This code of practice gives recommendations for the design and construction of flue linings and flue structures serving appliances (boilers and air heaters) of 45 kW output and above, burning solid, liquid or gaseous fuels. The recommendations contained in this code may be used for the design of a flue serving an incinerator, which should be provided with its own individual flue. Prior approval of the height of the chimney by the local authority is required by Section 6 of the Clean Air Act 1968 where the appliance(s) served by the chimney burn(s) (a) pulverized fuel or (b) 45 kg or more of solid fuel per hour or (c) liquid or gaseous fuel at a rate of 366 kW or more if: a new chimney is erected to serve a new or existing installation; or an installation (e.g. of boilers, incinerators) served by an existing chimney is enlarged, e.g. its combustion capacity is increased, or an additional appliance is added to the system; or an appliance served by an existing chimney is removed and replaced by one having a larger combustion capacity. The official publication Chimney heights. Second edition of the 1956 Clean Air Act Memorandum,* subsequently referred to in this code of practice as the \'Chimney Height Memorandum\', gives guidance for the calculation of heights likely to be suitable in most instances for solid or liquid fuel appliances. Useful information to facilitate calculation of possible chimney heights for gaseous fuel is contained in the CIBS Guide, Volume B, Section B13 (1970)†. This code applies to: linings that are built within a space enclosed by a building; linings that are built as an integral part of the building. This code does not, in general, apply to linings within a freestanding or independent chimney, but many of the conditions and criteria are applicable. The design of flue linings and flue structures in buildings is subject to increasing knowledge and experience, and should be undertaken only by those competent to evaluate the many aspects of satisfactory construction and performance. This code provides the building services engineer and the structural engineer with design criteria, suggested procedures and methods of design, recommendations for construction, and basic guidance data for the selection of lining materials. The code also seeks to augment the skills of experienced designers who will need to interpret the information and examples with appropriate professional expertise, giving due consideration to all the relevant issues involved. The examples and calculations in this code are given as one form of use of the data provided. They do not preclude other methods of computing satisfactory designs. Where appropriate, detailed information has been put in appendices.