Damaged chimney flues repaired in Christmas

The flue, the liner and why the words get mixed up

The flue is the passage smoke and combustion gases travel through on their way out of the house. The liner is the material that forms the walls of that passage, most often clay tile in a masonry chimney, sometimes a metal or cast-in-place lining added later.

People use the two words for the same thing, and repair offers blur them further. When we write up a flue repair, we name the passage, the lining material and the exact section that is damaged, so you know whether you are paying to fix part of the existing lining or to put in a new one.

The mortar that belongs between flue tiles

Ordinary masonry mortar is not what the code asks for inside a flue. Section R1003.12 calls for clay flue liners to be laid in medium-duty, water-insoluble refractory mortar conforming to ASTM C199, a material made to handle heat and moisture inside the passage.

Older chimneys were not always built that way, so we check what the joints actually contain and how it has held up. Where we repair joints, the mortar or listed repair product is named in your estimate, together with the sections it goes into.

When a repair changes the inside of the flue

Some repair methods add material inside the flue. A coating builds up the walls, a sleeve reduces the opening and patched joints can form small ridges. Each one changes the passage a little.

Those changes can be fine, or they can matter, depending on the appliance and the original flue size. We measure before recommending a method that changes the inside dimensions, and we write the expected result into the estimate. A small reduction may be harmless on a generous flue and a real problem on one already close to the minimum for its fireplace opening.

What a flue repair estimate should list

A flue repair estimate should be specific enough to compare with another quote. Vague lines such as repair liner leave you unable to tell what is being fixed.

Ours list each item below. Book within 72 hours after the inspection and 10% off is printed right on the estimate.

• Sections of the flue included in the repair
• The method and the named product or material
• Cleaning and preparation before the repair
• Related work such as a cap, crown or cleanout cover
• Curing time and when the fireplace can be used
• Anything that could not be seen or confirmed