A duct leak test, also known as a duct test, is a procedure used to measure the amount of air that seeps into all the ducts connected to the HVAC system. This test is typically done before or after construction and must achieve a leak rate of less than 4 CFM (cubic feet per minute) per 100 square feet of the home. Before the drywall is installed, it is important to visually inspect the duct system with a HERS meter to ensure that all connections are properly fitted and sealed. Additionally, an optional and additional test can be done with a duct blaster or theatrical smoke machine to detect air leaks and seal them.
The evaluator must also visually inspect that the ducts are completely insulated (up to the R-8 for the supply lines and the R-6 for the return lines and other ducts) along the entire length, including all connections. If these ducts have loose connections in the sections of the duct that connect to each other or to the duct covers, trunk lines, or the air purifier, air can leak through the cracks. As required by RESNET standards, the test is performed with a duct tester, such as the Minneapolis Duct Blaster or the Retrotec Duct Tester. If all of these conditions are met and all of the ducts are located in conditioned spaces, it is only necessary to test for total leaks.
An application known as total duct leak testing creates a negative pressure condition in the duct system and in the air handler, if installed. Some builders prefer to wait until all of their ducts are finished testing before installing drywall as it is likely that other businesses will move them in between. It is important to remember that there is no need to test for air leaks in the ducts if they are completely inside the building's thermal envelope.
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