Code requires cleaning the chimney, before installing another appliance or changing appliances using different fuel. Another point not brought forth, liners require spacers to keep the centered in the chimney and give the vertical run support. But spacers also serve to create airspace from the metal liner to the clay. Should the chimney not be as clean as it should, it is possible the spacers prevent direct contact with the existing creosote. How many know if there liners have spacers installed?