Why You Need To Fit A Flue or Liner
With a stove, 80% of the heat goes into the room, with an open fire 80% of the heat goes up the chimney and this means that the smoke from a stove is great deal cooler than from a fireplace. The result of this is that a stove will usually never manage to bring a masonry chimney up to operating temperature. This can mean that the stove will never draw properly, and the cooling smoke will condense, causing excess soot and, more dangerously, tar deposits. Tar deposits are not removed by a sweeps brush and cause chimney fires and you may get black tarry condensate leaking out through the bottom and sometimes even through the mortar joints of the chimney.
For this reason it is good practise to line and insulate a chimney whenever you fit a stove - see Flue Fitting & Lining.