carl spackler said:
Insulated liner, is in most cases, a nice to have but not a must have.
I couldn't disagree more with that statement. I seriously doubt that many with masonary flues have any idea how a properly built brick chimney is to be built.
carl spackler, please don't take this post personally, as you are likely one of the many who have been mislead and under informed. I have unfortunately had to take this issue on in my own home, and only after getting regulatory bodies involved, was I able to educate my certified installer. I am still waiting for him to get here and fix it.
A masonary flue that is lined without any insulation requires that the flue and liner have constant air gaps all the way from top to bottom, with provisions for firestop at floor and ceiling penetrations. These firestops need to be made of noncombustibles. If you don't have the required airspace (1 or 2 inches, depending on the specifics of the chimney), then your chimney doesn't meet safety requirements. Lining it doesn't change that, what changes it is insulating the liner.
Here is why - flue gases regularly reach 1200F. That means that in a 24/7 burning situation, your brick will reach really high temps. High enough in fact to, over time (weeks to a few years), cause pyrolysis (the process that creates charcoal) to occur in any wood framing etc that is to close to the flue. Insulating the liner helps isolate the heat from the surrounding brick, in turn isolating the wood outside the chimney.
What really kills me is that the same folks who flail at the thought of cutting corners on clearances around the stove or floor protection in front of the hearth don't even give a second thought to the risks associated with an uninsulated liner in a brick flue.
When you install a liner, unless you had a video scope done of the brick structure, you have NO IDEA what the brick is like, or even if bricks are missing. As well, most brick flues in North America were built to deal with oil furnaces - these have way lower temps, and a completley diffferent set of requirements when built. Installing a liner in a brick flue built for an oil appliance does not make it safe for burning solid fuels like wood.
From the Chimney Safety Institute of America, Six best practices for liner intallation
5. Insulate liners except in special circumstances.
The one key principle that applies to the question of insulating stainless steel liners is this: The only circumstance in which insulation is not required by the terms of the product listing is if the clearances of the existing chimney comply in all respects to the requirements of the NFPA 211 solid fuel installation code, or the building code enforced locally. Ideally, all masonry chimneys would comply with the simple clearance rules in the code, but most experts agree that the majority of masonry chimneys do not have sufficient clearance to combustible materials. Furthermore, a thorough inspection of a masonry chimney to confirm beyond doubt that it has adequate clearances can be a difficult and time-consuming job, depending on whether access to inspect clearances is straightforward or requires the removal of enclosures. Considering the challenges inherent in determining code compliance of an existing masonry chimney, the installer should assume that insulation is required around the liner if adequate clearances cannot be determined anywhere along the chimney’s length.
Even if clearances comply with the code there are other good reasons to insulate. Insulation around the liner reduces heat loss from the flue gases to the surrounding masonry. Natural draft is produced by the temperature difference between the flue gases and the outdoor air – the greater the temperature difference, the stronger the draft. Stainless steel liners have little mass and when they are backed up by insulation they heat up quickly and absorb a relatively small amount of heat from the flue gases. Therefore, the average flue gas temperature in an insulated stainless steel flue rises fast when a fire is started, meaning that fires kindle rapidly and smolder less. Strong natural draft also reduces smoke rollout when the stove, furnace or fireplace door is opened for reloading. Since both wood smoke and water vapor condense on cool surfaces, keeping the flue liner hot is a good way to reduce both creosote deposits and water damage in the form of staining and freeze-thaw deterioration. The use of insulation around chimney liners is an effective strategy for reducing maintenance costs and extending the life of the masonry chimney. For safety and performance reasons, chimney experts recommend that all liner installations include the insulation specified by the liner manufacturer.
