Twin wall flue installation cost is affected mainly by the total height of run needed, the number of directional bends, and whether the flue exits through an existing roof, a wall, or needs full external bracketing up the side of a building. Access difficulty, such as installations on multi-storey properties or awkward roof lines, can also add to labour costs. Because a twin wall flue is a Class 1 chimney alternative, it must be installed to strict clearance and support standards, which is not a task suited to DIY. A HETAS-registered installer can survey your property and provide an accurate cost breakdown for your specific flue run.