On Sunday 14 April 2013 01:08 The Natural Philosopher wrote in uk.d-i-y:
Unless the old chimney is dripping with tar I fail to see why?
Sure, the liner radiates a lot of heat[1], but I would be very impressed if that managed to cause hot jets of gas to blast through gaps in the mortar and still be hot enough to cause combustion.
[1] Which I regard as a good thing - it puts heat into the core of a huge lump of masonry right in the centre of my house which gives some background heat to other parts and acts as a buffer.Mine is a single skin SS liner installed in a brick flue by a HETAS installer, with no vermiculite. Obviously his judgement was based on seeing the chimney.