Builders use a number of methods for duct construction, depending on house layout and building needs and code, and they incorporate components such as diffusers, registers, flues, vents, roof penetrations and filters. In duct construction, there are three main types of duct systems: low-pressure ductwork, medium-pressure ductwork and high-pressure ductwork. Identifying the duct system will help identify the type of ductwork needed for the job.

All duct construction involves components that maximize ventilation efficiency. Major elements include corner closures, elbows, hangers, “T” connections, fire dampers, access doors and air intakes. Duct fittings include caps, offsets, single-branch take-off, square-to-round transitions, register box and round flexible duct reinforced with spiral wound wire. The galvanized sheet metal used should have a galvanized coating on both sides of the sheet equal to at least 1.25 ounces (35.4 g) of zinc per square foot (0.09 square meters) and should be of lock-forming quality.

The best duct design is one that utilizes these construction components to eliminate vibration, which is accomplished by tightly installing rigid, constructed metal ducts. Rectangular ducts should be made with standing seams and braced with angle iron to stiffen the sheet and avoid vibration. The velocity in the main duct and the remote branches should be lowered gradually to distribute the air uniformly. This is done by increasing the size of the duct, and it cuts down the friction in the smaller ducts.

Reducing “leakage” is another important aspect in duct construction. Air pressure loss within the duct system can be avoided by soldering, welding or otherwise sealing all joints and seams to make them tight. They also can be riveted or spot welded. The lap joints should be made so that the outlet of one length of ductwork fits into the inlet end of the next length of ductwork in the direction of air flow.