Selecting appropriate duct construction is an important part of minimizing duct leakage. A specification for duct construction and sealing needs to be determined early in the duct design process. The engineer should specify a leakage class [ASHRAE, 1997] to meet the requirements of the system application and local energy conservation code. Project specification should allow appropriate duct leakage and leakage testing following the "HVAC Air Duct Leakage Test Manual" [SMACNA, 1985]. Random testing should always be required as part of air balancing. More extensive testing is required if the random testing reveals irregularities. [ASHRAE, 2000]
Studies indicate that supply and exhaust laboratory ductwork should be specified to leakage Class 3, except in special conditions, and should always be leak tested. Using welded or flanged and gasketed duct construction in critical exhaust systems will make ductwork virtually leak free. Leakage Class 3 is achievable by most contractors with some practice and guidance in duct construction and sealing techniques.
Leakage occurs at transverse and longitudinal joints and at connections, e.g. at tees, at Variable Air Volume (VAV) boxes, and at flexible terminal sections. Sealing the duct joints, usually with silicone-based sealant, is a basic method for avoiding the leaks. All duct manufacturers provide methods of sealing ductwork.
In most cases, sealing ductwork is economical. Duct sealing is recommended whenever electricity cost is greater than $0.02/kWh and sealing cost is less than $1.5/m2 ($0.14/ft2) [Tsal and Varvak, 1992]. Sealing of ducts is always required for process exhausts. A simple rule is: the higher the system cost, the greater the need for ductwork sealing. Duct sealing is always recommended for laboratories in California where electricity cost is higher than $0.02/kWh for every utility. Poorly installed duct systems that must later be resealed can cost more than a usual installation. [ASHRAE, 2000] [ASHRAE, 1996].