11
Table 3 – Minimum Free Area Required for Each Combustion Air Opening or Duct to Outdoors
FURNACE
INPUT
(BTUH)
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH)
(1,100 SQ. MM/KW)
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH)
(734 SQ. MM/KW)
TWO OPENINGS OR
VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH)
(550 SQ. MM/KW)
Free Area of
Opening and
Duct
Sq. In (Sq. mm)
Round Duct
In. (mm) Dia
Free Area of
Opening and
Duct
Sq. In (Sq. mm)
Round Duct
In. (mm) Dia
Free Area of
Opening and
Duct
Sq. In (mm)
Round Duct
In. (mm) Dia.
26,000* 13 (8388) 4 (102) 9 (5807) 4 (102) 7 (4517) 3 (77)
40,000* 20 (12904) 5 (127) 14 (8696) 5 (127) 10 (6452) 4 (102)
60,000 30 (19355) 6 (152) 20 (13043) 5 (127) 15 (9678) 5 (127)
80,000 40 (25807) 7 (178) 27 (17391) 6 (152) 20 (12904) 5 (127)
100,000 50 (32258) 8 (203) 34 (21739) 7 (178) 25 (16130) 6 (152)
120,000 60 (38709) 9 (229) 40 (26087) 7 (178) 30 (19355) 6 (152)
140,000* 70 (45161) 10 (254) 47 (30435) 8 (203) 35 (22581) 7 (178)
*Not all families have these models.
EXAMPLES: Determining Free Area
FURNACE WATER HEATER TOTAL INPUT
100,000 + 30,000 = (130,000 divided by 4,000) = 32.5 Sq. I n. for each two Vertical Ducts or Openings
60,000 + 40,000 = (100,000 divided by 3,000) = 33.3 Sq. In. for each Single Duct or Opening
80,000 + 30,000 = (110,000 divided by 2,000) = 55.0 Sq. In. for each two Horizontal Ducts
Table 4 – Minimum Space Volumes for 100% Combustion, Ventilation and Dilution Air from Outdoors
AIR
CHANGES
PER
HOUR
(ACH)
OTHER THAN FAN-ASSISTED
TOTAL
(1,000’S BTUH GAS INPUT
RATE)
FAN-ASSISTED TOTAL
(1,000’S BTUH GAS INPUT RATE)
30 40 50 26 40 60 80 100 120 140
SpaceVolumeFt
3
(M
3
)
0.60
1,050
(29.7)
1,400
(39.6)
1,750
(49.5)
910 (25.8)
1,400
(39.6)
1,500
(42.5)
2,000
(56.6)
2,500
(70.8)
3,000
(84.9)
3,500
(99.1)
0.50
1,260
(35.6)
1,680
(47.5)
2,100
(59.4)
1092
(30.9)
1,680
(47.5)
1,800
(51.0)
2,400
(67.9)
3,000
(84.9)
3,600
(101.9)
4,200
(118.9)
0.40
1,575
(44.5)
2,100
(59.4)
2,625
(74.3)
1365
(38.7)
2,100
(59.4)
2,250
(63.7)
3,000
(84.9)
3,750
(106.1)
4,500
(127.3)
5,250
(148.6)
0.30
2,100
(59.4)
2,800
(79.2)
3,500
(99.1)
1820
(51.5)
2,800
(79.2)
3,000
(84.9)
4,000
(113.2)
5,000
(141.5)
6,000
(169.8)
7,000
(198.1)
0.20
3,150
(89.1)
4,200
(118.9)
5,250
(148.6)
2730
(77.3)
4,200
(118.9)
4,500
(127.3)
6,000
(169.8)
7,500
(212.2)
9,000
(254.6)
10,500
(297.1)
0.10
6,300
(178.0)
8,400
(237.8)
10,500
(297.3)
5460
(154.6)
8,400
(237.8)
9,000
(254.6)
12,000
(339.5)
15,000
(424.4)
18,000
(509.2)
21,000
(594.1)
0.00 NP NP NP NP NP NP NP NP NP 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
L12F012
Fig. 6 -- Air for Combustion, Ventilation, and Dilution for
Outdoors
* 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.
L12F013
Fig. 7 -- Air for Combustion, Ventilation, and Dilution from
Indoors