Is insulation necessary when lining my chimney with a stainless steel liner?
It is critical on the typical lightweight alloy chimney liners that are most common currently. The insulation allows the pipe to heat up at a rapid rate instead of actually being cooled by the airspace that is present if left un-insulated. The dangerous part of using wood fuel is the formation of creosote that occurs when temperatures are not hot enough to prevent it. Creosote is unable to form or greatly reduced at higher temperatures found in a properly insulated chimney liner, and buildup is multiplied in a non-insulated installation. If creosote develops to a dangerous degree within a lightweight liner setup it can require a much more aggressive clean which could damage or destroy the lightweight system. Properly insulated however, it is a rare occurrence that a buildup would develop that would take more then a standard wire brush and rod cleaning to remove.
When working in an application where space does not allow for adequate chimney liner insulation we recommend going with the original “Homesaver-Flex” "ArmorFlex" style pipe for maximum durability under aggressive cleaning methods.
What is a pre-fabricated chimney and can I use an insert with a chimney liner with one?
A pre-fabricated chimney (a.k.a. pre-fab chimney or zero clearance chimney) is a non-masonry chimney constructed out of metal components instead of brick and mortar and safely installed, typically in a standard wooden structure, by maintaining the clearances and guidelines from the manufacturer.
However the manufacturer sometimes specifically prohibits the installation of a wood stove/insert depending on the quality or UL certification of the chimney without voiding warranty and relieving them of any liability such an installation attempt. In fact the only way to get a UL listed chimney liner installation in a pre-fab is if the specific aplliance is tested and listed in the specific fireplace. This is very rare and probable only if both are made by the same manufacturer. The reason for not allowing this is usually the design of the chimney pipe is a double wall system with an air space between them that uses convection to draw cool air down between the inner and outer pipe that leads to a dangerously fast accumulation of creosote to a dangerous level. The chimney pipe is generally not suitable for any kind of aggressive cleaning it might take to remove the build-up without destroying it, making the fireplace chimney unusable or unsuitable to use after a short period of use.
If the chimney manufacturer does not exclude this option, the wood stove or similar appliance manufacturer must list it as acceptable in a factory built fireplace as well and the installation must adhere to very specific rules.
To install an approved insert into an approved pre-fab chimney an insulated chimney liner must be used from the appliance all the way through the top of the existing chimney. A special "Factory built liner adaptor" must also be used to seal the top of the existing chimney where the original cap is removed and the double wall pipe is exposed. This adaptor covers this opening, sealing the elements out, but also provides an air-space for the convection process designed by the manufacturer to keep the outer wall of the chimney cool working properly. Without this part, the chimney could overheat and be damaged or worse yet, cause a fire from excessive temperatures in the chase.