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FIGURE 6-2: DOUBLE THIMBLE - WALL OUTLET DETAILS
EXHAUST PIPING THRU WALLS AND ROOFS
Always check local and state laws pertaining to hot gas pipes before making plans for running
exhaust piping thru walls or roofs. If the exhaust pipe must pass thru combustible walls or roofs,
use an exhaust thimble to prevent exhaust pipe heat from being transmitted to the combustible
material. Construction details of a typical thru wall double sleeve thimble is shown above —
thimbles are not available. They are usually fabricated at local sheet metal shops to the
specifications furnished by the installation engineer. The minimum outside diameter of a double
thimble (inner and outer sleeves) should be at least six inches greater than the diameter of the
exhaust pipe — with single sleeve thimbles, the O.D. should be at least 12 inches greater than
the diameter of exhaust pipe. The minimum thimble diameters for some common exhaust pipe
sizes are listed in the table below. The minimum thimble diameter for some common exhaust pipe
sizes are listed in the table below. The thimbles should be constructed so that they extend at
least 10" both ways from the surface of the wall or roof. Holes are provided at both ends to allow
cooling air to circulate thru the thimble. If screening is used on the outer end to keep birds,
rodents, etc., from entering the thimble, make sure the mesh is large enough so it doesn’t impair
air circulation through the thimble. If the exhaust pipe must exit thru a roof,
EXHAUST THIMBLE - MINIMUM O.D.
MINIMUM OUTSIDE DIAMETER MINIMUM OUTSIDE DIAMETER
EXHAUST
PIPE
DIAMETER
SINGLE SLEEVE
THIMBLE*
DOUBLE SLEEVE
THIMBLE**
EXHAUST
PIPE
DIAMETER
SINGLE SLEEVE
THIMBLE*
DOUBLE SLEEVE
THIMBLE**
0.5"
0.75"
1.0"
1.25"
1.5"
2.0"
12.5"
12.75"
13.0"
13.25"
13.5"
14.0"
6.5"
6.75"
7.0"
7.25"
7.50"
8.0"
2.5"
3.0"
3.5"
4.0"
5.0"
14.5"
15.0"
15.5"
16.0"
17.0"
8.5"
9.0"
9.5"
10.0"
11.0"
*DIAMETER OF PIPE PLUS 12". **DIAMETER OF PIPE PLUS 6"