The flue liner is the interior layer that protects the flue and other parts of the chimney system from heat and fire damage as smoke and other combustion materials move through it. These days, flues are typically made from clay or metal.
As we mentioned above, combustion particles (like smoke) can release a lot of heat when rising through the flue. The liner’s job is to absorb this heat and protect the more vulnerable parts of the flue – as well as the rest of the chimney and even the interior walls of the home, which might otherwise be in danger of catching on fire.
Because of the stack effect – the temperature difference between the air in the fireplace and outdoor air – combustion particles are naturally drawn up and out. As they pass, they tend to lose a significant amount of heat, so flues need to be very tough to avoid fire damage.
Thanks for explaining why we should choose a company that does counter flashing to prevent leaks in the chimney. My sister and her husband purchased a small fixer-upper home with a few repairs need to the exterior, including some brickwork on the chimney. I’ll have to share this info so they can ask specific questions to ensure they find a reliable contractor for the job.