A flexible flue liner can be used only to reline a chimney and should never be used as the primary liner of a chimney. The liners are always installed in one length so there are no joints present within the flue structure. Flexible flue liners are used in place of standard rigid liners when there are bends and curves present in the chimney. Most flexible flue liners are made of stainless steel.
Insulation is necessary for the entire length of the flue liner. Many people end up not getting enough insulation, or getting the wrong type all together. If your flue is round, you can determine how much insulation you need by multiplying the diameter of the flue liner by 3.14. This will give you the circumference. (For rectangular or non-round flues, measure each side and add the measurements together.) Add one inch to the total you get. This is the amount of insulation you need at minimum. It's always a good idea to purchase additional insulation to allow for errors.
Even flexible flue lining can be sharp on the edges. Always make sure you have on a good pair of work gloves to avoid cutting your hands. Since you will need to cut the liner to size with snips, it's also important to wear eye protection. You will want to keep safety in mind for the roof as well, since you will need to work on the roof. Use a roofing harness to protect you from falls. Have a friend handy to hold the ladder and call for help if needed.
Weigh Down the Liner
Most liners are installed by dropping them from the top of the chimney down. It's possible to damage the insulation and the liner itself if there isn't a weight on the bottom. You can place a rope on the bottom of the liner and have someone at the other end guide the lining in to place. Or you can add a small weight to the bottom of the lining to prevent it from swinging and pulling down the insulation.
Type and Size of Liner
Liners are made for a variety of appliances. A liner for a wood stove will have different requirements than a liner for a fireplace. Using the incorrect size can lead to the build up of noxious fumes and gases. If you are unsure which liner is best for your needs, it's much safer to consult with a professional before guessing. You can generally get free or low cost estimates from chimney repair professionals, and the estimate will show you what materials are needed and the sizes of each.