How Duct Systems Are Tested Duct leak tests are performed  by pressurizing the duct system with a fan and then measuring the air that continues to go into the system once it has reached a stable pressure which is designated by the design engineer.  The flow is measured by taking a pressure drop across a calibrated orifice tube.  This pressure drop can then be correlated to the leakage rate by comparing it to the calibration data of the orifice tube.  Pass fail criteria are usually based on one of two methods which are specified by the design engineer. 1. Percent of design method - This is done by pressurizing the duct to a specified criteria and then determining if the percentage of leakage relative to the design flow rate of the system is within an acceptable range, typically 1-5% of the design system flow. 2. SMACNA method - Seeing a need to value a duct system more on its size versus its total flow Sheet Metal and Air Conditioning Contractors National Association (SMACNA) developed a method that would relate leakage to the total surface area of a duct.  This method allows systems which have long duct runs and low airflow to be tested in a manner that can fit into the realities of many large commercial projects. Using this method, the engineer will designate a seal class which defines how the duct will be sealed and a leakage class(CL) which designates the amount of leakage at 1.00" of water pressure.  For example, a duct with a leakage class of 24 would allow 24 cubic feet per minute (cfm) leakage per 100 ft2 of duct surface area.  To determine the leakage factor (F) at other static pressures (P), use the following equation.         Duct Leak Testing Welcome Why Duct Leak Test? With the exception of welded duct, virtually all duct systems leak.  With twenty-five years of testing experience, we have seen systems with leakage rates over 30% of their design airflow.  This leakage becomes a significant source of energy loss for the facility, and it is one that will bleed those costs for the life of the building.  Duct leakage can also contribute to reduced equipment life, indoor air quality problems, comfort complaints and reduced equipment capacity. Examples of problems from duct leakage. Return leakage in a garage or storeroom can draw in contaminants such as carbon monoxide and chemical fumes from stored cleaners or reagents.  These can result in serious indoor air quality issues that can result in severe illness or death. Return leakage can also dramatically increase the load on equipment by drawing in hot humid attic air that the equipment was not designed to handle.  If the dehumidification capacity of the coil is exceeded, it will result in the space being humid and can contribute to mold and mildew growth within the space. Return air leakage on systems with filter grills can result in the induction of particulates into the system which then clog the coil elements of the equipment reducing both efficiency and equipment life. Supply air leakage results in the loss of valuable energy by diverting the conditioned air’s path to the attic, crawl space or mechanical room as opposed to the occupied space where it was intended. Supply air leakage can also result in the depressurization of the conditioned space which can result in drawing combustion gases from fireplaces, furnaces and water heater flues into the space with the potential for carbon monoxide poisoning of the occupants.  This depressurization can also lead to the drawing in of hot humid outside air which can contribute to mold and mildew growth.