Chimney Flue Liners

William Kibbel III, The Home Inspector

Dear Home Inspector: We've been told that the fireplace flues in our 1890s home should be lined. We're getting estimates and getting confused. One contractor wants to install a formed concrete liner. Another recommends a stainless steel liner. Do we really even need a liner? And if so, which one is best?

Your chimney probably performed well for more than 115 years -- the house didn't burn down. But a flue liner will extend the life -- and safety -- of your chimney. Benefits include:

  • Easier maintenance
  • Reduction of heat transfer to aging combustibles near the chimney
  • Elimination of further wear inside the flue

There are more than the two options you mentioned for lining flues. Each has advantages and limitations. I'll discuss three popular lining methods in this column.


chimney inside
What's inside your chimney? A liner can prolong the life of your chimney -- and make it safer.


chimney outside
Cast-in-place liners can reinforce a weakened, older chimney.


Terra Cotta

This hard-fired clay was the first and most widely used chimney lining material. It's still used for new chimneys today. Terra cotta is very durable, often lasting 80 years or more with regular maintenance.

The downside? Installing terra cotta inside an existing chimney is very tricky. Many chimneys are too small or have offsets that won't accept this type of liner. Only experienced professionals can maneuver these sections of flue tile into alignment.

Stainless Steel

These lining systems have become popular in the past 20 years. Safety tested and listed by the Underwriter's Laboratory, they are available in flexible, or in straight, rigid sections. Stainless steel costs more than similar aluminum liners. However, aluminum cannot be used with solid fuels such as wood and coal that produce corrosive emissions.

Stainless liners are fairly easy for experienced contractors to install. There are some special requirements and some limitations:

  • Most manufacturers require insulation to be used around the liners for performance and safety.
  • The connection above the damper and the throat of the fireplace is the most difficult part of the installation. In my experience, this is often done improperly.
  • These liners work well with stoves or fireplace inserts. They may not be a good choice if you want to enjoy open wood fires in your fireplace. The inside dimensions of these liners are often too small to properly vent old, open fireplaces.

Cast-in-place

This lining method involves pouring lightweight, heat resistant concrete against the interior walls of the chimney. There are two application methods.

The most popular method uses an inflated cylindrical tube to form a void, or flue, in the middle of the concrete. This creates a seamless, smooth interior that's easy to maintain. It also reinforces a worn, weakened chimney structure. This method can also be used to create multiple flues inside a large chimney.

The downside of this method? The resulting small, round flue dimension can make certain types of older fireplaces unusable. I've seen "walk-in," or shallow, Rumford style fireplaces, which "vent large," rendered inoperable with this lining method. Alas, this installation is not reversible.

Fortunately, there is another method of casting a liner that creates a much larger cavity. A large steel box with beveled sides, or a large oval bell, is lowered by a winch down the chimney. The box or bell is slowly raised while the concrete is poured from the top. This forces the concrete against the interior walls, filling any gaps and reinforcing the chimney. The result is a flue with a large interior dimension. A big guy with a white beard and red velvet suit has informed me that this is his preferred method.

 

Now that I've listed some options, I would advise that you have your chimney evaluated by a few competent chimney experts. They can offer an opinion on the best solution to create a safe and properly functioning chimney flue for your fireplace. Check manufacturer Web sites -- they often have a list of trained and approved contractors for their products.


bad liner
This cast-in-place liner ended up being too small for a large venting fireplace. Unfortunately the installation of a cast-in-place liner is not reversible, and the fireplace was rendered unusable.


lining chimney
This method of pouring a cast liner results in a larger diameter flue.


About the Author
William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.


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