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Table 3 – Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH)
FURNACE
(1,100 SQ. MM/KW)
INPUT
(BTUH)
Free Area of
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)
(118.9)
6,300
0.10
(178.0)
(237.8)
0.00
NP
NP = Not Permitted
*Minimum dimensions of 3‐in. (76mm)
NOTE: Use any of the following combinations of openings:
A & B, C & D, D & E, F & G
Fig. 6 - - Air for Combustion, Ventilation, and Dilution for
Outdoors
Round Duct
Opening and 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
1,400
(39.6)
(49.5)
(39.6)
1,680
2,100
1,680
(47.5)
(59.4)
(47.5)
2,100
2,625
2,100
(59.4)
(74.3)
(59.4)
2,800
3,500
2,800
(79.2)
(99.1)
(79.2)
4,200
5,250
4,200
(148.6)
(118.9)
8,400
10,500
8,400
(297.3)
(237.8)
NP
NP
NP
L12F012
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH)
(734 SQ. MM/KW)
Free Area of
Round Duct
In. (mm) Dia
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
80
3
3
Space Volume Ft
(M
)
1,500
2,000
(42.5)
(56.6)
1,800
2,400
(51.0)
(67.9)
2,250
3,000
(63.7)
(84.9)
3,000
4,000
(84.9)
(113.2)
4,500
6,000
(127.3)
(169.8)
9,000
12,000
(254.6)
(339.5)
NP
NP
* Minimum opening size is 100 sq in. (64516 sq. mm) with
minimum dimensions of 3‐in. (76mm)
{ Minimum of 3‐in. (76mm) when type‐B1 vent is used.
Fig. 7 - - Air for Combustion, Ventilation, and Dilution from
11
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of Open-
Round Duct
ing and Duct
In. (mm) Dia.
Sq. In (mm)
10 (6452)
4 (102)
15 (9678)
5 (127)
20 (12904)
5 (127)
25 (16130)
6 (152)
30 (19355)
6 (152)
35 (22581)
7 (178)
100
120
140
2,500
3,000
3,500
(70.8)
(84.9)
(99.1)
3,000
3,600
4,200
(84.9)
(101.9)
(118.9)
3,750
4,500
5,250
(106.1)
(127.3)
(148.6)
5,000
6,000
7,000
(141.5)
(169.8)
(198.1)
7,500
9,000
10,500
(212.2)
(254.6)
(297.1)
15,000
18,000
21,000
(424.4)
(509.2)
(594.1)
NP
NP
NP
Indoors
L12F013
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