The external diameter of the Radiant Twin wall Flue is 150mm, with an internal diameter of 125mm. For more information please see this data sheet: https://www.reznor.co.uk/wp-content/uploads/2025/09/D301626-Reznor-125mm-Radiant-Twin-Wall-Flue-Data-Sheet-Aug-2024.pdf.
How big is the Radiant Twin Wall Flue?
Information about Reznor's range of flue