The main function of a chimney structure is “draft” through which, flue gases and smoke are removed from the building. Several parameters influence draft including the chimney height and the temperature differences between the outside air and combusted gases inside the flue. Flow resistance decreases the induced draft. Moreover, additional turns and sharper parts in a chimney also influence the process and often maximize the flow resistance and minimize the draft in the system. Furthermore, rain caps and other terminals increase flow resistance depending on the terminals’ materials, geometries, and size. Proper design of a venting system requires information and specific data that address flue gas flow through the venting system, especially in solid fuel appliances. Such information and pertinent data can be obtained from published materials describing modeling and reported testing of the entire venting system (including test results published by manufacturers on their products).
"Impact of Chimney-top Appurtenances on Flue Gas Flow" was authored by Pegah Farshadmanesh, Mehdi Modares and Jamshid Mohammadi with the Department of Civil, Architectural and Environmental Engineering at Illinois Institute of Technology. This report summarizes findings of a comprehensive search of available published papers on chimney-top device flow resistance including reported testing and modeling, fire accident investigations, and other related studies. The gap in the needed knowledge is determined in an effort to provide insight into what is needed for development of guidance for installing rain caps and other chimney-top devices with technical substantiation.