After a chimney sweep has inspected a chimney he/she may find damages to the original liner due to 
exposure to rain and acidic flue gasses, chimney fire, or other cause and recommend a new chimney liner. Chimney contractors often prefer to use steel flue liners due to their relative ease of installation compared to tile liner installation, and the fact that unlike vitreous clay tile flue liners, stainless steel chimney liners are a U.L. listed product with a warranty by the manufacturer.
Fireplace liners:
The critical issue with any flue liner is sizing it correctly for the appliance it is serving.
Correct sizing allows for the proper flow of flue gasses, good draft, and proper performance of a fireplace or wood-burning stove or insert. For fireplaces the interior cross-sectional dimensions of the liner should be 10% of the fireplace opening. For example: A fireplace that measures 36” w x 24” h has an area of 864 square inches, so requires a liner size of at least 86 square inches, and would likely have a 13 x 13” tile flue liner measuring 110 square inches.
A commonly used round liner for replacement of a tile flue liner is 10” in diameter, which is 78.5 square inches, so unless the flue is long to compensate for the under-sizing, the fireplace may not draft correctly.
The next size up, 11” x 11,” provides 121 square inches of air flow capacity, therefore, it is properly sized for the fireplace opening according to International Residential Code and NFPA 211 Standards. It is highly likely that a clay tile flue liner will need to be extracted from the chimney in order to make room for a properly sized liner for a fireplace. This requires scaffold and equipment to break out and remove the old liner and mortar, then install the new stainless steel flue liner and the required insulation around it.
Stainless steel chimney liners come in various grades and thicknesses. For instance, a 304 grade liner is very heavy, somewhat flexible, and has been available for many years. A newer type of liner is the 316Ti, which is a combination of Titanium and Stainless steel, but is very lightweight, very flexible, and much easier to handle. Your chimney sweep will advise you on what type of liner he/she recommends based on the configuration of the chimney and the intended use.
Most chimney sweeps are using a ceramic wool insulation wrap with a foil face, or a newer product which already has the insulation wrapped around the liner.
An alternative insulation is Thermix, which is poured around the liner after it is installed. Insulation is required by all of the manufacturers because the liner was tested and listed to U.L. 1777 using insulation. A top plate, bottom plate, and chimney cover may also be required.
In the case of a flue liner with sharp offsets (bends in the flue) some demolition work may be required of the damper and fireplace or removal of bricks on the exterior in order to access the bottom part of the flue liner. Brick extraction and replacement should be done by a qualified mason. Many chimney sweeps have learned or are learning the masonry trade for this reason.
The brick smoke chamber should be addressed at the same time. If unparged, the chamber should be parge coated with a high-temperature insulating mortar per IRC and NFPA 211 requirements. Parging fills holes and gaps in the mortar, smoothes out the chamber for the smooth transition of smoke, and lowers the temperature of the exterior of the chimney. The flue liner should be sealed to the smoke chamber so there are no gaps around the outside of the liner.
Wood-burning Stove Liners:
When a new flue liner is needed for a wood-burning fireplace insert or a free-standing wood-burning stove, the measurements are quite different.
Older wood-burning inserts installed 30 years ago do not meet current standards. They are dirty burning and not EPA compliant. At the time these stoves came on the market the installers often didn’t follow manufacturer instructions and installed them without a stainless steel flue liner sized for the stove (called a “Slammer” by chimney sweeps). “Slammers” are responsible for excessive creosote accumulation, glazed creosote, and many chimney fires across the U.S. and Canada.
Today’s high-efficiency wood stoves require a much smaller flue liner than a tile flue liner used for a fireplace or older wood stoves. Currently, the most common flue size for stoves is 5.5” or 6” in diameter, and the flue take-off will be the size required for the liner. Without the proper size liner the stove performance will be affected and excessive creosote may accumulate. If a stove insert is installed in a fireplace the new steel liner can usually be inserted inside the old tile liner.
Who Should do the Job:
A professional chimney sweep is the person most qualified to size and install a stainless steel flue liner.
Ask your chimney sweep what type of liner he suggests using and why, what size he plans to use, and if for a fireplace – if he will take out the tile liner before inserting the new steel flue liner. If not, ask why. If you are not satisfied with the answer it might be a good idea to get a second opinion before proceeding with the work. It is advisable to ask for a warranty on workmanship and always check references, the Better Business Bureau, and the company’s website to check for experience and methods.
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