When using low-pressure VAV and CAV systems, the total allowable leak must not exceed 2 percent, including the box and downstream ducts. The box and downpipes should be tested in the lower static position of 1 inch. This is the minimum for most systems currently used in current design practices. SMACNA and NEBB recommend using the leak class to specify allowed leaks.
To calculate the allowed leaks, you must know the surface area of the system, the required test pressure, and the specified leak class. In large residential buildings, testing individual apartment doors with blowers often makes more sense than testing the entire building. New research on the leak rates of sealed and unsealed ducts has revealed the need for a better method for evaluating leaks in ducts. European countries introduced an assessment approach that uses duct surface area and duct pressure as basic parameters.
SMACNA has concluded that this approach is far superior to the arbitrary assignment of a percentage of fan flow as a leak criterion. The surface area base highlights the effect of system size and is now the key to SMACNA's new pipeline leak classifications. In the future, it is expected that the industry will have classes of leaks correlated with the performance of certain sealing methods used in individual gasket systems. The new SMACNA leak classifications are based on this relationship between escape factors.
Whether the designer uses the identified speeds or prefers other constants, it is practical to evaluate leaks using this method. An example of a leak classification analysis is given in Appendix B. The SMACNA duct construction standards or this leak testing manual do not require any leak testing. When the designer has only required that leak tests be performed in accordance with the SMACNA air conditioning duct leak testing manual to verify that the leak classifications in table 1 are met (and has not indicated other criteria or scopes), it will be considered that he has not fulfilled the responsibilities described in section 2.1 to provide a clear scope of work. When the construction pressure classes of the ducts are not identified in the contract drawings and the amount of the leak tests is not established in the contract documents, any implied obligation of the installer to comply with the responsibilities set out in section 2.2 in relation to leaks will be considered waived if the specification is defective.
Verification of compliance consists of testing the duct sections at the specified pressure level, determining the leak in cfm and comparing it with the allowable amount associated with the leak class. If, at the specified test pressure, the leak factor (F), depending on the test, is lower than or equal to that associated with the specified leak class, the duct meets the requirements. The apparent differences of about ten percent between the fan supply and the sum of the airflow measurements at the terminals do not necessarily imply poor sealing or excess leaks. Table 4-1 represents the expected leak using seal classes A, B and C, as indicated in the construction of the ducts of the types that are normally selected for each pressure class.
The contractor will follow the manufacturer's application recommendations for all hardware and accessory elements and select them in a manner that is consistent with the classification and services of the ducts. Compare the numerical value of the leak class obtained by this calculation with the suggested leak classes for the type of duct construction and the extent of the seal used. This document or publication is prepared for voluntary acceptance and use within the limits of application defined here and, if not, as deemed appropriate by those who adopt or apply it. in the openings of a section of the duct and connecting a fan and a flow meter to the sample in such a way that, when pressurizing the sample, all the air that escapes from the sample passes through the flow meter.
Read the flow meter and compare the leak in cfm per square foot with the allowable rate determined in step 3.In addition, investigations (in Europe and independently in the United States) have led to the conclusion that, within acceptable tolerances, a leak factor on the duct surface can be identified by the following relationship. On the other hand, the industry does not recognize to what extent equipment that is inserted in-line in ducts leaks. Unless otherwise specified, steel sheets and strips used for ducts and connectors must be made of galvanized steel with a sealing grade G-60 coating that meets ASTM A653 and A924 standards. When testing the compliance of several separate segments within the same system and pressure class, the total leak must not exceed what is allowed, even though the amount in one or more segments may slightly exceed the permitted cfm indicated for each segment.
They are generally known as “peel and seal” and have been used between flanges and outside ducts. When the fan capacity of the test apparatus is marginal, the total pressure loss across the orifice gauge can make it difficult to obtain the required test pressure level in the duct. Researchers investigating leaks in homes and compartments of commercial buildings have frequently used tracer gas methods.