A top quality stainless steel high temperature insulated twin wall flue pipe guaranteed for 10 years against defects due to faulty manufacture this stainless steel system consists of a 25mm high density insulated core surrounded by 316 grade inner liner is 05mm thick 25 thicker than some products and the 304 grade outer casing is 05mm thick they are bsria approved to bs en 1856 parts 1 amp 2 and hetas approved for solid fuel burning it is supplied with a bright annealed mirror finis