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Adjustable Length Section
An 18" Adjustable Length Section (18A) is
available for Large Diameter Type B Gas
Vent is to allow for installation where fixed-
length sections do not produce the desired
dimensions. Sizes 10-inch and above use
two sheet metal screws equally spaced
around the mating section. The Adjustable
Length telescope over a fixed length, to
accommodate odd distances between vent
lengths or connectors.
Adjustable Length Section may also be used
to compensate for linear thermal expansion
between two fixed points in the vent system.
Examples of this are shown in Figure 36. In
these examples the fastening devices are not
used. This allows for expansion between two
fixed supports or between a fixed support and
the appliance. Note that the fastening means
should be left loose only if the Vent and the
Adjustable Length Sections are installed
vertically. If the Adjustable Length Sections
are used in a position other than vertical,
such as between offsets, the fastening means
should be used. Finally, when joining an
Adjustable Length Section to another section
of Type B Gas Vent, make sure that there is
a minimum overlap of two inches. DO NOT
USE Adjustable Lengths to suspend any
weight of pipe below.
USE OF GAS VENT FITTINGS
Model BV tees, elbows, increasers, and short
lengths are specially designed to facilitate
interconnections.
Tees used to start vertical vents must use a
tee cap to prevent air leakage. All unused
openings in a gas vent must be sealed to
prevent loss of effective vent action.
A Wall Thimble is available in diameters of
10" and 12" only. It is used where a Type
B Gas Vent passes through a vertical wall.
The Wall Thimble will provide for the proper
clearance to combustible material as the vent
passes through the wall, and will also act
as a firestop for the vent. NOTE: For larger
diameters (14" to 30") a Firestop Support
Plate (FSP) must be used installed in a fully
framed opening at the required air space
clearances to combustible materials.
NOTE: DIAGRAMS & ILLUSTRATIONS ARE NOT TO SCALE.
Elbows are used to offset installation to
avoid obstructions such as rafters and joists.
Diameters 10" to 14" are fully adjustable
from 0° to 45°. Diameters 16" to 30" are
non-adjustable.
Termination of Vent Must be Securely
Guyed or Braced if it Extends more than
5 Feet Above Roof
Seal Around
Collar and
Flashings
Maintain Proper
Clearance
Terminate
Vent
At Least 5
Feet
Draft Hood
Above Draft
Connector
Hood
Listed
Appliance
Whenever the vent is changed from a
vertical direction, the vent system must be
resupported before and after the change in
direction. The support should provide rigid
stability, both vertical and horizontally (see
Fig. 37). Check with your local Building
Officials to make sure any alternative support
method is acceptable.
See
Table 1 -
page 14
Lateral Support
Firestop
Support
Assembly
Vertical Vent
Fig. 37
Elbow
Resupport
Adequate
Air Supply
23
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