Midtherm HTS Twin Wall Flue - 250Mm Length (0.9mm Inner)Midtherm HT S Starter Length has a 0. 9mm inner liner so that it can be fitted directly to a stove where permitted by the stove manufacturer. Stove manufacturers installation instructions and building regulations must be followed. The Midtherm HTS Twinwall 1000mm Length is a single length of twin wall pipe that can be used vertically or at an angle. Available in a choice of stainless steel or painted black finishes, once fitted this piece of twin wall