Instructions / Assembly

Heating and Cooling Essentials
Figure 28-33. Installing a top-mounted takeoff. A—Draw the outline for the opening. B—Punch a hole to use when starting
the cutting snips. C—Cut around the outline.
starting collar or takeoff is used. Make a template of
the opening required and draw its outline accurately on
the metal to be cut, Figure 28-33A. Punch a hole through
the sheet metal within limits of the opening, using a
hammer and chisel or similar tool, Figure 28-33B. Insert
aviator snips and cut to the outline, then around the
outline until the opening is complete, Figure 28-33C.
When a branch pipe will be run from the top of an
extended plenum, the opening must be cut and the
takeoff or starting collar installed before the plenum is
fastened in place. Openings on the sides of the plenum
can be made at any time.
28.10.3 Rectangular 90° Elbows
Rectangular elbows are not adjustable, but are
available in 90° "long-way" or 90° "short-way" forms,
Figure 28-34. They are also available in 45° configu-
rations. A long-way elbow changes the direction of the
Long-way
Short-way
long dimension, and the short-way elbow changes the
direction of the short dimension. To form any smaller
angle needed, 45° and 90° rectangular elbows can be
cut.
28.11 Duct Support
All duct should be run in a straight line and
supported at regular intervals — typically, 2'-5' — to
prevent any sags. The best support material is woven
polypropylene strap that comes in various widths. It is
sometimes called "webbing strap" and is wide enough
so it won't bite into flex duct. Perforated metal strap is
a second choice, with steel wire being least desirable.
Steel wire is still often used to support round pipe for
branch runs. The wire should be wrapped once around
the pipe before anchoring to a joist. See Figure 28-35.
Window
Perforated
pipe strap
Steel wire
Figure 28-34. Rectangular elbows are made in two vari-
eties: long-way and short-way. They are not adjustable.
Figure 28-35. Three methods of duct support include woven
polypropylene strapping, perforated pipestrap, and wire.