Posted: 28/12/2015 at 10:25
My ex was a builder and specialised in restoration of old timber-framed houses - after wood-burning stoves became popular in the 70s there were many times that he had to restore or even rebuild lovely old rural cottages etc because the chimney had caught fire due to the layer of tar which had soaked into the brickwork - the tar would sometimes even be visible in the bedrooms, as dark staining on the chimney breast !
Fortunately modular chimney liners were introduced and most people use them and this has really reduced the number of chimney fires from woodburning stoves. However, some people still try to manage without in order to save money ... it's a dangerous and false economy.
It may also be to do with insufficient updraught in the chimney and there may be some blowback which is coming down your chimney - again a properly qualified chimney sweep is the best person to advise you.