An existing chimney must firstly be given a visual inspection,and make sure it is of a suitable size,a smoke test should be carried out to make sure it is clear of obstruction and check for gas tightness,your chimney should be swept before installing a new appliance,do not attempt to fit a liner without firstly sweeping the chimney,if your chimney passes the the visual inspection and passes the smoke test,it is deemed sound and not a legal requirement to line,lining your chimney has advantages such as increased draw and better efficiency,reduces the risk of chimney fire,and piece of mind having a sealed system