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Table 2-Minimum
Free Area Required for Each Combustion Air Opening or Duct to Outdoors
TWO HORIZONTAL
DUCTS
SINGLE DUCT OR OPENING
TWO OPENINGS
OR VERTICAL
DUCTS
FURNACE
(1 SQ. IN./2,000 BTUH) (1,100 SQ. IvIM/KW)
(1 SQ. IN./3,000 BTUH) (734 SQ. MM/KW)
(1 SQ. IN./4,000 BTUH) (550 SQ. IvIM/KW)
iNPUT
Free Area 01
Free Area 01
Free Area of
(BTUH)
Opening and Duct
Round Duct
Opening and Duct
Round Duct
Opening and Duct
Round Duct
(Sq. In.)
(in. Dia)
(sq In.)
(in. Dia)
(Sq In.)
(In. Dia)
44,000
22
6
14.7
5
11
4
66,000
33
7
22
6
16.5
5
88,000
44
8
29.3
7
22
6
110,000
55
9
36.7
7
27.5
6
132,000
66
10
44
8
33
7
154,000
77
10
51.3
9
38.5
8
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 3-Minimum
Space Volumes
for 100% Combustion,
Ventilation,
and
Dilution from Indoors
OTHER THAN FAN=ASSiSTED
TOTAL
FAN-ASSISTED
TOTAL
{1,000'S BTUR GAS iNPUT RATE)
(1,000'S BTUR GAS iNPUT RATE)
ACH
30
I
40
!
50
44
!
66
I
88
I
110
!
132
I
154
Space Volume (ft 3)
0.60
1,050
1,400
1,750
1,100
1,650
2,200
2,750
3,300
3,850
0.50
1,260
1,680
2,100
1,320
1,980
2,640
3,300
3,960
4,620
0.40
1,575
2,100
2,625
1,650
2,475
3,300
4,125
4,950
5,775
0.30
2,100
2,800
3,500
2,200
3,300
4,400
5,500
6,600
7,700
0.20
3,150
4,200
5,250
3,300
4,950
6,600
8,250
9,900
11,550
0.10
6,300
8,400
10,500
6,600
9,900
13,200
16,500
19,800
23,100
0.00
NP
NP
NP
NP
NP
NP
NP
NP
NP
NP = Not Permitted
2. Fig. 7 illustrates
how to provide
TWO
OUTDOOR
OPEN-
INGS, one inlet and one outlet combustion
and ventilation
air
opening,
to the outdoors.
a. One opening
MUST
commence
within
12" (300 ram)
of
the
ceiling
and the
second
opening
MUST
colnmence
within
12" (300 rain) of the floor.
b. Size openings
and ducts per Fig. 7 and Table
2.
c. TWO
HORIZONTAL
DUCTS
require
1 square
inch
of
free area per 2,000 Btuh
(l,lO0
nnn2/kW)
of cembined
input R)r all gas appliances
in the space per Fig. 7 and Table
2.
d. TWO
OPENINGS
OR
VERTICAL
DUCTS
require
1
square
inch ol free area per 4,000
Btuh (. 50 mm-/l<W)
l_r
combined
input of all gas appliances
in the space
per Fig.
7 and Table 2.
3. ONE OUTDOOR
OPENING
requires:
a. 1 square
inch of free area per 3,000 Bmh
(734 elnl2/kW)
liar combined
input of all gas appliances
in the space
per
Table
2 and
b. Not less than the sum of the areas ef all vent connectors
in
the space.
The opening
shall commence
within
12" (300 nnn) of the ceiling.
Appliances
in the space
shall have clearances
of at least l" (25
ram) l_'om the sides and back and 6" (150 nnn) from the l_'ont. The
opening shall directly comnmnicate
with the outdoors or shall
communicate thamgh a vertical or horizontal duct to the outdoors
er spaces (crawl
or attic) that l]'eely conmmnicate
with the
eutdoors.
Indoor
Combustion
Air{) NFPA & AGA
Standard
and Known-Air-Infiltration
Rate Methods
Indoor air is permitted R)r combustion, ventilatiom and dilution,
if the Standard
or Known-Air-Infiltratlon
Method is used.
CARBON
MONOXIDE
POISONING
HAZARD
Failure
to lollow
this warning
could
result
in death
and/or personal
injury.
Many
homes
require
air to be supplied
l]'om outdoors
lk_r
furnace
combustion,
ventilation,
and dilution
of flue gases.
The
furnace
combustion
air
supply
must
be
provided
in
accordance
with this instruction
manual.
The Standard
Method:
1. The space
has no less vohnne
than 50 cubic
IL'et per l,I)I)I)
Btuh
of the
maxinmm
input
ratings
lk)r all gas
appliances
installed
in the space
and
2. The air infiltration
rate is not known
to be less than 0.40 air
changes
per hour (ACH).
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