Minimum Clearance: 2 in. (51mm) to combustibles
System D. Solid-Insulated, listed factory-built chimney length with an inside diameter 2 in. (51 mm) larger
than the chimney connector and having 1 in. (25.4mm) or more of insulation, serving as a pass-through for
a single-wall sheet steel chimney connector of minimum 24 gauge [0.024 in. (0.61 mm)] thickness, with a
minimum 2 in. (51 mm) air space between the outer wall of chimney section and combustibles.
Minimum length of chimney section shall be 12 in. (305 mm). Chimney section concentric with and spaced 1
in. (25.4 mm) away from connector by means of sheet steel support plates on both ends of chimney section.
Opening shall be covered, and chimney section supported on both sides with sheet steel supports of
minimum 24 gauge [0.024 in. (0.61 mm)] thickness.
Supports shall be securely fastened to wall surfaces on all sides and shall be sized to fit and hold chimney
section. Fasteners used to secure chimney section shall not penetrate chimney flue liner.
Minimum Clearance: 6 in. (152mm) to combustibles
System C. Sheet steel chimney connector, minimum 24 gauge [0.024 in. (0.61 mm)] in thickness, with a
ventilated thimble, minimum 24 gauge [0.024 in. (0.61 mm)] in thickness, having two 1 in. (25.4 mm) air
channels, separated from combustibles by a minimum of 6 in. (152 mm) of glass fiber insulation. Opening
shall be covered, and thimble supported with a sheet steel support, minimum 24 gauge [0.024 in. (0.61 mm))]
in thickness.
Supports shall be securely fastened to wall surfaces on all sides and shall be sized to fit and hold chimney
section. Fasteners used to secure chimney section shall not penetrate chimney flue liner.
SYSTEM C
Minimum 6” (152 mm)
glass fiber insulation
Two ventilated air channels,
each 1” (25.4 mm);
construction of sheet steel
Chimney connector
Masonry chimney
constructed to NFPA 211
C
hi
m
ne
y
flu
e
Minimum chimney clearance
from masonry to sheet steel supports
and combustibles 2” (51 mm)
Sheet steel supports
Two ventilated air channels,
each 1” (25.4 mm)
SYSTEM D
Sheet steel
supports
Chimney
section
Chimney
connector
Air space:
2” (51 mm) min.
C
hi
m
ne
y
flu
e
Minimum chimney clearance
to sheet steel supports
and combustibles 2” (51 mm)
Minimum clearance 2” (51 mm)
1” (25.4 mm) air space to
chimney length
Masonry chimney
constructed to NFPA 211
Sheet steel supports
Chimney connector
Chimney length
Figure 9: System C
Figure 10: System D
ALT 4-LE 040523-28
20
100003978
