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b. Combining spaces on same floor level. Each opening shall
have free area of at least 1 in.
of the total input rating of all gas appliances in the space,
but not less than 100 in.
commence within 12 in. (300 mm) of the ceiling and the
second opening shall commence within 12 in. (300 mm)
of the floor. The minimum dimension of air openings shall
be at least 3 in. (80 mm). See Fig. 7.
c. Combining space on different floor levels. The volumes of
spaces on different floor levels shall be considered as com-
municating spaces if connected by one or more permanent
openings in doors or floors having free area of at least 2
2
in.
/1,000 Btuh (4,400 mm
all gas appliances.
2. An attic or crawlspace may be considered a space that freely
communicates with the outdoors provided there are ad-
equate permanent ventilation openings directly to outdoors
having free area of at least 1- -in.
rating for all gas appliances in the space.
3. In spaces that use the Indoor Combustion Air Method, in-
filtration should be adequate to provide air for combustion,
permanent ventilation and dilution of flue gases. However,
in buildings with unusually tight construction, additional air
MUST be provided using the methods described in the
Outdoor Combustion Air Method section.
4. Unusually tight construction is defined as Construction
with:
Table 3 – Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH)
(1,100 SQ. MM/KW)
FURNACE
INPUT
Free Area of
(BTUH)
Opening and
Duct
Sq. In (Sq. mm)
40,000*
20 (12904)
60,000
30 (19355)
80,000
40 (25807)
100,000
50 (32258)
120,000
60 (38709)
140,000*
70 (45161)
*Not all families have these models.
FURNACE
WATER HEATER
100,000
+
30,000
60,000
+
40,000
80,000
+
30,000
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)
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)
6,300
0.10
(178.0)
0.00
NP
NP = Not Permitted
440 01 4541 00
2
/1,000 Btuh (2,000 mm
2
2
(0.06 m
). One opening shall
2
/kW) of total input rating of
2
/4,000 Btuh of total input
Round Duct
In. (mm) Dia
5 (127)
6 (152)
7 (178)
8 (203)
9 (229)
10 (254)
EXAMPLES: Determining Free Area
TOTAL INPUT
=
(130,000 divided by 4,000)
=
(100,000 divided by 3,000)
=
(110,000 divided by 2,000)
40
50
1,400
1,750
(39.6)
(49.5)
1,680
2,100
(47.5)
(59.4)
2,100
2,625
(59.4)
(74.3)
2,800
3,500
(79.2)
(99.1)
4,200
5,250
(118.9)
(148.6)
8,400
10,500
(237.8)
(297.3)
NP
NP
Specifications subject to change without notice.
a. Walls and ceilings exposed to the outdoors have a continu-
2
/kW)
ous, sealed vapor barrier. Openings are gasketed or sealed
and
b. Doors and openable windows are weatherstripped and
c. Other openings are caulked or sealed. These include joints
around window and door frames, between sole plates and
floors, between wall- -ceiling joints, between wall panels,
at penetrations for plumbing, electrical and gas lines, etc.
Combination of Indoor and Outdoor Air
1. Indoor openings shall comply with the Indoor Combus-
tion Air Method below and,
2. Outdoor openings shall be located as required in the Out-
door Combustion Air Method mentioned previously and,
3. Outdoor openings shall be sized as follows:
a. Calculate the Ratio of all Indoor Space volume divided by
required volume for Indoor Combustion Air Method be-
low.
b. Outdoor opening size reduction Factor is 1 minus the Ra-
tio in a. above.
c. Minimum size of Outdoor openings shall be the size re-
quired in Outdoor Combustion Air Method above multi-
plied by reduction Factor in b. above. The minimum di-
mension of air openings shall be not less than 3 in. (80 mm).
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH)
(734 SQ. MM/KW)
Free Area of
Opening and
Round Duct
In. (mm) Dia
Duct
Sq. In (Sq. mm)
14 (8696)
5 (127)
20 (13043)
5 (127)
27 (17391)
6 (152)
34 (21739)
7 (178)
40 (26087)
7 (178)
47 (30435)
8 (203)
= 32.5 Sq. In. for each two Vertical Ducts or Openings
= 33.3 Sq. In. for each Single Duct or Opening
= 55.0 Sq. In. for each two Horizontal Ducts
FAN-ASSISTED TOTAL
(1,000'S BTUH GAS INPUT RATE)
40
60
3
3
Space Volume Ft
(M
)
1,400
1,500
2,000
(39.6)
(42.5)
(56.6)
1,680
1,800
2,400
(47.5)
(51.0)
(67.9)
2,100
2,250
3,000
(59.4)
(63.7)
(84.9)
2,800
3,000
4,000
(79.2)
(84.9)
(113.2)
4,200
4,500
6,000
(118.9)
(127.3)
(169.8)
8,400
9,000
12,000
(237.8)
(254.6)
(339.5)
NP
NP
NP
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of
Opening and
Duct
Sq. In (mm)
10 (6452)
15 (9678)
20 (12904)
25 (16130)
30 (19355)
35 (22581)
80
100
120
2,500
3,000
(70.8)
(84.9)
3,000
3,600
(84.9)
(101.9)
3,750
4,500
(106.1)
(127.3)
5,000
6,000
(141.5)
(169.8)
7,500
9,000
(212.2)
(254.6)
15,000
18,000
(424.4)
(509.2)
NP
NP
Round Duct
In. (mm) Dia.
4 (102)
5 (127)
5 (127)
6 (152)
6 (152)
7 (178)
140
3,500
(99.1)
4,200
(118.9)
5,250
(148.6)
7,000
(198.1)
10,500
(297.1)
21,000
(594.1)
NP
11
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