HVAC Air Duct Principles

Calculations

• The size of a home’s climate control system and its duct work determine both the comfort of the homeowner and the home's energy use. If the system is too small, it will not keep the house comfortable. If it is too large, it will cycle off and on too frequently and use more energy than it would if it were smaller. HVAC professionals can size a home's ducts using Manual D from the Air Conditioning Contractors of America. Air conditioners or furnaces can be sized with Manual J from the same source. Both manuals take into account general climate, location and design of the house, and the number of occupants in the home.

Codes

• Local, state and federal building codes define the rules of air duct construction and use. The codes are based on the International Residential Code (IRC), which was developed by the International Code Council, a member organization founded in 1994. The IRC is updated as needed. It standardizes construction techniques and approvals so that a contractor who learns his trade to correct code specifications in Boston can perform the same job in Los Angeles and receive the same building approvals.

Thickness

• The 2012 IRC requires that above-ground HVAC ducts are made of either galvanized steel or aluminum with a wall thickness that varies by size. For example, galvanized steel round and enclosed rectangular ducts that are 14 inches or under need a minimum thickness of 0.0157 inches, while aluminum versions need 0.0145. At 16 to 18 inches, galvanized steel thickness goes up to 0.187 inches, while aluminum reaches 0.018. At 20 inches and more, galvanized steel minimums are 0.0236 inches, and aluminum becomes 0.023. If the ducts are exposed and rectangular, then those 14 inches or fewer can be 0.0157 inches in galvanized steel or 0.0145 inches in aluminum. At more than 14 inches, galvanized steel thickness reaches 0.0187 inches, and aluminum goes to 0.018 inches.

Others

• Equipment connected to HVAC ducts must discharge air at no greater than 250 degrees Fahrenheit. Coverings, linings and adhesives used for ducts must have flame-spread indexes of 25 or under, and they must not glow, smolder or smoke at the operating temperature of the climate control system. Metal ducts require support every 10 feet or fewer by 1/2-inch wide 18-gauge metal straps or 12-gauge galvanized wires. Joints, seams or connections in duct work require fastening with adhesives, welds, gaskets or tape to avoid air loss. Contractors must adhere to manufacturer’s instructions when sealing metal duct work. The instructions typically specify that round metal ducts need at least three equally spaced metal screws of rivets for mechanical fastening.