If you need to install a new chimney liner inside your flue, you need to take care to choose the correct size of liner to use. Some devices that burn solid fuel or natural gas, such as woodstoves or baseboard hot water heat system boilers, will come with manufacturers specifications that describe venting requirements and correct sizing of the chimney liner or piping used to vent the exhaust gases.
If you use a vent piping system that is a smaller diameter than the manufacturer’s specifications call for, you run the risk of having the device not vent properly, which can lead to poor combustion, low fuel efficiency, dirty vent piping, and even carbon monoxide buildup in your home. Carbon monoxide is a clear, odorless gas which is undetectable without special alarms or other electronic devices. Several hundred deaths per year occur due to improper ventilation of fuel burning appliances that allow carbon monoxide buildup to reach lethal levels inside a building.
One of the important considerations in sizing your chimney liner for use with a fireplace or woodstove is the effect it will have on your chimney’s drafting ability. Good draft brings sufficient air to the fire for complete combustion, so you get more heat from your wood, your chimney stays cleaner and you don’t have smoke backing up into your house. The rule of thumb for a good drafting fireplace is to have area of the firebox opening no more than ten to twelve times the area of the flue. Some designers say an even better ratio is eight to one. So, if your fireplace opening is twenty four inches wide and twelve inches high, giving an area of 288 square inches, you want the area of the inside of your flue or chimney liner to be no less than 24 square inches, and preferably closer to 36 square inches. This would mean that your flexible steel chimney liner should be no less than about six inches in diameter.
Most existing chimneys with clay tile liners have sufficient space inside the flue to accommodate a new flexible liner of adequate size for the fireplace. If the fireplace opening is too large to allow a flexible chimney liner to be inserted inside the existing flue, then you can either do things to effectively reduce the fireplace opening area, or you can remove and replace the clay tile liner. This is a job for a skilled mason with experience building and repairing fireplaces and chimneys. A good alternative would be to install a wood burning insert in your fireplace which requires a smaller liner and burns more efficiently than your open fireplace.