Cutting down a twin wall flue pipe is not recommended, as it can compromise the integrity and safety of the flue system. Each section of a twin wall flue pipe is precisely manufactured to ensure proper insulation and secure connections. Cutting the pipe will damage the insulating layer and make it almost impossible to achieve a proper seal when reassembling the sections. If you need a specific length of flue pipe, it’s best to purchase an adjustable length or pre-manufactured section that meets your requirements.