Easy to assemble and can be combined to obtain the required installation height. Couplers overlap 1-1/2" when joined. Assembled length is 1-1/2" less than listed lengths. Size refers to the inside diameter of the pipe or fitting. The outside diameter of RV is approximately 1/2" larger. Type B gas vent. Size: 4" x 18" Material: Alum Liner/Galv Steel Jacket Temperature Rating: 550 F Grade: UL Listed