Figure

HVAC

Double duct offsets

Separate plan and elevation calculations and correct the sloping length for a double offset.1

A double-offset layout separates plan and elevation information before finding true length. A length seen in plan can still include an elevation slant. Record intermediate results with their view and meaning before transferring them.

Plan, elevation and true length

Keep plan and elevation measurements separate. Record each result before starting the next stage.

Start a new calculation

Use 2nd if a unit is above its button.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Select length units.

Plan view

Plan θ = 180° − A; b is the hypotenuse. x is horizontal offset; y is still a slant length affected by elevation.

Plan right triangle: x is beside Theta, y is opposite Theta, and the hypotenuse r is measured side b. Theta is 180 degrees minus angle A of the measured triangle.IIIAθacr = bxy
Schematic, not to scale.

1. Enter side a

Skip Close if no mode is open.

  1. Press the keys

    Press Close calculation mode, then Open Oblique Triangle.

  2. Plan side a

    Press 2509, then millimeters, then Store side a.

Enter side b

  1. Plan side b

    Press 2096, then millimeters, then Store side b.

Enter side c

  1. Plan side c

    Press 1067, then millimeters, then Store side c.

  2. Press the keys

    Press Close calculation mode.

Find the pitch angle

Skip Close if no mode is open.

  1. Subtract angle A from 180°

    Press 180, then Subtract, then Close calculation mode, then Open Oblique Triangle, then Close calculation mode.

  2. Complete the subtraction

    Press Equals.

  3. Open Right

    Press Close calculation mode, then Open Right Triangle.

  4. Assign Pitch

    Press Store pitch.

Read the stored side

Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Oblique Triangle.

Use = to select Side b.

Use b as the diagonal

Skip Close if no mode is open.

  1. Press the keys

    Press Close calculation mode.

  2. Complete the value

    Press Equals.

  3. Press the keys

    Press Close calculation mode, then Open Right Triangle.

  4. Press the keys

    Press Store diagonal.

2. Horizontal offset

  1. Press Equals.
  1. Example result357.7207 millimeters

Use = to select Run.

3. Plan slant length

  1. Press Equals.
  1. Example result2065.249 millimeters

Use = to select Rise.

4. Enter side a

Record the plan results first. In elevation, θ = 90° − A. Use b as the diagonal.

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Press the keys

    Press Close calculation mode, then Open Oblique Triangle.

  3. Elevation side a

    Press 2088, then millimeters, then Store side a.

Enter side b

  1. Elevation side b

    Press 2096, then millimeters, then Store side b.

Enter side c

  1. Elevation side c

    Press 356, then millimeters, then Store side c.

  2. Press the keys

    Press Close calculation mode.

Find the pitch angle

Skip Close if no mode is open.

  1. Subtract angle A from 90°

    Press 90, then Subtract, then Close calculation mode, then Open Oblique Triangle, then Close calculation mode.

  2. Complete the subtraction

    Press Equals.

  3. Open Right

    Press Close calculation mode, then Open Right Triangle.

  4. Assign Pitch

    Press Store pitch.

Read the stored side

Skip Close if no mode is open.

  1. Press Close calculation mode, then Open Oblique Triangle.

Use = to select Side b.

Use b as the diagonal

Skip Close if no mode is open.

  1. Press the keys

    Press Close calculation mode.

  2. Complete the value

    Press Equals.

  3. Press the keys

    Press Close calculation mode, then Open Right Triangle.

  4. Press the keys

    Press Store diagonal.

5. Elevation horizontal leg

  1. Press Equals.
  1. Example result2083.887 millimeters

Use = to select Run.

6. Elevation offset

  1. Press Equals.
  1. Example result225.0112 millimeters

Use = to select Rise.

Correct the plan slant

The plan slant is r; the elevation offset is y. Their right triangle gives the true length x.

True-length right triangle: the slant measurement r is the hypotenuse, elevation offset y is perpendicular to the horizontal true length x. Find x from r and y.r · slantx · true lengthy
Schematic, not to scale.

7. Find true length

  1. Start fresh

    Press Clear entry, then Clear entry.

  2. Recorded plan slant

    Press 2065, then millimeters, then Store diagonal.

  3. Recorded elevation offset

    Press 225, then millimeters, then Store rise.

  4. Complete the value

    Press Equals.
  1. Example result2052.706 millimeters

Keep in mind

Plan slant is not true length. Keep plan and elevation labeled. Transferring rounded lengths changes the final answer slightly.

Display assumptions

  1. Fraction precision: 1/16″; Fraction format: Simplified; Angle decimal places: Variable. ↩

Keep the guide beside your calculation.

These examples are built into Figure. Open Help while you calculate, then return to the same guide and reading position.

See Help in Figure →