Yesterday, I picked up a Jotul 550 C Rockland insert. I plan on doing the install myself. I have a 25 foot masonry chimney that is lined with clay or terra cotta flue liner. The terra cotta flue measures 13 X 8.5 outside dimension and 11.5 X 6.75 inside dimensions. The flue is fairly straight and appears to be in good shape. The insert calls for a 6 inch liner. The installers at the store recommended that I use 5.5 chimney liner because of the inside dimension of 6.75. They claim there may be mortar or other obstructions that will impede the liner from sliding down the chimney and may cause the liner to tear. However, they also said, if there are no obstructions the 6 inch will go fairly easily. Jotul recommends the 6 inch liner. The installers at the store said they use the 5.5 all the time when the flue is tight fitting with the liner. I am confused. What size do you guys think I should use? I am not familiar with the different drafting issues between the 5.5 or the 6.0 liner.