9
TABLE 2—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 Open-
ing and Duct
(Sq. In.)
Round Duct
(In. Dia)
Free Area of
Opening and Duct
(Sq. In.)
Round Duct
(In. Dia)
Free Area of Open-
ing and Duct
(Sq. In.)
Round Duct
(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
ACH
OTHER THAN FAN---ASSISTED TO-
TAL
(1,000’S BTUH GAS INPUT RATE)
FAN---ASSISTED TOTAL
(1000’S BTUH GAS INPUT RATE)
30
40
50
44
66
88
110
132
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
PG
8J
/M
