Osburn 4" x 10' Insulated Flex Pipe for Fresh Air Intake

Ten feet of insulated 4-inch flexible duct for the outside-air connection on an Osburn Everest III or Stratford II — exactly the minimum run length between the fireplace's 4-inch adapter and the outside wall register (the maximum is 30 feet). 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. $75.