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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)
(1,100 SQ. MM/KW)
FURNACE
INPUT
Free Area of
(BTUH)
Opening and
Duct
Sq. In (Sq. mm)
26,000*
13 (8388)
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
AIR
(1,000'S BTUH GAS INPUT
CHANGES
PER
RATE)
HOUR
30
(ACH)
1,050
1,400
0.60
(29.7)
(39.6)
1,260
1,680
0.50
(35.6)
(47.5)
1,575
2,100
0.40
(44.5)
(59.4)
2,100
2,800
0.30
(59.4)
(79.2)
3,150
4,200
0.20
(89.1)
(118.9)
6,300
8,400
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
440 01 6101 00
Round Duct
In. (mm) Dia
4 (102)
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
26
1,750
910 (25.8)
(49.5)
1092
2,100
(59.4)
(30.9)
2,625
1365
(74.3)
(38.7)
1820
3,500
(99.1)
(51.5)
2730
5,250
(148.6)
(77.3)
5460
10,500
(297.3)
(154.6)
NP
NP
NP
Specifications subject to change without notice.
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)
9 (5807)
4 (102)
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
(39.6)
(42.5)
1,680
1,800
(47.5)
(51.0)
2,100
2,250
(59.4)
(63.7)
2,800
3,000
(79.2)
(84.9)
4,200
4,500
(118.9)
(127.3)
8,400
9,000
(237.8)
(254.6)
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.
L12F012
Fig. 7 - - Air for Combustion, Ventilation, and Dilution from
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of
Opening and
Duct
Sq. In (mm)
7 (4517)
10 (6452)
15 (9678)
20 (12904)
25 (16130)
30 (19355)
35 (22581)
80
100
120
2,000
2,500
3,000
(56.6)
(70.8)
(84.9)
2,400
3,000
3,600
(67.9)
(84.9)
(101.9)
3,000
3,750
4,500
(84.9)
(106.1)
(127.3)
4,000
5,000
6,000
(113.2)
(141.5)
(169.8)
6,000
7,500
9,000
(169.8)
(212.2)
(254.6)
12,000
15,000
18,000
(339.5)
(424.4)
(509.2)
NP
NP
NP
Indoors
Round Duct
In. (mm) Dia.
3 (77)
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
L12F013
11
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