Determining Fireplace Liner Sizing

When it comes to determining the liner size for a masonry fireplace, it's just mathematics. Once you understand how the formula works then it becomes easy to size the liners. 

Some manufacturer's such as Duravent provide a liner sizing guide as shown in the example with the fireplace.

Step 1: Find the width of the fireplace opening on the chart (look for Step 1, at right). In the example they use a 36" width. Hint: Follow the red lines in the example.

Step 2: Find the height of the fireplace opening. In the example above they use 30".

Step 3: Find the intersect of these two measurements. Then follow the horizontal line over to the appropriate chimney height. In this example they use 20'.

Step 4: Where the width/height of the fireplace opening and the chimney height line intersects the liner diameter size is given.

Another way to determine the liner size is by following some mathematical formulas. It starts with the fireplace opening. In this example we will use the 36" W X 30" H example used above.

Step 1: Find the cross-sectional area of the fireplace opening. Multiply the W=36" X H=30" and that equals 1080 square inches of fireplace opening.

Step 2: If you are using a round liner and the chimney height is more than 8' above the fireplace opening you can divide the 1080 by 12. This amount equals 90".

Step 3: Find the liner size by comparing the 90" to the cross-sectional area chart for round liners in Chart A. A 10" liner only has 78.5" of area and will be too small. An 11" liner has 95" of area so it should work.

If you are installing an oval liner for a fireplace, calculate the fireplace opening in the same manner as the above formula, but instead divide by 10. See Chart below for oval specs.

When sizing a fireplace for a square or rectangular liner the 1/10 ratio should be used. This means that the area of the liner should be 1/10th of the fireplace opening. So in the example above where the fireplace had 1080" of area, it should be divided by 10 which means a liner with 108" of area is needed.

The exception to this rule is when the liner is twice as long as the depth, like a 6 X 12 liner. Then the fireplace opening should be divided by 8 instead of 10.

To determine the area in a square or rectangular pipe just multiply the length by the depth. For example a liner that is 6" X 10" would be 6 X 10 = 60" of area. For the fireplace above that needs 108" of area, this liner would be way too small. A liner size of 9" X 12" would equal 108" and should just provide enough draft for the above fireplace.