On the surface, the chimney seems a simple concept. Smoke rises and goes up the chimney and out the top of the flue. In reality, however, the fairly complex principles of draft are at play in your chimney’s proper functioning. The size of your fireplace opening, the width of the flue and the height of your chimney all play into the ability of your chimney to carry smoke out of, rather than allowing it to flow back into, your home.
How chimney draft works
The force that keeps smoke flowing up your chimney is called draft. It is based on the simple principle that warm air will rise because it is less dense than cool air. When a chimney drafts properly, the warm air rises up and out of the chimney flue, and the cooler air from within your living space is pulled into the fireplace to fuel the flames. A chimney flue that isn’t sized right for the fireplace, however, will interfere with the draft and can make lighting a fire unbearable. A poorly drafting chimney will allow smoke to flow backward into your home. Smoking chimneys due to a bad draft tend to be more common when fires are burning at lower temperatures, as they do before they burn out, or when chimneys are built on the exterior walls of a home.
How to determine proper flue size
Improper flue sizes are a leading cause of poorly drafting fires. A flue that is too small won’t allow the fireplace gases to exhaust properly, and a flue that is too large will be cool down too quickly, causing condensation and creosote buildup within the flue. As a general rule of thumb, the area of a rectangular chimney flue should be no less than one-eighth of the area of the fireplace opening, and the area of a circular flue should be no less than one-tenth the area of the fireplace opening. Chimney height also plays a role, and taller chimneys generally draft better than shorter ones. With that in mind, the height of the chimney should be at least 15 feet, from the floor of the fireplace to the top of the flue.
Solving your chimney’s math
Figuring out the proper size for your chimney flue can seem like a challenge. In the end, you just want a chimney that drafts properly and allows you to enjoy your fireplace. Fortunately, the chimney experts at Environmental Chimney Service will do the math for you. When we install a fireplace for you, we will make sure your flue is sized right. If your existing chimney and fireplace have drafting problems, we can help! We can evaluate your fireplace and your flue and help develop a solution to fix your flue size and have your chimney drafting properly in no time.