How to Plan Ductwork

Obtain or draw a house plan showing the location of the heating/cooling unit and all the elements that must be worked around, like hot water heaters, hot water pipes, electrical boxes and fixtures or other obstructions. Mark places where ducts likely can be placed, such as between joists in a basement or attic and up through walls or closets to get from one story to another. Diagram every vent on outside walls and mark places where return inlets can be placed on interior walls.

Determine the size of ductwork needed. Start by researching the heating/cooling unit's airflow output. Size the main supply duct to accommodate that airflow and size branch ducts to individual vents so the total is equal to the output from the unit. Place return openings on interior walls, either in the same space as supply vents or in a central area where air flows naturally. Size return ducts so their total airflow is balanced with the output.

Draw lines to connect the supply or output side of the unit to every vent in the shortest, straightest, most direct paths. Then test that pattern by examining the joists, walls and other framing elements to make sure the ductwork fits in those spaces. Use a tape measure to confirm. Avoid any sharp turns or tight fits, which might crimp or squeeze the duct. Try to go under floors, or in a basement or deep crawl space if possible. Use an attic if necessary, and find the most direct path to get air up to that level.

Locate places to secure the duct along each duct's path; flexduct, the kind most commonly used in houses, must be secured in under-floor installations with metal straps that surround the duct and fasten to joists on either side. Use natural supports like gas lines or plumbing drain pipes where available. Plan ductwork in an attic to lay between joists and with enough fasteners to keep it from moving with any wind gusts.

Plan special sections to be made with rigid duct board. Do this for any areas too tight for flexduct. Duct board can be cut and formed into rectangles to fit in small spaces, but make sure the volume of those ducts is equal to the flexduct on either end of it. Remember that air flows more efficiently through a cylinder than a rectangle, so use duct board only when absolutely necessary.