Do It Yourself Ductwork

Draw a diagram of the house floor plan on graph paper. Locate where all vents should be on the outside walls of rooms, usually near windows or other openings. Mark where return ducts, to take "used" air from the house back to the heating unit, can be placed on interior walls, either in a room with a supply vent or in an area like a hallway where air will flow naturally. Identify any special obstructions or barriers like water heaters.

Lay out duct routes on the diagram, using as many straight lines as possible. Try a "trunk and branch" arrangement, with a central supply duct running the length (or width) of the house from the heating/cooling unit to the far wall, with individual ducts extending out to the vents. Use a similar approach to the returns, with collectors gathering air to be returned to the unit trough a large central duct. Ideally, put a return in every room with a vent; practically, install large returns in central collecting points like interior hallways.

Balance the supply and return ducts, so the return system will collect as much air as the supply side is pushing into the house. Return ducts must have the same total capacity of cubic feet per minute as the supply side is delivering. The unit should have a rating telling its output capacity; you can get formulas to calculate the volume of ducts. Basically it is high school math: pi/R/squared, pi times the radius of the duct squared.

Route ducts as straight as possible; avoid bends or very sharp turns. Plan ductwork to run between ceiling joists or under floor joists in a basement; put ducts in an attic between joists. Fasten ducts in a basement or under a floor or in a ceiling with metal straps that cross the ducts and fasten to joists about every 6 feet if there are no natural supports like gas lines. Secure ducts in a few places in an attic to prevent ductwork from blowing around. Avoid running ducts over hot water pipes or in areas subject to heat and moisture.

Connect ductwork joints with metal sleeves that fit inside the ducts. Fasten joints with heat-resistant tape, wrapped securely around the duct. Don't use duct tape; despite its name, its adhesive will not hold up on flex duct. Attach flex duct to the sheet metal plenums or big openings in the unit -- where supply and return elements enter the house -- with sheet metal screws, then cover those seams with tape.

Build special ducts of rigid foam board for areas where flex duct cannot be fitted, like in a tight wall cavity. Foam board can be cut and built with tape into squares, rectangles or other shapes to fit narrow openings. Fasten it to flex duct with metal sleeve connectors and tape.