Question from Clive T Johns, via e-mail:
In regards to flexible SS flue liners for solid fuelled appliances, I always thought that they should be packed in a chimney void from top to bottom with insulation, to stop the risk of explosion, due to flue gas cooling and dropping? Has there been a change in this procedure? Only, I have come across several such installations, one of which had an explosion.
Answer from Martyn Bridges, director of marketing and technical support at Worcester, Bosch Group:
To my understanding the requirement is still the same, please see below the relevant section from Approved Document Part J published in May for implementation in October this year.
1.28 Liners should be installed in accordance with their manufacturer’s instructions. Appropriate components should be selected to form the flue without cutting and to keep joints to a minimum. Bends and offsets should be formed only with matching factory-made components. Liners need to be placed with the sockets or rebate ends uppermost to contain moisture and other condensates in the flue. Joints should be sealed with fire cement, refractory mortar or installed in accordance with their manufacturer’s instructions. Spaces between the lining and the surrounding masonry should not be filled with ordinary mortar. In the absence of liner manufacturer’s instructions, the space could be filled with a weak insulating concrete such as mixtures of:
A) One part ordinary Portland cement to 20 parts suitable lightweight expanded clay aggregate, minimally wetted; or
B) One part ordinary Portland cement to six parts Vermiculite; or
C) One part ordinary Portland cement to 10 parts Perlite.