Smoke Problems
A chimney that is producing too much smoke is often seen as broken by homeowners. It is very rare for the actual fireplace or stove attached to the chimney to cause the draft problem that leads to too much smoke, though. Your chimney pulls combustion air in through the mouth of the attached fuel-burning appliance to perform its critical role of providing a proper draft. There are multiple potential causes of a draft problem in your chimney; here are some of the ones that we encounter most often, as well as some of their solutions.
Undersized or Oversized Flues
Sizing of the flue in proportion to the opening of the stove or fireplace is important. For years, masons have used the general “rule of thumb” when correctly sizing flues: the Cross Sectional Area, or CSA, of the opening for the masonry flue is to be at least 1/10 the CSA of the mouth of the fireplace below. Using that rule, a 16″ x 24″ flue liner would be on a chimney that would be venting a fireplace with a mouth smaller than 80″ by 48″. For good airflow, the size of the stove or fireplace opening determines the minimum size of the flue. For smoke problems stemming from an improper flue sizing, see if temporarily decreasing the size of the fireplace opening with stainless steel plates will help. If temporarily restricting the airflow at the opening effectively solves the smoke issue, you can fabricate a permanent and more visually appealing solution that restricts the opening by the same factor, or have a professional technician complete the work for you.
Flue sizes for wood-burning stoves vary by model and are determined by the manufacturer. The manufacturer tests each stove to make sure it is safe for use before it is released to vendors. Both appliance openings and flue openings are required to be sized appropriately. But unlike fireplaces, there is typically no difference in the size between these two openings. In fact for a typical stove to function optimally, the openings should be the exact same size. If your chimney is the smaller of the two, there is a significant chance that it will not be able to provide enough room for all of the air coming up through the flue in order to meet the minimum requirements for the stove. An exhaust opening that leads to an oversized chimney can exhaust a great deal of heat along with the smoke, diminishing the amount of heat felt in your home. Mismatched flue sizing should be fixed as soon as possible because it is a main cause of inefficiency. If for some reason the intake and exhaust openings have a difference in size, there are a limited number of solutions. A chimney that is too small can either be replaced, or its opening can be made to match that of the appliance it is attached to. For large chimneys, you can install a flue liner to match the size of the appliance’s exhaust opening.
Obstruction in the Flue
A blocked chimney can cause inefficiency, smoke problems, or even a significant health risk. Loose bricks, falling leaves, or even nests left by birds can all cause chimney obstructions. Obstructions like this can be a fire hazard, and greatly hinder proper air flow. Keep your flue clear by having it swept at least once a year.
Air Pressure Differences
If you are certain your chimney is free from blockage, then it is time to check the combustion air levels for your fire. In many cases an obstructed chimney can cause negative air pressure to develop. In this case air rushes down your chimney and counteracts the natural draft of your chimney, keeping the smoke from your fire inside. This has become such an issue that Washington state enacted a law that prohibits any new fireplaces or wood burning stoves from being installed or built without the addition of an external air combustion chamber. If you have a taller home you may have experienced drafts caused by warm air from the area around the fireplace; as the hot air rises it can escape through cracks or crevices in the upper floors. It is quite uncommon, but in a few cases rising hot air within a home can cause negative pressure below to become stronger than your chimney’s regular updraft. You can sometimes eliminate draft problem by slightly opening a window or door near your fireplace. If bathroom or kitchen exhaust fans may be the cause, it is also possible to build an outside air source for your fireplace to alleviate the problem.
Elevation Difference Between Fire and Chimney Top
Nearly any chimneys will allow some air to enter or exit your home no matter whether you have a fire going. We refer to this as ambient draft. Ambient chimney draft occurs because the top of the chimney flue sits in a thinner atmosphere than that present at the location of the fireplace. The reduced density of the air above forces air from below up through the chimney. Current housing codes typically require that your chimney extends at least two feet above your roofline. A chimney that meets all the requirements may still have trouble establishing ambient draft due to the topography and atmosphere surrounding it. Waterfront homes or those in heavily wooded locations will need significantly higher chimneys to get the right ambient draft.
Mismatched or improperly sized components, or even air flow issues throughout your home can cause smoke problems. Fortunately the diagnosis is typically straightforward and there are solutions available. Schedule an annual cleaning and examination of your chimney smoke and flue with a chimney smoke repair professional at 866-345-4751 or email us.
