Masonry Chimneys

This is your traditional chimney. This diagram is missing a very important part; the cap.Masonry Chimney

Common problems for masonry chimneys include spalling brick, cracks in the chimney structure, cracked flue liners, deteriorated mortar joints in the flue and shell, cracks in the crown and cracked and/or deteriorated fire brick. Log lighters and grates often burn out.

Over time every chimney needs some type of maintenance even if you do not use it.  I’ll use this example: if you bought a car and parked it in your driveway for 2, 5, 10 or 20 years would it fire right up when you turned the key? Probably not and so it goes with your chimney too.

Over time the crown will soften from the weather, the chimney shell and liners can crack from settling and seismic activity. If your masonry chimney does not have a proper chimney cap rain water will run down the flue spalling liners and pool in the smoke shelf leaching down in to the fire box, softening the fire brick and rusting the damper solid.

The birds and the bees will make your chimney home building nests and hives in the flue. Prefabricated, factory built chimney spark caps, terminations and chase covers rust out and corrode through allowing water intrusion in to the fire place and the chase itself. This can cause water damage, corrosion of metal fireplace parts and mold problems.

Couple these problems with use and the corrosive nature of wood burning byproducts, especially creosote, and you can have some major chimney problems. Simple maintenance is the key to avoiding costly repairs and even fireplace and chimney replacement.

“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.” Courtesy of the CSIA website.

I never use my chimney so it doesn't need to be inspected.

“I never use my chimney so it doesn’t need to be inspected”.

Causes and Remedies for Smokey Fireplaces

Smoke stains above the fireplace are proof of a smoking problem. Investigation is needed to determine the cause of the problem before it can be fixed. Soot stains are carbon particles. If you have soot stains inside your home, you have also had carbon monoxide (CO) in your home. Soot stains are not only unsightly; they indicate air in your home has been exposed to dangerous fumes. Take care of this problem (which may not be caused by your fireplace) before using it again. Soot and carbon monoxide are a natural byproduct of burning wood, gas, coal and oil – any carbon based fuel.
The first thing to check is that the damper has been opened before starting a fire! Next, try preheating the flue. Keep in mind that in the winter the chimney may be filled with cold air. Since warm air rises and cool air falls,you must reverse the air flow and send warm air up the flue. You can do this by using a bit of newspaper, tightly rolled and lit like a torch and held up through the fireplace damper. Or use a hair dryer to blow warm air up the chimney for a few minutes before lighting your fire.

Fireplace Size

Large fireplaces are a stunning and beautiful feature in any room. However fireplaces must built within a general set of proper sizing guidelines. Standard American fireplaces should be designed with a 10:1 ratio of fireplace opening to flue size (square or rectangular flues). Length x width of fireplace opening provides the fireplace size; length x width of flue tile provides the cross-sectional area of the chimney. Severe rectangles may need a 8:1 ratio. Round flues should provide a minimal 12:1 ratio as a round flue drafts more easily.

When the fireplace opening is overly large for the size of the flue, the chimney cannot adequately remove the byproducts of combustion; the amount of air entering the fireplace must be equal to the amount of air that is exiting the chimney. It’s much easier to reduce the size of the fireplace opening versus enlarging the chimney, several options are available:

  • Install a smoke guard, which decreases the size of the fireplace opening at the top
  • Install glass doors so the frame overlaps the firebox opening, again reducing the size of the fireplace opening
  • Raise the fire by laying a row of brick on the floor or using a taller fire grate or tall andirons
  • Rebuild the firebox to the appropriate dimensions
  • Install a fireplace insert (wood, gas, pellet) with a new chimney liner sized properly for the insert

Multi-Sided Fireplaces

Because see-through or multi-side fireplaces have much bigger openings, they almost always smoke to some extent. The flue serving such fireplaces must be considerably larger to carry the volume of air needed by the multi-sided fireplace. We’re frequently asked about removing the rear wall of an existing fireplace to make it a see-through to the room behind it. A chimney that was designed to vent a fireplace with a single opening cannot handle having its fireplace opening size doubled. Even if you find a mason who will do this job for you, it’s not a good idea and never functions properly.

Chimney Design

An exterior chimney stays colder in the winter, especially in harsh climates. Before a chimney can draft properly it needs to be warmed, so an exterior chimney, although a common design, can present drafting problems. If you’re building a home, consider a chimney built to the interior of the home if possible.

Chimney Height

The chimney needs to be at least 3′ higher than the roof where it penetrates AND 2′ taller than anything within 10′ away. This is known as the 3, 2 in 10 rule.

On chimneys with multiple flues the flue terminations should be staggered in height, (one higher than the other), to help prevent one flue from sucking smoke downward from the other one. This is known as flow reversal and can bring smoke down the unused chimney and in to the home. It’s best practice to have the chimney built near the peak of the roof rather than on the lower side to minimize stack effect problems. This is where there is a big difference in air pressure between the indoor and outdoor air. Warmer air inside the house rises and exits the home from the upper sections like the roof vent or upstairs windows. Make up air has to come from somewhere to replace the air that is exiting. Make up air likes to take the path of least resistance, such as down the chimney. If the chimney is in use this can cause a competition between cold air being sucked down the chimney while smoke is simultaneously trying to vent from it causing back puffing.

Chimney flues should be built as straight as possible; turns and offsets increase resistance in the draft and slow the smoke and fumes which can cause heavy build up.

Negative Pressure

In our compulsive obsession to design energy-efficient “air tight” homes, we often don’t consider there must be means for outdoor air to enter the house to operate the devices we use in them. Chimneys must pull air from somewhere to provide combustion air for the fire AND allow an updraft so smoke and fumes can exit, so air must be supplied at an equivalent rate to replace the air leaving the chimney. Extremely airtight homes can prevent chimneys from operating properly, especially where other air-moving devices are being used such as furnaces, bathroom or kitchen vents, attic fans, clothes dryers, etc. Again, replacement air for these devices AND for the chimney may be entering through the chimney.

It is possible to have cold air dropping down one side of flue while warm smoke or fumes are also trying to exit at the same time. Try cracking the closest window to the fireplace to provide extra air for the fireplace and make sure no exhaust fans or clothes dryers are being used in your home at the same time. If your chimney used to work well and now doesn’t, consider what has changed that affects its ability to draft weatherstripping, replacement windows, new siding, extra insulation, room additions, new appliances or anything else to tighten up the home.

High efficiency clothes dryers that dry clothes more quickly can use drastically more air to operate.

Chimney and fireplace professionals may be able to suggest alterations to improve or cure your smoking or malfunctioning fireplace in some situations. If your new stove or fireplace is not working properly, have it checked by a pro for advice. But remember, using the best principles of design for a fireplace or stove installation may not be overcome by the design of the house and the way that air enters and leaves it. Simply put, we in the fireplace industry cannot solve smoking or draft problems in every situation.


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