We often hear the complaint that someone has a “bad” stove or fireplace because it smokes. In our experience, however, draft problems are almost never caused by the fireplace or appliance. The chimney is the engine of every wood-burning system: in order for the fire to burn properly, the chimney must pull combustion air through the fireplace or stove. Here are some common causes of chimney draft problems, and possible solutions.
- Improper Flue Sizing
- Flue Blockage
- Resistance From Below
- Insufficient Chimney Height
- Cold Flue Temperatures
- Air Inversion
- Down Drafting or Cross Drafting
- Possible Problems
- Adding a Smoke-Guard
- Chimney Extension
- Chimney Termination Heights
- Unused Fireplace Smokes while Another is being Used
Improper Flue Sizing
Masonry fireplace flue sizing is determined by the size of the fireplace opening below. Masons have for many years sized fireplace flues using a “rule of thumb” that the CSA (Cross-Sectional Area) of the masonry flue be at least 1/10 the CSA of the fireplace opening. For example, an 8×12 (id) flue liner is used in a chimney venting a fireplace with an opening measuring up to 40″ wide x 24″ tall: larger fireplaces require larger liners. If your fireplace smokes because the flue is too small, try temporarily reducing the size of the fireplace opening with pieces of sheet metal; if this works, use masonry materials or heavy steel to accomplish a permanent solution.
Proper wood stove flue sizing is determined by the stove manufacturer for each model during the testing process. In order for a wood stove chimney to do the best possible venting job, the flue opening must have exactly the same cross-sectional area as the vent opening on the appliance. If the chimney is too small, it may not have room for the volume of rising air the stove requires. If it is too big, it may draw too slowly for the appliance, and may never heat up enough to compensate. For this reason, both flue under sizing and flue over sizing should be avoided. If your chimney is too small, replace either the chimney or the appliance. If your chimney is too big, install a masonry or stainless steel flue liner that has the same CSA as the vent opening on the appliance.
If the chimney is the proper size and still isn’t providing sufficient draft, the first thing to do is check the stovepipe and chimney flue for blockage: bird nests, fallen bricks, frisbees, leaves, etc. These items can block or partially block a chimney flue, interfering with proper draft. Make sure the flue is clean: even the thinnest coating of creosote or soot reduces the flue diameter, and can interfere with proper draft.
Resistance From Below
Having determined that the chimney is clean, make sure sufficient combustion air is being provided to the fire. As the chimney pulls air through a fireplace or wood stove, negative air pressure (a partial vacuum) can be created in the house, which fights against the chimney draft and can actually draw smoke backward down the chimney. This problem has become so prevalent in today’s tightly-constructed homes that Washington State Law now requires that an outside combustion air supply be provided directly to all new wood stove or fireplace installations. Another problem sometimes occurs in tall houses that leak large amounts of air in the upper stories: heated air rises, so the warm air inside the house wants to flow upstairs and escape through the leaks. In extreme examples, this can result in negative air pressure below that is stronger than the chimney updraft. If opening a door or window near the fireplace or wood stove eliminates the draft problem, or if the problem only occurs when the kitchen stove or bathroom exhaust fans are evacuating air, vent the stove or fireplace to outside air.
Insufficient Chimney Height
Chimneys draw a small amount of air, even when there is no fire below: this phenomenon is called ambient updraft. Ambient chimney draft occurs because the top of the flue extends upward several feet, into a thinner atmosphere than exists at the bottom. Thus, air is drawn up the chimney in much the same way as liquid is drawn up a soda straw when you reduce the air density inside your mouth.
Code requires that all chimneys must extend at least two feet above any part of the roof within ten feet. Given that topographical and atmospheric conditions can vary considerably from house to house, it is possible for a chimney to comply with this minimum code specification but still fail to extend upward into air of a low-enough density to establish ambient updraft. It is not uncommon for fireplaces and stoves in houses that are surrounded by hills or trees, for example, or that are located in the high-density air that often surrounds large bodies of water, to need more chimney height than the minimum required by code.
Cold Flue Temperatures
The ambient updraft created by the air pressure differential from the top to the bottom of a chimney is often not sufficient to exhaust the smoke from a wood fire. To “prime” a cold chimney, heat from a paper or kindling fire must first be drawn up the flue by the ambient updraft, where it warms the flue and creates the much stronger thermal updraft needed to exhaust the smoke from a wood stove or fireplace. Creating and maintaining the temperatures needed inside the chimney to sustain thermal updraft can be difficult, especially with the low exhaust temperatures created by today’s high-efficiency airtight appliances.
Masonry materials are terrible insulators: it would take a brick chimney with sidewalls 28″ thick to provide the same insulation as the one inch of spun ceramic blanket used in today’s prefabricated insulated metal chimney. A masonry chimney bleeds precious heat away from the smoke and transmits it through to the outside of the flue, slowing the thermal updraft and promoting creosote formation. Masonry chimneys that extend up through the house stay warmer than those that are entirely exposed to outside temperatures, but all chimneys lose precious flue temperature above the roof line
Air-cooled metal chimneys, which are designed to vent manufactured zero clearance fireplaces, actively cool the smoke. While cooling the smoke is a good idea when venting the super-hot exhaust created by zero clearance fireplaces, which send most of the heat from the fire up the chimney, it is the worst possible method for venting the already-cool exhaust from an efficient airtight stove. For this reason, the code authority has outlawed the use of air-cooled chimneys to vent airtight appliances.
If you have a choice, the best possible venting method for a wood stove is insulated stainless steel chimney. If you are venting into a masonry chimney, install a stainless steel liner and provide dead-air or spun ceramic insulation.
There is an atmospheric condition, known as air inversion, which causes high-density air to be trapped at flue top altitudes normally occupied by the low-density air that creates ambient updraft. During periods of air inversion, chimneys in the affected area simply don’t draw properly. One way to tell if air inversion is causing temporary draft problems is to look at the smoke that exits the flue: if it eddies around the top of the chimney or flows downward onto the roof instead of rising as heated air normally would, an inversion layer is most likely present.
Having established that a draft problem is being caused by air inversion, several solutions present themselves:
Don’t attempt to start a fire during inversion days. These don’t happen very often in most areas, and seldom occur during the long periods of winter cold when we use our wood stoves most. Air inversion episodes occur most often when cold weather turns warmer, or when warm weather suddenly turns cooler, as sometimes happens in the Spring or late Fall. Some areas are more subject to air inversion than others: if your house is totally surrounded by tall trees, hills or buildings, you may experience local “inversion” every time the wind blows across the top of the taller obstruction, pressurizing the air below.
During air inversion episodes, remove all possible draft resistance at the bottom of the chimney. Today’s wood stoves have very small air intakes and very restrictive baffle systems through which air must be drawn by the chimney. Opening a nearby door or window a crack often reduces this resistance considerably, and may allow the stove to be burned even on heavy inversion days.
Elevate the top of the chimney to a point above the inversion layer. This is kind of a hit-or-miss solution, for three reasons: (1) nobody can accurately predict exactly how high the inversion air tends to stack over a given neighborhood, (2) the density of an inversion layer can vary from one episode to the next, and (3) there is a limit to how high a chimney can extend before it gets too top-heavy to support. If there’s a chimney in your neighborhood that is taller than yours, you might ask the owners if the additional height overcomes the effects of inversion you are experiencing. If so, try extending yours to the same height. Note: never extend your chimney with uninsulated metal pipe, or excessive creosote formation will result.
Down Drafting or Cross Drafting
Quite often, chimney draft failures are caused by wind, blowing down or across the top of the chimney. If the problem only occurs when the wind blows, replace your rain cap with a draft inducing cap that works on Bernoulli’s principle, such as the old-fashioned “rooster tail”, or the more effective Vacu-Stack. These caps are designed to reverse the effects of wind-induced downdraft or cross draft, but will only work when the wind is blowing directly onto them. Draft inducing caps will not correct a downdraft caused by increased air density at the top of the chimney due to air inversion, or the type of chimney-top pressurization which can occur when the wind blows across the top of a cylinder formed by nearby tall trees, hills, or buildings that extend above and totally surround the chimney.
There are a number of problems a chimney may have, which actually make establishing a good draw difficult. But, 9 out of 10 people who think they have “smoke problems” really don’t. Most people just don’t realize that they need to take more time to get their fireplace working in the first place. But, there are a number of possible problems that need to be considered.
Adding a Smoke-Guard
Sometimes, the opening of the fireplace is a little too big for the size of the flue in the chimney. In cases like this, smoke perpetually spills out into the room. There is a way to test to see if your chimney has this problem. Tape up a 4″ strip of aluminum foil across the top of the opening of your fireplace. The object is to cut down the size of the opening of the fireplace, so that the flue doesn’t have to exhaust so much incoming air. Use your fireplace several times with this temporary “smoke guard.” If the 4″ aluminum foil strip eliminates your problem, install a real Smoke-Guard.
If a chimney is too short, the siphon (draw) which must be created, is easily broken by wind changes, or other factors. A chimney needs to be a least 10-ft high. The best chimney’s are between 12 and 16 feet high. If your chimney is less than 10 feet high, and a carefully established draw cannot be maintained for an extended period, you may need to extend your chimney.
Chimney Termination Heights
Minimum chimney height: If the chimney is within 10-ft of the peak, the chimney must extend 2-ft higher than the peak. If the chimney is over 10-ft from the peak, the chimney should be 2-ft higher than anything within 10-ft horizontally. (NFPA 211). Sometimes nearby foliage needs to be removed. Remember that most types of smoking problems can be “fixed” simply by taking more time to establish a good draw.
Unused Fireplace Smokes while Another is being Used
Often a fireplace will smoke while another in the house is being used due to the “inadequate air supply”. This problem can at times be easily solved by raising the flue height, 12-24 inches, of the fireplace used the most. And will in most cases solve the problem.