Selkirk RV Round Gas Vent Pipe is designed for easy assembly and can be combined to achieve the required installation height. The couplers overlap by 1-1/2 inches when joined, and the assembled length is 1-1/2 inches less than the listed lengths. The size refers to the inside diameter of the pipe or fitting, with the outside diameter approximately 1/2 inch larger. This Type B gas vent features an aluminum liner and galvanized steel jacket, ing a temperature rating of 550 degrees Fahrenheit. It is UL Listed for safety and reliability. Available in multiple sizes ranging from 3 inches to 6 inches in diameter and lengths from 18 inches to 5 feet, this vent pipe suits various RV venting needs. Model Number: 3RV-18