Troubleshooting Chimney Draft Problems

We often hear the complaint that someone has a "bad" stove or fireplace because it smokes into the house. 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

Masonry fireplace flue sizing is determined by the size of the fireplace opening below. Masons traditionally calculate the size of 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 8x12 (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 products such as the Smoke Guard to accomplish a permanent solution.

Wood stove flue sizing is determined by the stove manufacturer for each model during the testing process. In order for a woodstove 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 undersizing and flue oversizing 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. Here's a chart showing recommended masonry liner sizes for woodstoves and inserts

Improper stovepipe sizing or configuration

In order for the connector pipe (stovepipe) between a woodstove and the chimney to do the best possible job, it must be the same size as the vent collar on the stove. If the stovepipe is too small, it won't handle the exhaust volume the test laboratory determined that the stove requires: if it is too big, the exhaust column exiting the flue collar will have to expand to fill the oversize pipe, slowing the flow. Either condition can interfere with proper chimney updraft.

Elbows and horizontal lengths in the connector pipe (stovepipe) can also interfere with chimney updraft, and should be avoided if possible. When a woodstove is installed in front of a fireplace or chimney, locate it as close to the chimney as its rear clearance listing allows, to minimize the horizontal length required to make the connection. When connecting a top-vent stove to a chimney located in the wall behind it, eliminate the 90-degree turn if possible by substituting two 45-degree elbows with a short length between them so the exhaust can continue to rise as it flows toward the chimney. When a horizontal length must be used, install it with a slight upward tilt towards the chimney thimble (at least 1/4" rise per foot of run). When venting into a manufactured chimney at the ceiling, locate the stove directly under the chimney so the pipe goes straight up. If you must offset the pipe, use 45-degree elbows instead of 90's.

Flue blockage

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. can block or partially block a chimney flue, interfering with proper draft. Make sure the flue is clean: it doesn't take much soot or creosote buildup to reduce the flue diameter enough to interfere with proper draft.

While you're up there looking at the chimney, don't forget to make sure the spark mesh in your chimney cap isn't plugged with creosote or flyash. If it is, clean it out and snip larger holes in the mesh to prevent a recurrence.

Air intrusion into the flue

Each wood-burning stove and fireplace needs its own flue. If another appliance is venting into the same flue, air can leak in through the second appliance (or through cracks in its connector pipe or thimble fitting) and reduce the chimney updraft in much the same way as opening the thumb slide on a vacuum cleaner's hose reduces the suction at the floor. Second appliances should be disconnected from the flue, and the thimble opening mortared up. The same goes for any other openings in the flue, such as unused thimbles. Cleanout doors in masonry chimneys should be tightly gasketed, or sealed shut with hi-temp silicone or metal tape. Tip: if your stove vents into a manufactured "tee" chimney (through the wall and up the outside of the house), make sure the cover at the bottom of the tee is tightly sealed.

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 woodstove, 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 woodstove or fireplace installations. Another problem, called The Stack Effect 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 woodstove eliminates the draft problem, the best solution is to provide outside combustion air directly into the firebox.  If the design or location of the stove or fireplace doesn't allow for this, an openable window or makeup air intake like the Condar Air Supply Ventilator, located in the nearest outside wall, is the next best solution.

Mechanical depressurization

Bathroom and kitchen exhaust fans, attic ventilation fans, clothes dryers, etc. often cause negative pressurization in the house that can draw exhaust backward down the chimney. Outside combustion air coupled with tight-fitting glass fireplace doors and good gasketing on your woodstove door can help solve this problem, as can opening a window between the exhaust fan and the fire. Furnace blower systems often cause negative pressurization in the house as well, as in cases where the return air intake isn't perfectly balanced with the flow back into the house through the heat registers. To minimize room depressurization caused by a properly balanced forced air system, make sure all registers connected to the forced air system are open. If this doesn't balance the system, consider calling in an HVAC professional.

Insufficient chimney height

Chimneys often draw at least 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 lower density 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.

Most wood stove manufacturers require a minimum stack height of at least 13-15 feet, measured from the floor of the stove. Code requires that all woodstove and wood fireplace 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 these minimum specifications but still fail to extend upward into air of 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 drop in air pressure from the bottom to the top of a chimney is often not sufficient to exhaust the smoke from a wood fire. In colder weather, for example, an unused flue can fill up with low temperature, high density air which can completely block the flow of smoke up the chimney. When this happens, any attempt to light a wood fire will result in a house full of smoke. To "prime" a cold chimney, line the backwall of the stove or fireplace with loose balls of newspaper and light them. Replace the newspaper and relight repeatedly until the hot paper exhaust pushes the cold air plug out of the chimney (you'll see the smoke from the paper fire suddenly disappear up the flue when this happens). Our more impatient customers with super-cold chimneys also report good results by lighting a can of Sterno or gelled alcohol inside the firebox. Once an updraft is established, build a paper and kindling fire and add progressively larger pieces of fuelwood gradually as the flue continues to heat up and create the thermal updraft needed to exhaust the smoke from the wood fire. Caution: if you're using Sterno or gelled alcohol to prime your flue, remember to extinguish and remove the can before starting your kindling fire!

Masonry chimneys can be hard to prime, because 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 insulating blanket used in today's prefabricated insulated metal chimneys. A masonry chimney bleeds precious heat away from the exhaust 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 exposed to outside temperatures for their entire length, but all chimneys lose precious flue temperature above the roofline.

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 today's super-efficient airtight stoves. For this reason, code authorities have outlawed the use of air-cooled chimneys to vent airtight wood stoves.

If you have a choice, the best possible venting method to ensure adequate flue temperatures for a woodstove is insulated stainless steel chimney. If you are venting into a masonry chimney that just doesn't seem to want to heat up, install an insulated stainless steel liner.

Air inversion

The atmospheric condition known as air inversion causes high-density air to be trapped at fluetop 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:

(A) 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 woodstoves 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.

(B) During air inversion episodes, remove all possible draft resistance at the bottom of the chimney. Today's woodstoves 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.

(C) Elevate the top of the chimney to a point above the inversion layer. This is a sort of 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 woodstove chimney with uninsulated metal pipe, or excessive creosote formation will result.

Downdrafting or Crossdrafting

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 Bernouli's principle, such as the old-fashioned "roostertail", or the more effective Vacu-Stack. These caps are designed to reverse the effects of wind-induced downdraft or crossdraft, 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.

Excessive Updrafting

The preceeding sections all relate to insufficient updraft. Sometimes, too much updraft can be a bad thing. If your airtight woodstove is burning out of control, make sure the door and glass gaskets are making a good seal, and that the air control mechanism on the stove is operating properly. If all else fails, you might consider attempting to reduce the chimney updraft using mechanical dampers.

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Great info about chimneys, thanks!
As a retired bricklayer I guaranteed my chimneys would suck the hat off my customers head.........

Phil Smith, KY

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Guys, you have a very informative site. I am a bricklayer with almost 30 yrs. under his belt and have done many fireplaces and wood burning ovens and until today I never knew there was a mathematical formula for calculating flues.

Thanks for the info.

John DeMeo
Mason /Project Manager
Quality general Contracting Co. Inc.

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Q: Our Country Comfort CC150 woodstove has served well for 16 years. Until this fall, we have not had a smoke- into-the-house problem when first starting a fire with a cold chimney and stove. Now we are. The stovepipe and chimney flue are clean, and the stove has been serviced and checked out. The problem doesn't seem to be inversion because the smoke goes straight up and out of the chimney. I can't think of anything different this year than in past years. No changes have been made in the stove, piping or chimney since last year. We use kiln dried kindling and small wood with plenty of newspaper to start the fire. If I input additional air into the controlled airway with an air pump, I can overcome the smoke into the house and eventually the smoke stops and a good fire burns with no more problems. I'm thinking of adding a small output electric blower which I could turn on for a few minutes until the fire burns well and updrafts correctly. I'm mystified about why I have this problem now and not in past years.

Thanks for your fine informational website.

Ray VanTassel
Portland, OR

Hi Ray,
There are several things that might be contributing to your problem, but based upon the mild Fall temperatures the Pacific Northwest is experiencing this year and your statement that eventually the backdrafting stops and a good fire burns, I'm going to take a stab at it. My guess is, it's not as cold outside just now as it has been when you started using your woodstove in past years.

Essentially, thermal updraft in a chimney flue is created by the tendency for heated air to "float" above cooler air. The greater the difference in temperature, the faster the heated air rises. In milder weather, when the temperature difference from inside the house to outside the house isn't very pronounced, chimney draft can be somewhat sluggish, even after the flue has been primed. During these times, special care must be taken to kindle the fire SLOWLY. Prime your flue as outlined above by burning several loose balls of newspaper, three or four at a time, until you get the flue heated up enough to start an updraft, but don't be so quick to pile on the kindling and fuelwood. GRADUALLY add SMALL pieces of kindling a few at a time, criss-crossing them to create lots of airspace between pieces and waiting until the smoke exhausts before adding more. Your goal is to create an extra-hot kindling fire to create as much temperature difference as you can between your exhaust gases and the not-very-cold outside air. As the flue temperature continues to rise, add increasingly larger pieces of wood until a hot, briskly-burning fire is established. If your stove isn't directly connected to outside combustion air, it can be helpful to open a door or window a crack to prevent depressurization of the house during the kindling process.

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Thank you so much for the extremely valuable help you gave me. Using more newspaper and for a longer start-up period did the trick to counteract the cold down-draft.

Thanks again,

Ray VanTassel

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I wanted to thank you very much in regards to your chimney draft trouble shooting page. I had a draft problem that nobody could figure out. After reading your advice we found that a pull down attic staircase was leaking air and that the whole house was acting like a chimney (the "stack effect"). I sealed the stairs and the problem is gone.

Thanks again,

Gene

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Thanks for your wonderful webpage/article re: draft problems. All of a sudden my wonderful woodstove seemed like it was plagued by draft problems--smoke would pour out of the door when opened, sluggish fire, etc... I went up and cleaned the "birdscreen" (hardware cloth) in the cap of creosote. Confident that would solve the problem, I came down to restoke the fire. But, the problems persisted. Then I went to the local home store and bought a cleaning brush and rods. Just before going up to the roof to begin, for some reason I went into the "room" behind the hearth and, lo and behold, the bottom of the chimney tee had partially fallen off! ! ! Well, I don't need to tell you the rest of the story--I'm sure you know. I am, however, still going to clean the chimney--but I will wait for spring to do it.

Thanks again for your wonderful webpage.

Clark Tomlin

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I want to add a follow-up to Clark’s comment and add a Big Warning about the tee cap falling off.

A year and a half ago I noticed a slow draft with my wood stove. It was early in the burning season and the chimney had just recently been cleaned, so I suspected it had to do with the quality of the wood and that the weather had not gotten really cold outside yet. Two days later, the carbon monoxide alarm in my son’s bedroom went off (his room is over the garage, which is the reason we had a detector in his room). Long story short, the cap at the bottom of the tee had fallen off, allowing carbon monoxide to vent into the attic. My wood stove is configured with an insulated steel chimney that is enclosed on the exterior of the house in a framed chase (covered by siding). Structurally, the inside top of the chase has a small opening into the attic which is the only place the gas could escape, other than through the vent pipe.

The thing to also understand is that my son’s room over the garage is on the OTHER side of the house over 50’ away, and the carbon monoxide detector is IN his room, so the carbon monoxide was gradually building throughout the top of the house. When we were burning wood without knowing the tee cap had fallen off, we never smelled smoke and none of the many smoke detectors in the house went off. It was a frightening discovery.

Needless to say, I have mounted a combination smoke and carbon monoxide detector in the attic, and I inspect the cap tee regularly. From my experience, I suggest your site visitors consider this ounce of prevention as well.

Ward

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You nailed it in your "Air Intrusion Into the Flue" section. I have a like new Vermont Castings Catalytic Encore 2550 wood burning stove that is the best looking thing in my house, except it has always leaked smoke into the room whenever I add fuel, whether I load it through the top or through the front doors (a beautiful but bad stove).

My masonry chimney has a metal cleanout door near gound level on the outside. Open it up and you have a 6" square opening into the flue tile liner to remove the swept-down creosote. Even though the door was closed, it was apparently leaking enough air that it worked like the 'open thumb slide on a vacuum cleaner hose' you describe. I packed the hole with foam insulation (always cool at this level 5' below the thimble opening where smoke enters the flue) and voila! No smoke on reloads!! In fact the draft is so much stronger it will pull a candle flame sideways when held next to the cracked front door of the stove, even when the stove isn't burning. Oh, another big benefit, the door glass is staying clean much, much, longer.

Again, thanks for your most helpful information: it saved me a bundle and now I truly love my "beautiful and not bad" stove.

Steve in North Carolina

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E A Murphy
Ann Arbor, MI

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Your explanation of the "stack effect" is spot on. I want to offer another example. I am gutting and renovating my master bath, including some ceiling space and lighting. It is on the second floor. My fireplace is on the first floor. I had noticed significant cold air coming from the fireplace when opened to light it, but didn't think much of it. The exhaust fumes from the gas logs and the flames had begun not going up the chimney for the first minute or so, but eventually corrected. On a very cold night, I was unable to get the fumes/flames to reverse at all and we could see the flames being blown downward, until it occurred to me the exposed attic space in the bathroom where some insulation and ceiling has yet to be replaced was causing the chimney to draw replacement air down faster than the fireplace heat could reverse it. So I opened a nearby door for about thirty seconds. Voila. The draft reversed correctly up the chimney, the flames and fumes reversed, and I closed the door. Need to get the ceiling closed up (or insulated) very soon! Tells how much heat is being lost.

Mike Dean
Montgomery, AL