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Minimum Opening (Square Inches)
Minimum Permanent Free Opening, Square Inches
for Masonry Built Fireplaces
Chimney Height (Feet)
6
8
10
15
20
30
18” Log Set
49.2
45.5
41.7
37.7
34.3
31.2
24” Log Set
56.6
52.4
48.2
43.2
39.8
35.9
Table 1
Minimum Opening (Square Inches)
Minimum Permanent Free Opening, Square Inches
for Factory Built Fireplaces
Chimney Height (Feet)
10
15
20
25
30
35
40
18” Log Set
35.3
26.4
22.1
18.1
17.3
15.9
15.2
24” Log Set
44.2
32.2
26.4
22.9
20.4
18.9
18.1
Table 2
3