Small Chimney Requirements
On small traditionally constructed chimneys it is unlikely there will be sufficient room to add a flue liner and maintain the minimum required 200mm distance from the inside of the flue liner to the thatch of a tightly thatched chimney. In these situations with a traditional lining system the only option is to request the thatch be cut back away from the chimney before any work is started. Traditional lining systems include pre-manufactured clay, refractory concrete and pumice flue liners, cast in-situ concrete linings and twin-wall flexible metal liners.
To obtain the required 200mm distance to combustible material on a 150mm diameter lining, the external brickwork of the stack would need to be at least 550mm square, with the flue liner carefully centred. On a traditional 440mm square two brick stack, it is not possible to add a traditional liner and maintain the required distance to combustibles without adding the 40mm air gap to the outside of the stack.
Large Chimney Requirements
On larger chimneys where a distance between the inner surface of the flue liner and outer surface of the chimney-stack of 200mm or larger can be achieved ADJ permits combustible materials to touch the outside of the brickwork. Great care however must be taken when relining a chimney to make sure that the 200mm dimension is maintained on all sides of the chimney that are in contact with combustibles.This is especially true of flexible lining systems such as twin-wall flexible stainless steel liners or the flexible former of cast in-situ concrete lining systems, which can snake within the chimney if care isn’t taken to make sure they are correctly centred over their full length during the lining process.
If the liner snakes to a position closer than the allowed 200mm to combustibles, the chimney will not comply with ADJ and may cause a localised hot spot that can cause a fire.