You must remember that the internal diameter of an existing chimney would be much larger than that required for the stove. All stoves have a certain sized outlet at either the top or the back of the stove. The typical diameters on this are 125mm and 150mm. As a chimney is much larger in diameter, in some cases it would give too much draught to the stove and would cause it to run inefficiently. Solid fuels and wood typically need a lot of draught.