Rules to Build in a Chimney Flue

Flue Shape and Surface

• When smoke rises up the flue, the draft air makes it swirl. Its circular pattern flows much better through a rounded flue with no edges than in rectangular or square shapes where the angles impede the smoke’s upward progress. The interior surface of the flue should be smooth to aid the flow of the smoke passing through it. Metal flues with flat, unstamped surfaces work best. If the flue is made from concrete or other rough materials, line it with smooth metal or insert a prefabricated flue liner in the flue. Regularly check the lining of the flue for creosote buildup, which can slow the path of the smoke, decrease the draft necessary to keep the fire going and cause smoke to back up into the room where the fire is built.

Flue Size

• For the flue to operate at maximum efficiency, it has to be precisely configured in relation to the size of the pipe that joins the stove to the chimney. The diameter of the flue should be 25 percent bigger than that of the pipe. For instance, if the pipe has an 8-inch diameter, the flue should be 10 inches in diameter, according to University of Missouri Extension. Flues the same size as the chimney are most efficient. If the chimney diameter is more than three or four times that of the flue, insert a smaller chimney liner into the original one to make the flue work better.

Flue Troubleshooting

• If the fire in your stove or fireplace won’t stay lit, check the new flue to ensure it is securely attached to the pipe and drawing properly. If the problem occurs after it has been installed for several weeks, check for blockage or obstructions. In addition to creosote, pests sometimes build nests in chimneys, pipes and flues, especially if the unit is not used for several consecutive months. Chimneys that have been remodeled or rerouted often have twists and turns between the pipe and chimney top that even the best designed flue cannot overcome. Extending the height of the chimney might make the flue and pipe draw better because of the additional space for air to build up, which increases the force of the draft.

Multiple Flues

• If you are building flues for a stove and fireplace, they must be separate units. The National Fire Protection Association guidelines prohibit using the same flue for two appliances, and doing so could void your fire insurance coverage.