When a chimney was designed originally for an open fire it was typical for a fire to burn 12kW of fuel per hour and give nominally 2kW into the room. When a stove is fitted, for every 2kW it gives into the room there is less than 1kW of heat going up the chimney. Thus there is insufficient heating of the clay or stone masonry if a stove is fitted without a lining. In this situation it is typical to have tar condensation and an incorrect draught for the stove to work correctly and efficiently. A chimney lining will also prevent smoke and fumes from your fire from leaking into other rooms of the house.