Comfort Flame's 18" Section Double Wall Pipe is designed for use with FMI wood burning fireplaces only. This pipe has an 8 inch inner diameter and a 12 3/8" outer diameter, with a snap lock design. All Direct Vent and Wood Burning products require venting to the outside of the home to achieve proper ventilation and firebox combustion. FMI Products offers a full assortment of venting components to properly vent the Direct Vent or Wood Burning fireplace.