11
b. Combining spaces on same floor level. Each opening shall
have free area of at least 1 in.
2
/1,000 Btuh (2,000 mm
2
/kW)
of the total input rating of all gas appliances in the space,
but not less than 100 in.
2
(0.06 m
2
). One opening shall
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
in.
2
/1,000 Btuh (4,400 mm
2
/kW) of total input rating of
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.
2
/4,000 Btuh of total input
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:
a. Walls and ceilings exposed to the outdoors have a continu-
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 abovemulti-
plied by reduction Factor in b. above. The minimum di-
mension ofair openingsshallbe not lessthan 3 in. (80 mm).
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
Opening and
Duct
Sq. In (Sq. mm)
Round Duct
In. (mm) Dia
Free Area of
Opening and
Duct
Sq. In (mm)
Round Duct
In. (mm) Dia.
40,000* 20 (12904) 5 (127) 14 (8696) 5 (127) 10 (6452) 4 (102)
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)
*Not all families h ave these models.
EXAMPLES: 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
SpaceVolumeFt
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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