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Laying Out Rectangular Duct Transitions

August 30, 2005
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Figure 1. Transition – flat on top.
Transitions are needed in almost every duct run to change the size or shape of the ductwork. This article explains how to lay out rectangular transitions.

Two-Piece Transition

A duct run is often kept flat on the top to keep it against the overhead. A transition is also likely to be flat on top. If the duct is relatively small and the change is only in the height of the duct, such as a 12 x 12 duct changing to a 12 x 6 duct (Figure 1), the transition is often made in two pieces. A two-piece transition can be cut out of a sheet economically and has only two seams.

Figure 2. Patterns for two-piece transition – flat on top, shown in Figure 1.
Figure 2 shows the layout for the two-piece transition shown in Figure 1. The finished fitting is 14 inches long, but the bottom piece is longer than this. This is because it must be long enough to fit the slanted side of the fitting. Use dividers or a rule to transfer dimension X from the slanted side of the pattern to the straight bottom piece pattern.

Of course, allowances for seams and connections have to be added to make a complete pattern.

Figure 3. Four-piece transition.

Four-Piece Transition

More often, transitions are made in four pieces in order to save both time and sheet metal. Figure 3 shows the plan and elevation of a typical four-piece transition. Each side of this transition is different. You must learn how to lay out these transitions and how to check your layouts by matching sides.

Always lay out the patterns for each side so that the inside is the side you see. This is so the marking instructions for forming will be on the inside of the fitting when it is finished. Also, it is less confusing if you see all the patterns from the same side. To lay out the patterns with the inside up, picture yourself as being inside the fitting and looking at the side. You can then visualize whether an edge slants to the left or to the right.

The length of the finished fitting in Figure 3 is 6-1/2 inches, but none of the patterns will be 6-1/2 inches long. Because each side slants, the length of each pattern piece will be determined by the true length of the centerline of that side.

Figure 4 shows a plan view of the fitting marked to explain the layout. Each side is identified by a number. The corners are identified with letters.

Figure 4. Plan view used for layout for four-piece transition.

Lay Out Side 1

To lay out side 1:

  • Draw a true length triangle (Figure 5A) to determine the true length of the centerline (KL) of Side 1 in Figure 4. Since the fitting is 6-1/2 inches (Figure 3), the true length triangle is 6-1/2 inches high. Since this side slants in 1 inch, draw a 1 inch side on the true length triangle (A in Figure 5). The third side of the triangle is the true length of line KL. By measurement on the triangle the true length is found to be 6-9/16 inches.

  • To start the layout, draw the bottom of side 1 at 11 inches (Figure 5A). This is line DC in the plan view (Figure 4). Note that D is marked on the left end of the line, since you want to see the inside of the pattern.

    Figure 5. Patterns for four-piece transition in Figure 3.
  • Square up a line from point D to provide a line to measure from.

  • Draw a line parallel to line DC and 6-9/16 inches above it (the true length of side 1).

  • Measure over 1 inch from the square line to establish point H, because side 3 slants in 1 inch.

  • From point H, measure 7 inches to establish point G, because the top opening is 7 inches on Side 1.

  • Draw lines DH and CG to complete the layout of Side 1.

    To complete the pattern, add allowances for seams and connectors. Mark the pattern as shown to help you keep everything straight in your head. Mark the following:

  • SIDE 1

  • IN

  • The dimensions of the top and bottom openings.

  • Any bending instructions required.

  • An X near line DH and an XX near line CG. (You will see in a bit how they are helpful in matching up corners.)

    In actual shop work, you usually won't have a plan view to work from. And once you learn the method, you won't need the letters. They are only part of the learning process. What you will generally have is a shop ticket or a sketch to work from like the one in Figure 6. As you gain experience, this is all you will need. If you have trouble visualizing the fitting, make a freehand sketch of the plan view and letter the corners.

    Figure 6. Shop ticket for Figure 3 transition.

    Lay Out Side 2

    Lay out side 2 (Figure 5B) as you laid out side 1:

  • Draw a true length triangle to determine the true length of the centerline MN. Calculation shows that side 2 slants in 2 inches:

    9 – (6 + 1) = 2 inches

    The third side of the triangle is the true length of MN (by measurement, 6-3/4 inches) (Figure 5B).

  • To start the layout, draw the bottom of side 2 at 11 inches (line BA). B is on the left end to make the inside of the pattern up.

  • Square up a line from point B to provide a line to measure from.

  • Draw a line parallel to line BA and 6-3/4 inches above it (the true length of side 2).

  • Measure over 3 inches from the square line to establish point F, because side 4 slants in 3 inches:

    11 – (7 + 1) = 3 inches

  • From point F, measure 7 inches to establish point E, because the top opening is 7 inches.

  • Draw lines BF and AE to complete the layout (Figure 5B).

    Add allowances for seams and connections. Mark the pattern. Add XXX near edge BF and XXXX near edge AE.

    Lay Out Sides 3 And 4

    The patterns for sides 3 (Figure 5C) and 4 (Figure 5D) are laid out in the same way. Add the X marks as shown.

    Check The Patterns

    You won't have the corner letters on your pattern (they are just for explanation), so use the X marks on the edges to check the patterns and make sure the mating edges will fit together:

  • A single X was marked on edge DH on side 1 and edge DH on side 3. These two edges must fit together, so they must be the same length. If they are not, one of the patterns is wrong.

  • Edge CG on side 1 and CG on side 4 are marked XX. Check to see if these are the same length.

  • Check edges BF and AE in the same way.

    The X marks can also be used to avoid confusion when the fitting is assembled. The two edges with XXXX must be assembled together, the two edges with XXX must be assembled together, and so on.

    Excerpted and reprinted from Layout for Duct Fittings by Leo A. Meyer, one of the books in the Indoor Environment Technician's Library series published by LAMA Books. For over 30 years, Meyer has been writing and publishing training materials for the HVAC industry. His books cover a wide range of topics, including heating and cooling, indoor air quality, sheet metal work, electricity basics, safety, and others. For more information, visit www.lamabooks.com.

    Publication date: 09/05/2005

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