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Specifications are subject to change without notice.
441 01 3700 00
9
dimension of air openings shall be at least 3 in. (80
mm). (See Fig. 8.)
c. Combining space on different floor levels. The vol-
umes of spaces on different floor levels shall be con-
sidered as communicating 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
adequate permanent ventilation openings directly to out-
doors 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,
infiltration should be adequate to provide air for combus-
tion, permanent ventilation and dilution of flue gases.
However, in buildings with unusually tight construction,
additional air MUST be provided using the methods de-
scribed 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 con-
tinuous, 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 below.
b. Outdoor opening size reduction
Factor
is 1 minus the
Ratio
in a. above.
c. Minimum size of Outdoor openings shall be the size
required in
Outdoor Combustion Air
Method above
multiplied by reduction
Factor
in b. above. The min-
imum dimension of air openings shall be not less than
3 in. (80 mm).
Table 1 – Minimum Free Area Required for Each Combustion Air opening of Duct to Outdoors
FURNACE
INPUT
(BTUH)
TWO HORIZONTAL DUCTS
SINGLE DUCT OR OPENING
TWO OPENINGS OR VERTICAL DUCTS
(1 SQ. IN./2,000 BTUH) (1,100 SQ. MM/KW)
(1 SQ. IN./3,000 BTUH) (734 SQ. MM/KW)
(1 SQ. IN./4,000 BTUH) (550 SQ. MM/KW)
Free Area of Opening
and Duct
Sq. In. (Sq. mm)
Round Duct
Dia.
In. (mm)
Free Area of Opening
and Duct
Sq. In. (Sq. mm)
Round Duct
Dia.
In. (mm)
Free Area of Opening
and Duct
Sq. In. (Sq. mm)
Round Duct
Dia.
In. (mm)
44,000
22 (14194)
6 (152)
14.7 (9484)
5 (127)
11 (7096)
4 (102)
66,000
33 (21290)
7 (178)
22 (14193)
6 (152)
16.5 (10645)
5 (127)
88,000
44 (28387)
8 (203)
29.3 (18903)
7 (178)
22 (14193)
6 (152)
110,000
55 (35484)
9 (229)
36.7 (23677)
7 (178)
27.5 (17742)
6 (152)
132,000
66 (42580)
10 (254)
44 (28387)
8 (203)
33 (21290)
7 (178)
154,000
77 (49677)
10 (254)
51.3 (33096)
9 (229)
38.5 (24839)
8 (203)
Note: Not all models have these sizes.
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 from Indoors
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
44
66
88
110
132
154
Space Volume Ft
3
(M
3
)
0.60
1,050
(29.7)
1,400
(39.6)
1,750
(49.5)
1,100
(31.1)
1,650
(46.7)
2,200
(62.2)
2,750
(77.8)
3,300
(93.4)
3,850
(109.0)
0.50
1,260
(35.6)
1,680
(47.5)
2,100
(59.4)
1,320
(37.3)
1,980
(56.0)
2,640
(74.7)
3,300
(93.4)
3,960
(112.1)
4,620
(130.8)
0.40
1,575
(44.5)
2,100
(59.4)
2,625
(74.3)
1,650
(46.7)
2,475
(70.0)
3,300
(93.4)
4,125
(116.8)
4,950
(140.1)
5,775
(163.5)
0.30
2,100
(59.4)
2,800
(79.2)
3,500
(99.1)
2,200
(62.2)
3,300
(93.4)
4,400
(124.5)
5,500
(155.7)
6,600
(186.8)
7,700
(218.0)
0.20
3,150
(89.1)
4,200
(118.9)
5,250 (148.6)
3,300
(93.4)
4,950
(140.1)
6,600
(186.8)
8,250
(233.6)
9,900
(280.3)
11,550
(327.0)
0.10
6,300
(178.3)
8,400
(237.8)
10,500
(297.3)
6,600
(186.8)
9,900
(280.3)
13,200
(373.7)
16,500
(467.2)
19,800
(560.6)
23,100
(654.1)
0.00
NP
NP
NP
NP
NP
NP
NP
NP
NP
NP = Not Permitted
Note: Not all models have these sizes.