If you need to install a new chimney liner inside your flue, it is important to choose the correct liner size for your fireplace, woodstove, or other fuel-burning appliance. Some solid-fuel and natural-gas appliances, including woodstoves and boilers used in baseboard hot-water heating systems, come with manufacturer specifications that outline the required venting configuration and the appropriate size of chimney liner or vent piping.
Using vent piping with a smaller diameter than specified by the manufacturer can prevent the appliance from venting properly. This can lead to incomplete combustion, reduced fuel efficiency, increased buildup inside the vent system, and potentially dangerous carbon monoxide accumulation in your home. Carbon monoxide is a colorless, odorless gas that cannot be detected without a carbon monoxide alarm or other specialized equipment. Improperly vented fuel-burning appliances can allow carbon monoxide to reach dangerous or potentially fatal concentrations inside a building, making proper chimney liner sizing essential for safe operation.
One of the most important considerations when sizing a chimney liner for a fireplace or woodstove is its effect on chimney draft. Proper draft supplies the fire with enough oxygen for efficient combustion, helping you get more heat from your fire, keep the chimney cleaner, and prevent smoke from entering your home. As a general guideline, the area of a fireplace's firebox opening should be no more than ten to twelve times the cross-sectional area of the flue. Some chimney designers recommend an even more conservative ratio of eight to one. For example, if your fireplace opening is 24 inches wide and 12 inches high, the opening has an area of 288 square inches. The flue or chimney liner should therefore have a cross-sectional area of at least 24 square inches, with approximately 36 square inches being preferable under the eight-to-one guideline. This would mean that your flexible steel chimney liner should be approximately six inches in diameter or larger.
Most existing chimneys with clay tile liners have enough space inside the flue to accommodate a properly sized flexible liner for the fireplace. If the fireplace opening is too large for an adequately sized flexible chimney liner to fit inside the existing flue, you may need to reduce the effective area of the fireplace opening or remove and replace the existing clay tile liner. Replacing a clay liner is a job best handled by a skilled mason with experience repairing and constructing fireplaces and chimneys. Another effective option is to install a wood-burning insert, which typically requires a smaller liner and can burn wood more efficiently than an open fireplace.