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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)
40,000*
20 (12904)
60,000
30 (19355)
80,000
40 (25807)
100,000
50 (32258)
120,000
60 (38709)
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
0.60
1,050 (29.7)
0.50
1,260 (35.6)
0.40
1,575 (44.5)
0.30
2,100 (59.4)
0.20
3,150 (89.1)
6,300
0.10
(178.0)
0.00
NP
NP = Not Permitted
Fig. 6 - - Air for Combustion, Ventilation, and Dilution for
Outdoors
Round Duct
In. (mm) Dia
5 (127)
6 (152)
7 (178)
8 (203)
9 (229)
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
1,750
1,400 (39.6)
(49.5)
2,100
1,680 (47.5)
(59.4)
2,625
2,100 (59.4)
(74.3)
3,500
2,800 (79.2)
(99.1)
4,200
5,250
(118.9)
(148.6)
8,400
10,500
(237.8)
(297.3)
NP
NP
L12F012
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)
14 (8696)
5 (127)
20 (13043)
5 (127)
27 (17391)
6 (152)
34 (21739)
7 (178)
40 (26087)
7 (178)
= 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 (39.6)
1,500 (42.5)
1,680 (47.5)
1,800 (51.0)
2,100 (59.4)
2,250 (63.7)
2,800 (79.2)
3,000 (84.9)
4,200
4,500
(118.9)
(127.3)
8,400
9,000
(237.8)
(254.6)
NP
NP
Fig. 7 - - Air for Combustion, Ventilation, and Dilution from
9
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of
Opening and
Round Duct
In. (mm) Dia.
Duct
Sq. In (mm)
10 (6452)
15 (9678)
20 (12904)
25 (16130)
30 (19355)
80
100
2,000 (56.6)
2,500 (70.8)
2,400 (67.9)
3,000 (84.9)
3,750
3,000 (84.9)
(106.1)
4,000
5,000
(113.2)
(141.5)
6,000
7,500
(169.8)
(212.2)
12,000
15,000
(339.5)
(424.4)
NP
NP
Indoors
4 (102)
5 (127)
5 (127)
6 (152)
6 (152)
120
3,000 (84.9)
3,600
(101.9)
4,500
(127.3)
6,000
(169.8)
9,000
(254.6)
18,000
(509.2)
NP
L12F013
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