square inch
(25.4 square mm) of free area per 2,000 Btuh (1,100
mm
2
/kW) of combined input for all gas appliances in
the space per Figure 6 and Table 3.
d. TWO OPENINGS OR VERTICAL DUCTS require 1
square inch (25.4 square mm)of free area per 4,000
Btuh (550 mm
2
/kW) for combined input of all gas
appliances in the space per Figure 6 and Table 3.
3. ONE OUTDOOR OPENING requires:
a. 1 sq. in. (25.4 square mm)of free area per 3,000 Btuh
(734 mm
2
/kW) for combined input of all gas
appliances in the space per Table 3 and
b. Not less than the sum of the areas of all vent
connectors in the space.
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.
Table 3 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 (Sq. mm)
Round
Duct
In. (mm)
Dia
Free Area of Open-
ing and Duct
Sq. In (Sq. mm)
Round
Duct
In. (mm)
Dia
Free Area of Open-
ing and Duct
Sq. In (mm)
Round Duct
In. (mm)
Dia.
60,000
30 (19355) 6 (152) 20 (13043) 5 (127) 15 (9678) 5 (127)
80,000
40 (25807) 7 (178) 27 (17391) 6 (152) 20 (12904) 5 (127)
100,000
50 (32258) 8 (203) 34 (21739) 7 (178) 25 (16130) 6 (152)
120,000
60 (38709) 9 (229) 40 (26087) 7 (178) 30 (19355) 6 (152)
140,000
70 (45161) 10 (254) 47 (30435) 8 (203) 35 (22581) 7 (178)
EXAMPLE: Determining Free Area
FURNACE WATER HEATER TOTAL INPUT
100,000 + 30,000 = (130,000 divided by 4,000) = 32.5 Sq. In. for each two Vertical Ducts or Openings
60,000 + 40,000 = (100,000 divided by 3,000) = 33.3 Sq. In. for each Single Duct or Opening
80,000 + 30,000 = (110,000 divided by 2,000) = 55.0 Sq. In. for each two Horizontal Ducts
Table 4 Minimum Space Volumes for 100% Combustion, Ventilation and Dilution Air from Outdoors
OTHER THAN FAN‐ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE)
FAN‐ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE)
ACH
30 40 50 40 60 80 100 120 140
Space Volume Ft
3
(M
3
)
0.60
1,050
(29.7)
1,400
(39.6)
1,750
(49.5)
1,400
(39.6)
1,500
(42.5)
2,000
(56.6)
2,500
(70.8)
3,000
(84.9)
3,500
(99.1)
0.50
1,260
(35.6)
1,680
(47.5)
2,100
(59.4)
1,680
(47.5)
1,800
(51.0)
2,400
(67.9)
3,000
(84.9)
3,600
(101.9)
4,200
(118.9)
0.40
1,575
(44.5)
2,100
(59.4)
2,625
(74.3)
2,100
(59.4)
2,250
(63.7)
3,000
(84.9)
3,750
(106.1)
4,500
(127.3)
5,250
(148.6)
0.30
2,100
(59.4)
2,800
(79.2)
3,500
(99.1)
2,800
(79.2)
3,000
(84.9)
4,000
(113.2)
5,000
(141.5)
6,000
(169.8)
7,000
(198.1)
0.20
3,150
(89.1)
4,200
(118.9)
5,250
(148.6)
4,200
(118.9)
4,500
(127.3)
6,000
(169.8)
7,500
(212.2)
9,000
(254.6)
10,500
(297.1)
0.10
6,300
(178.0)
8,400
(237.8)
10,500
(297.3)
8,400
(237.8)
9,000
(254.6)
12,000
(339.5)
15,000
(424.4)
18,000
(509.2)
21,000
(594.1)
0.00
NP NP NP NP NP NP NP NP NP
NP = Not Permitted
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