An Exit for the Smoke

If there’s a wood stove in your life (or in your future) and you already have a masonry fireplace in your home, it’s likely that you have used (or wish to use) your fireplace’s chimney as the chimney for your wood stove or wood burning fireplace insert. Such a choice would seem both sensible and economical.

However, any heating system works best when all its parts are designed – at the outset – to work together. Just as a furnace operates best when the flue size of the chimney is carefully matched to furnace capacity, so a wood stove is safest and most efficient when attached to a chimney whose flue size most closely matches the flue collar outlet of the stove.

While wood stoves can be successfully connected to fireplace flues (a flue is the inner section of a chimney and is designed to carry away smoke and other toxic products of combustion), certain standards must be met. Here’s a look at what they are and why they are important.

Today’s Wood Stoves & Inserts

There are two types of wood stoves that can be connected to fireplace flues: freestanding stoves and fireplace inserts. Freestanding stoves can be connected to chimneys built especially for them. The chimneys may be of masonry construction or be a factory-built metal system that’s been designed, tested and listed for use with wood burning appliances. Freestanding stoves also can connect to an existing fireplace chimney, if the height and position of the stove’s flue collar permits it. When this type of installation is done, the stoves may be called hearth stoves.

Fireplace inserts are a special type of wood stove and are specifically designed to fit into the firebox (where logs normally go) of an existing fireplace and to use the fireplace flue to vent smoke and other by-products of combustion. Since the insert must be smaller than the fireplace opening, there is usually a surround panel attached to the stove which extends out around the fireplace opening to seal the firebox from room air.

Since 1984, national codes and standards – as well as many manufacturer’s installation instructions – have dictated that a connector pipe extend from the flue collar outlet of the stove or insert…up through the fireplace damper…and into the first flue tile of the masonry chimney.

How an Open Fireplace Works

Fireplaces aren’t designed to vent (or carry away combustion by-products from) wood stoves or wood burning inserts. They are a uniquely designed solid fuel burning system in their own right. The fireplace system consists of the firebox, a damper (the mechanism that regulates air flowing up the chimney), a smoke chamber (the area between the damper and the flue) and a flue (a passageway inside the chimney through which the smoke rises). Together, they draw off the smoke and gases produced by burning wood. How well the firebox, smoke chamber and flue are in correct proportion to each other determine the system’s performance.

The applicable rule of thumb (which is included in many local codes) is that the area of the firebox opening should only be about 10 times larger than the area of the flue’s interior dimension – a 10:1 ratio.

Stoves & Inserts Connected to Fireplaces: A Different Equation

Whenever a wood stove or insert is vented through a masonry fireplace system – even if the code requirements laid down in 1984 are followed – the ratios on which efficient operation of that masonry system initially was based may be changed. For example, the size of an insert’s firebox is smaller than that of the masonry firebox, so the existing masonry flue may now be proportionately too large. An over-sized flue causes a reduction in the speed at which air moves out of the chimney. This lets the smoke that exits the wood stove linger inside the chimney, cool down and deposit condensed creosote on the chimney interior.

Creosote is a brown or black combustible deposit – given off when smoke condenses – which must be monitored and swept out to keep your system safe.

Major creosote deposits are created when wood stoves or inserts do not meet the 1984 “first flue tile” standard, and vent smoke directly into the fireplace or smoke chamber. Smoke condenses inside both the firebox and smoke chamber and may produce a ceramic-hard glaze of condensed creosote – which is hazardous, difficult and potentially expensive to clean…And which damages masonry materials through the corrosive action of acids it contains.

Never permit continued use of this type of installation, even if your stove is old enough that manufacturer’s instructions do not require the connector pipe to extend into the first flue tile. Insist on a safer installation for yourself and your family.

Exceeding Minimum Requirements

The “first flue tile” installation method for wood stoves and inserts, which are vented through masonry fireplaces, solved many problems. For those committed to the use of a wood stove or insert for heat, a more efficient, easier to maintain and ultimately safer method is available, which your chimney specialist may recommend for you. That method calls for the vent pathway to extend from the stove collar all the way to the top of the flue creating a new chimney liner within the chimney.

This new chimney liner is sized correctly for the wood stove or insert, so drafting the smoke out of the system is less likely to be a problem. In addition, it is an easy and economical way to extend the life way to extend the life of your chimney, since the new liner protects the existing structure from heat deterioration and acid-based smoke condensation.

Types of Lining Systems

There are three types of tested and listed lining systems available: stainless steel; poured-in-place or pumped; and ceramic. The one you select may be determined by the kind of wood stove or insert you have and the condition of your chimney’s structure. Your chimney specialist will help you decide which system will serve you best.

Stainless steel chimney liners – tested and listed to Underwriters’ Laboratories (U.L.) standards – run from your stove to the top of your chimney and should include an insulating system to assure stable temperatures within the flue and help prevent heat transfer to combustible parts of the house

Pumped liners create a newly-sized flue by pumping a special slurry mixture around a form, allowing it to harden and then removing the form. Poured-in-place systems use a vibrating “bell” to compress the mixture into place. The mixtures are tested and listed to U.L. standards for chimney liners. They result in a smooth, hardsurfaced flue that runs from the top of the chimney to the smoke chamber and sometimes continues to the damper area. A length of connector pipe makes the final connection from the new flue to the stove.

When You Properly Size Masonry Fireplace Flues for Stoves & Inserts…

If you want to use the fireplace in the future, a removable stainless steel liner is the appropriate choice. If the proper sizing of the flue is achieved in a more permanent manner in order to accommodate a wood stove or insert, future use of the fireplace may depend on some modification. Ceramic liners are installed inside the chimney with a special setting tool, and a connector is extended to the wood stove. An insulator is poured around the liner. The liner and insulator together are tested and listed to U.L. standards.

Operation & Maintenance

An approved wood stove-to-fireplace installation will help assure your safety. Annual inspections and cleaning of these systems by a CSIA Certified Chimney Sweep® will enhance their safety and efficiency.

About CSIA & CSIA Certified Chimney Sweeps

The Chimney Safety Institute of America (CSIA) is a non-profit educational institution that has established the only nationally recognized certification and accreditation program for chimney sweeps in the United States. The program was developed in keeping with CSIA’s commitment to the safety of chimney and venting systems and to the elimination of residential chimney fires, carbon monoxide intrusion and other chimney related safety hazards. CSIA devotes its resources to educating the public, chimney service professionals and other fire prevention specialists, and the insurance industry about the prevention and correction of chimney venting system hazards.

Think about it for a moment. All the brick and other materials that make up your home are protected by the roof and eave, all that is, except your chimney. The chimney bravely sticks up above the roof constantly exposed to all the elements; rain, snow, and freeze/thaw cycles.

A masonry chimney is constructed of a variety of masonry and metal materials, including brick, mortar, tile, steel and cast iron. All of these materials will suffer accelerated deterioration as a result of prolonged contact with water.

Masonry materials deteriorate quickly when exposed to the freeze/thaw process in which moisture that has penetrated the materials periodically freezes and expands, causing undue stress. Water in the chimney also causes rust in steel and cast iron, weakening or destroying the metal parts.

Water penetration can cause interior and exterior damage to you home and masonry chimney including:

• Spalled and broken brickwork
• Deteriorated metal or masonry firebox assemblies
• Rusted damper assemblies
• Rotted adjacent wood and ruined wall coverings
• Deteriorated central heating system
• Decayed mortar
• Cracked flue liner systems

Preventing Water Damage

Following are the four main ways to prevent water damage.

1. Install a Chimney Cap

Chimney caps, also called rain covers, are probably the most inexpensive preventive measure that a homeowner can employ to prevent water penetration and damage to the chimney. Chimneys have one or more large openings (flues) at the top that collect rain water and funnel it directly to the chimney interior. A strong, well designed cap not only keeps this water out, but will also prevent birds and animals from entering and nesting in the chimney.
Caps also function as spark arrestors, preventing sparks from landing on the roof or other nearby combustible material.



2. Repair or replace a damaged chimney crown

The chimney crown, also referred to as the chimney wash, is the top element of a masonry chimney. It covers and seals the top of the chimney from the flue liners to the chimney edge. Most masonry chimneys are built with an inadequate crown constructed from common mortar mix, the same mixture used to lay the bricks of the chimney. This mortar is not designed for and will not withstand years of weather abuse without cracking, chipping or deteriorating; situations that allow water to penetrate the chimney. In fact, most sand and mortar crowns crack almost immediately after installation because of shrinkage.

A proper chimney crown should be constructed of a portland cement based mixture and cast or formed so it provides an overhang, or drip edge, projecting beyond all sides of the chimney by a minimum of two inches. This drip edge directs the runoff from the crown away from the sides of the chimney, helping prevent erosion of the brick and mortar in the chimney’s vertical surfaces.

There are also some modern waterproof, non-shrinking, cement like coatings for repairing damaged mortar crowns that seem to work fine.



3. Repair or replace flashing

Flashing is the seal between the roofing material and the chimney. Flashing prevents rain water from running down the chimney into living spaces where it can damage ceilings or walls, or cause rot in rafters, joists, or other structural elements. The most effective flashing is made up of two elements, the flashing and the counter- flashing.

The base flashing is an L shaped piece of metal extending up the chimney side and under the roofing shingles. The counter flashing, which overlaps the base flashing, is imbedded and sealed in the chimney’s masonry joints. This two element flashing allows both the roof and the chimney to expand or contract at their own rates without breaking the waterproof seal in either area.



4. Waterproof your chimney

Most masonry materials are porous and will absorb large amounts of water. Common brick is like a sponge, absorbing water and wicking moisture to the chimney interior. Defective mortar joints or the use of improper mortar or brick can greatly increase the tendency to absorb and convey water to the interior of the masonry chimney structure.

Several products have been developed specifically for use as waterproofing agents on masonry chimneys. These formulas are vapor permeable which means that they allow the chimney to breathe out, but not in. Thus water that has penetrated the chimney, or moisture that has originated from inside, is allowed to escape, while the waterproofing agent prevents water from entering from the outside.

Paint, or any non vapor permeable water sealer, should never be used as a waterproofing agent because it will trap moisture inside the chimney, accelerating deterioration.



In Conclusion: Water damage to masonry chimneys is usually a slow, subtle process. The problem is often not evident until it has become quite serious.

Although these water prevention measures may cost a few dollars initially, they will save you the major expense of large masonry repairs or rebuilding of the entire chimney in the not too distant future, and as such represent a wise investment in your home.

In the 1940′s and again in the 1980′s, masonry chimneys were tested by the National Bureau or Standards for durability due to rising concerns about their performance and safety. The tests revealed that unlined chimneys were so unsafe that researchers characterized building a chimney without a liner as “little less than criminal”.

Liners in chimneys serve three main functions:

1) The liner protects the house from heat transfer to combustibles. In the NBS tests, unlined chimneys allowed heat to move through the chimney so rapidly that the adjacent woodwork caught fire in only 3 1/2 hours.

2) Liners protect the masonry from the corrosive byproducts of combustion. In the tests it was determined that if the flue gases were allowed to penetrate to the brick and mortar, the result would be a reduction in the usable life of the chimney. The flue gases are acidic in nature and literally eat away at the mortar joints from inside the chimney. As the mortar joints erode, heat transfers more rapidly to the nearby combustibles and dangerous gases such as carbon monoxide can leak into the living areas of the home.

3) Liners provide a correctly sized flue for optimum efficiency of appliances. Modern wood stoves and gas or oil furnaces require a correctly sized flue to perform properly. The chimney is responsible for not only allowing the products of combustion a passage out of the house, but the draft generated by the chimney also supplies the combustion air to the appliance. An incorrectly sized liner can lead to excessive creosote buildup in woodburning stoves, and the production of carbon monoxide with conventional fuels.

Types of chimney liners Chimney liners come in three main types: Clay Tiles, Metal, and Cast-in-place

1) Clay tiles are the most common type of masonry chimney liners. They are inexpensive, readily available, and perform quite well for open fireplace chimneys that are properly maintained. There are two disadvantages to clay tiles. The first is that, being a ceramic product, they cannot rapidly absorb and evenly distribute heat during the rapid temperature rise that occurs during a chimney fire. This uneven heating produces an unequal expansion which in turn causes the flue tiles to crack and split apart. This is similar to immersing a cold drinking glass in very hot water. It will instantly shatter. A chimney with cracked chimney liners must be repaired before use. The second disadvantage is that tiles cannot adequately contain the liquid combustion byproducts produced by modern gas appliances.

2) Metal chimney liners, usually of stainless steel or aluminum, are primarily used to upgrade and repair existing chimneys. These liner systems are U.L. tested and listed, and if properly installed and maintained are extremely safe and durable. Stainless steel is suitable for woodburning, gas, or oil applications, while the aluminum is an inexpensive alternative for certain medium efficiency gas applications only. It is usually required that high temperature insulation be used in conjunction with the liners for safety and performance considerations.

3) Cast-in-place chimney liners are lightweight, castable, cement like products that are installed inside the chimney forming a smooth, seamless, insulated passageway for the flue gasses. They can improve the structural integrity of aging chimneys, and are permanent liners suitable for all fuels.

Considering the dangers of old unlined or damaged chimneys, and the many cost effective options now available to make these chimneys safe components of the home heating system, may we suggest you have your chimney professionally inspected to be sure it meets modern safety standards.

Simply put, fireplaces work mainly because hot air rises. When you start a fire, the air inside the chimney becomes warmer and less dense than the air outside the chimney, and consequently it starts to rise. As the warm air rises, cooler air from the room flows into the firebox, fanning the fire, creating more heat in an ongoing cycle. There are also some pressure differentials produced as wind moves across the top of your chimney.

There must be at least 100 reasons why your fireplace may not function properly. We will try to cover some of the basics here starting with the easy obvious solutions and working towards the more arcane. Please bear in mind this is a very simplified list of the more common reasons that fireplaces don’t work A true understanding of fireplaces requires extensive knowledge of air flow patterns, pressure differentials, and actual fireplace construction techniques. If the information provided here does not help you solve the problem with your fireplace, consider hiring an experienced, certified chimney sweep in your area. Often the problem is obvious to someone with enough experience once they can acutally look over the entire situation.

1.) Is your damper fully open? Everybody eventually forgets to open the damper. Many dampers also cease to fully open because of water damage or soot buildup behind them on the smoke shelf. A good professional cleaning can usually solve this problem.

2.) Is your firewood green or wet from rain or snow? Remember the main reason your fireplace works at all is the heat inside the chimney. If your wood is not dry and well seasoned it makes more smoke than heat and there simply may not be enough heat for the chimney to work properly.

3.) Is your chimney dirty? The gradual accumulation of soot can seriously affect the way your chimney performs. Thick layers of soot of course can physically restrict the flue so there is no longer enough free area to vent the fireplace properly, (see problem 5) but as little as a 1/4″ to 1/2″ inch buildup can make more difference than you might think. Consider that a 1/2″ buildup will restrict the air flow by 17% for a typical masonry fireplace chimney, and by a whopping 30% for the average prefab. Birds and small animals also think your chimney looks like a hollow tree in which to set up housekeeping. Sweeps often find chimneys literally packed full of leaves, twigs and baby animals. The solution of course is a good cleaning and a chimney cap.

4.) Is your chimney tall enough? To function properly, the chimney should be at least 10 or 12 feet in overall height. Where it projects above the roof, the chimney should be at least 3 feet tall, and at least 2 feet higher than anything within 10 feet of it-including other buildings, trees, etc. If your fireplace smokes because your chimney is too short, the problem is usually worse when the wind blows.

5.) Is your flue large enough for the fireplace opening? There are many variables that can affect this including; overall chimney height, how warm the flue stays, throat configuration, etc., but the basic rule of thumb here is that the area of the fireplace opening can be no more than 10 times the area of the flue (12 times for round flues). An undersized flue simply can’t handle the volume of smoke produced, and some of it will spill back into the room. Since there is no practical way to make the flue size larger, the solution may be to make the room opening smaller with metal smoke guards or some creative masonry work. In fact there are now some premanfactured refractory firebox retrofits that work well with a 15 to 1 ratio and deliver twice the heat of conventional fireboxes.

6.) Is your chimney on the outside of the house? Remember that warm rising air is the basic engine involved here. If you have a large masonry chimney on the outside of the house, and it’s cold outside, the air inside of the chimney will also be very cold, and it will want to fall down the chimney instead of rising. This can even happen a day or two after it’s warmed up outside. These chimneys may be hard to start and they may smoke as the fire burns low. To help get the fire started many people light some rolled up newspaper and hold it up near the damper to get that cold plug moving upwards. Keeping a moderate sized but bright, actively flaming fire can also help this situation. Remember that as the fire dies down, it will revert back to the original direction of flow.

7.) Is your home too tight? Fireplaces require large volumes of air to burn. Visualize a 12″ x 12″ column of air rising up your chimney and exiting the top the entire time your fireplace is working (but don’t visualize your heat bill!). This air comes from inside the living area and must somehow be replaced. With modern energy efficiency concerns most houses have been carefully insulated and weather-stripped to keep out the cold drafts, but an undesirable side effect is that there is often nowhere for all that air leaving the chimney to get back in. This can lead to fireplaces that burn sluggishly and smoke. A temporary solution is to open a window to let in a little make up air, preferably on the windward side of the house. It can also lead to very dangerous carbon monoxide buildup if your fireplace and furnace must compete for combustion air, and a permanent solution should be found at once.

8.) Your house can also be too loose! A house that leaks too much air to the outside, especially a multistory house that leaks air in the upper levels, can actually set up its own draft or chimney effect strong enough to overpower your fireplace chimney, particularly if the fireplace is located in the basement on a cold exterior wall. Be sure the attic access door is in place and that all upstairs windows are tightly closed.

9.) Is there a return air grill in the same room as the fireplace? As the fireplace consumes air and cold air moves into the house to replace it, the furnace is likely to come on. When the furnace comes on, air is drawn into the return competing directly with the needs of the fireplace.

10.) The other 91 reasons your fireplace can smoke have to do mainly with design problems when the fireplace was built. Aside from the chimney being too short, or too small, the chimney can also be too large, too tall, too crooked, etc. ad infinitum! Most of these details are fairly technical in nature, and again a good sweep may be your best bet.

Click here to visit the Pellet Fuels Institute’s handy calculator to compare fuel costs (Values shown are national averages)

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1. Always check your source. You can never be sure how much wood you are buying when you buy by the truckload. Buying a cord or a rick ensures that you will get what you pay for. A standard cord of firewood is 128 cubic feet of wood, generally measured as a stack 8 feet long by 4 feet tall by 4 feet deep. A rick is 8 feet long by 4 feet tall by the depth of the wood (approximately 18 inches).

2. Buy dry wood. Well-seasoned firewood, with an average moisture content of 20-25 percent, is easier to start,produces more heat and burns cleaner. An inexpensive handheld moisture meter, available at home improvement stores or local specialty hearth retailers, will allow you to test your own wood. In a pinch, you can simply bang two pieces together. When two pieces of seasoned wood are banged together, they make a “clunk” sound. When two pieces of green wood are banged together, they make a “thud” sound.

3. Nevermind the species. It is far more important that the fuel be dry as compared to the species. You don’t have to burn only premium hardwoods. Less dense woods like elm and even soft maple are abundant and make fine firewood as long as you’re willing to make a few extra trips to the woodpile.

4. Store smart. Wood should be stored off the ground if possible and protected from excess moisture when weather threatens.  Also remember that your woodpile looks like heaven to termites, so it is best to keep only a small amount of wood near the house.

5. Check your chimney. heck your chimney. Have chimneys inspected annually, and cleaned as necessary, by a qualified professional chimney service technician. This reduces the risk of fires and carbon monoxide poisonings due to creosote buildup or obstructions in the chimneys.

In 2005, there were 24,500 residential fires in the United States originating in chimneys, fireplaces and solid fuel appliances, according to the United States Consumer Product Safety Commission. These fires resulted in 20 deaths and $126.1 million in property damage. (Click here for more stats.)

Virtually all of these fires were preventable according to the Chimney Safety Institute of America (CSIA), a non-profit institution dedicated to public and chimney professional education. Both CSIA and the National Fire Protection Association recommend yearly chimney inspections to help prevent these hazards.
Many American homeowners think their chimneys only need to be cleaned and inspected if they burn wood in their fireplaces or wood stoves. But almost all heating appliances, whether they burn gas, oil, wood or coal, rely on the chimney to safely carry toxic gases produced by the heating system of the house.

A carbon monoxide detector can warn homeowners of potential poisoning after the deadly gas has already entered the living area, but an annual chimney check can help prevent carbon monoxide from entering the home in the first place.

Each fall, homeowners shift into home-improvement mode. They clean gutters, garages and basements — preparing homes for winter. But they usually don’t inspect, repair or clean their chimneys, despite the potential for damage to their property or even to their lives.

An annual chimney inspection by a CSIA Certified Chimney Sweep® is a modest investment that can reduce the danger of chimney fires or carbon monoxide poisoning. CSIA Certified Chimney Sweeps have earned the industry’s most respected credential by passing an intensive examination based on fire codes, clearances and standards for the construction and maintenance of chimneys and venting systems.

In fact, when chimney fires occur, many insurance investigators rely on CSIA Certified Chimney Sweeps to determine whether a fire originated in – or damaged – the chimney system. The CSIA, established in 1983, is a non-profit, educational institution, dedicated to educating the public about the prevention of chimney safety hazards.

For more information about chimney safety, or for a list of CSIA Certified Chimney Sweeps, please visit the CSIA web site at www.csia.org. The CSIA is located at 2155 Commercial Drive, Plainfield, IN 46168.