A flue liner is advisable but not mandatory and you may well have been offered differing advice on chimney lining. It is important to note that stoves have different operational requirements for the open fires for which most UK chimneys were designed. Because stoves are more efficient, less heat is transferred to the top of the chimney. It is this heat that, in an open fire, contributes to the ‘draw’ on the chimney pulling the smoke out.
A reduced draw can have a number of potentially catastrophic impacts on your stove. While blacked glass and occasional smoke backdraughts may be a minor inconvenience, the fire risk associated with tar accumulation in a low draw chimney and the significant risk of a lethal carbon monoxide backflow caused by a ‘cold plug’ at the top of the chimney are why all manufacturers and accredited installers will recommend that the flue serving a stove, (multi-fuel or wood burning), should be lined.
An unlined chimney may also affect you house insurance.