Building a masonry chimney for a wood-burning fireplace is not very difficult if you understand how a chimney works. A well-built chimney will draw in a sufficient amount of oxygen into the fireplace while expelling the exhaust as the hot air rises to the top without causing a backdraft due to the stack effect. The stack effect occurs when there are significant temperature differences between the cold outside air and the warmer inside air. The negative pressure of the colder air can cause a backdraft. Getting the chimney height right is critical to producing the right amount of draft.
One of the most critical aspects of building a new chimney is correctly calculating its height. An incorrectly sized flue will cause drafting and venting issues and inefficient heating. Check with the heating appliance manufacturer for the correct flue size, so you don’t build a chimney that is either too short or too tall. The Brick Industry Association also has charts and calculation formulas for correctly sizing masonry chimneys.
A cement foundation is also necessary to support the weight of the chimney. Chimneys are top-heavy by design and require a strong footing to prevent it from toppling over.
The 8-inch to 12-inch concrete footing needs to have a minimum 6-inch clearance around the chimney to avoid heat transfer to any adjacent combustible materials.
Although most chimneys are constructed on an external wall on the side of the home, the cooler winter weather chills the chimney due to the stack effect. Also, a cold chimney takes longer to heat the home. Another option is to place the fireplace on an interior wall with a chimney that protrudes through the top of the roof. The building envelope insulates most of the chimney significantly, reducing the stack effect resulting in more efficient heating.
Just like the flue needs to be the right size for the chimney, the liner needs to be the right size for the flue. The flue liner protects the masonry walls surrounding the flue pipe to contain the extreme temperatures of the fireplace and prevent its spread throughout the home. An incorrectly sized flue liner increases the risk of fire and exposure to carbon monoxide exhaust.
Your masonry chimney will need to comply with local building ordinances. Your local Building and Zoning department may require its approval of the architectural design before issuing a building permit for chimney construction. An inspection is usually done before pouring the footing. The chimney will likely need to pass additional inspections while it is being built and upon completion, before it is approved for use.
National fire safety professionals recommend homeowners with solid or liquid fuel heating appliance to have annual chimney inspections and cleanings. Having a chimney sweep inspect and clean your chimney every year will minimize unexpected repairs and help ensure your fireplace or heating stove will continue to provide years of warmth and enjoyment.