Osburn 4" x 25' Insulated Flex Pipe for Fresh Air Intake

Twenty-five feet of insulated 4-inch flexible duct for the outside-air connection on an Osburn Stratford II — enough for a long run between the fireplace's 4-inch adapter and the outside wall register (the run must be at least 10 feet and no more than 30). The intake duct must be HVAC type, ULC S110 / UL 181 class 0 or 1, rated to 250 °F; clamp it at both ends and tape the joint airtight, and Osburn recommends an insulated run long enough to hold a P-trap loop to avoid condensation. Pair it with the 4-inch intake register with airtight damper. $200.