For discussing chimney performance problems, we have divided these problems into four main areas.

	Fireplace or woodstove smokes all the time.
	Fireplace or woodstove smokes sometimes.
	Fireplace or woodstove difficult to start.
	Foul odors from fireplace or woodstove.

Years of handling fireplace performance complaints have led us to group problems into the four areas above. We feel most problems will center around one of the these topics.

FIREPLACE OR WOODSTOVE SMOKES ALL THE TIME

Constant smoking problems are often the easiest to diagnose while being the hardest to correct. Many constant smoking problems are the result of improper construction or design and may require extensive modification of the chimney system to correct.

Below is a description of common design problems which may lead to constant smoking problems.

Fireplace opening too large - The fireplace opening should be sized based on a relationship with the chimney flue. An ideal fireplace opening would not be more than ten times the cross sectional area of the chimney flue. Example - If the inside measurement of the flue is 10 inches by 10 inches equaling 100 square inches, then the fireplace opening should not exceed ten times this, or 1000 square inches. You will have to go to the chimney top to accurately measure the chimney flue. Determine the fireplace opening by multiplying the fireplace opening height times the width. Example - A fireplace opening 36 inches wide by 27 inches high equals a fireplace opening of 972 square inches. This would be a proper matchup for the 10 x 10 flue. If the fireplace opening is too large, it can draw in more combustion air than the chimney flue can vent. This problem can best be solved by reducing the fireplace opening. The fireplace opening can be reduced with a noncombustible shield at the top of the fireplace opening to lower the effective opening height. Often a three to four inch reduction in opening height will make a noticeable improvement in fireplace performance. Another method to reduce the fireplace opening is to install glass doors. The fireplace opening could also be reduced by rebuilding the firebox to smaller dimensions.

Firebox too shallow - If the firebox is too shallow, the smoke may roll out near the top of the fireplace opening rather than entering the smoke chamber normally. The best solution to these problem would be installing a "hood" at the top of the fireplace opening which protrudes about four to six inches past the fireplace profile. This would help capture the smoke and direct it up the flue rather than into the room.

Lintel too high - If the fireplace lintel (the steel bar supporting the top of the fireplace opening) is too high, it may allow smoke to roll out of the fireplace opening instead of going up the flue. The lintel should be at least six inches below the damper frame. If it is not, try installing a shield as described above in "Fireplace opening too large".

Fireplace throat improper - If the fireplace throat (opening into the smoke chamber) is not constructed properly, severe smoking problems may result. The throat may be located too far forward or simply not be large enough for the fireplace. Throat problems generally require rebuilding the firebox and throat area to correct this problem. The opening of the throat should be at least one tenth the fireplace opening, however, a larger throat is common.

Smoke chamber improper - The smoke chamber serves to funnel smoke from the fireplace opening into the flue. Improper design here can lead to numerous problems. The smoke chamber can be too deep, too tall, or too wide. It may also be extremely rough and irregular in construction. NFPA 211 (National Fire Protection Association's codebook for Chimneys, Fireplaces, Vents, and Solid Fuel Appliances) recommends that smoke chambers should not be taller than the fireplace opening width, nor deeper than the fireplace opening. It further states that the smoke chamber walls should not incline more than 45 degrees from vertical. The walls of the smoke chamber should be smooth to reduce turbulence. Most building codes require that smoke chamber walls be parged. "Parging" is a coating of mortar applied to the brick to give the smoke chamber a smooth surface. The flue should start at or very near the top center of the smoke chamber and should not have any radical offsets or bends. Corner fireplaces are a special problem here as the smoke chambers tend to be quite large and irregular. Most smoke chamber design problems are difficult to correct and will require specialized skills.

Chimney flue too small - If the chimney flue is too small, it will not allow smoke to exit the woodstove or fireplace fast enough and can lead to serious smoking problems. As stated above, the fireplace and flue must be sized in a proper relationship. (See "Fireplace opening too large") It will be easier to reduce the fireplace opening than to enlarge the fireplace flue. If you have a woodstove, the flue should be at least as large as the flue collar on the stove. If it is not, you must either replace the flue with a larger one properly sized for the woodstove or replace the woodstove with one designed to operate with a smaller flue. Sorry, no cheap fixes here!

Flue too short - Short chimneys can often lead to smoking problems. A short chimney is defined as one less than ten feet tall above the fireplace opening or woodstove flue outlet. The best thing to do here is raise the chimney.

Improper termination height A chimney should terminate at least three feet above the point it passes through the roof so that any sparks exiting the chimney have time to cool before landing on the roof. The chimney should also terminate at least two feet higher than any portion of the building within ten feet. This rule keeps the chimney above nearby obstructions that could cause smoking problems. Please realize that this is only the minimum accepted termination height and may not always be enough. If your chimney is lacking in either of these areas, you should consider raising it.

Proper Termination Height

Airtight homes - Smoke cannot rise up the chimney any faster than air can be drawn into the home to replace it. Normally the home will allow sufficient air leakage around doors and windows to satisfy the chimney. However, if your home is especially airtight, it may not allow enough makeup air to enter the home at a rate sufficient to promote good flow in the chimney. If you suspect this problem, open a window near the fireplace the next time you have a fire. If this cures the problem, consider installing an outside air supply so you don't have to leave a window open when having a fire in the fireplace.

Obstructions and blockages - If the chimney is blocked or obstructed the smoke cannot exit. The chimney could be obstructed with leaves and pine needles which fell into the chimney or were carried into the chimney by an animal or bird building a nest. An accumulation of creosote can block the chimney and possibly block the screen in the chimney cap, if you have one. Creosote blockages are often indicated by a problem, which gradually worsens, to the point that smoking is constant. Sudden blockages can occur in older, deteriorating chimneys if a portion of the chimney walls collapse. An experienced chimney sweep should be able to remove any blockages or obstructions in your chimney.

Damper - Make sure the fireplace damper is open! This is an easy thing to forget if you don't have fires on a regular basis.

FIREPLACE OR WOODSTOVE SMOKES SOMETIMES

Erratic or occasional smoking problems can often be the hardest to detect but often are easy to eliminate once detected.

Competing vents - Anything which removes air from the home can cause smoking problems, especially in fireplaces. Draft in fireplaces is quite low compared to draft in woodstoves and as a result fireplaces are more prone to smoking. A competing vent could be the kitchen or bathroom exhaust vents or attic exhaust vents, especially if they are of the powered type. The furnace return vent can also cause problems if the duct system is not properly designed and installed. By drawing air from the house this can cause a flow reversal. If you suspect that competing vents are causing your woodstove or fireplace to smoke, make sure all vents are off during your next fire. If this cures the problem, install an outside air supply to the house, so that it can draw outside air as needed.

Temperature - Draft can be defined as the difference between indoor and outdoor air temperature. A fireplace that works well when it is very cold outside may smoke when the outdoor temperature approaches the indoor temperature. If you suspect this problem, simply do not use the fireplace or woodstove on moderate days. Usually a temperature difference of twenty degrees between indoor air and outdoor air is sufficient.

Wood Supply - Green or wet wood can cause smoking problems. If you must burn green wood, mix it with some dry, well seasoned wood. Also keep enough wood for your next fire in a dry location or well covered to prevent rain or snow from getting to it. (Also see section on buying and storing firewood.)

Wind Induced Downdrafts - Wind related smoking problems are the most common type of occasional smoking problem. Wind induced downdrafts only occur when the wind is blowing and usually only affect a particular chimney when the wind is blowing from a certain direction. (Example - chimney only smokes when wind blows from the east, works fine all other times.) Two distinct types of wind related downdrafts occur when either high pressure or turbulence surrounds the chimney top. High pressure around the chimney can occur when the chimney is located upwind of some obstruction taller than the chimney. As the wind flows toward the obstruction it tends to backup, causing a higher pressure. If this pressure is higher than the pressure in the chimney, it can force smoke down the chimney. On the other hand, if the chimney is located downwind of an obstruction, turbulence will be created as the wind flows over and around the obstruction. If the chimney top is located within the region of turbulence, wind can be forced down the chimney. Wind induced smoking problems usually result in short, "gusty" bursts of smoke coming out of the fireplace or woodstove. A chimney cap will often eliminate problems resulting from wind diverted directly down a chimney flue and is a good thing to try first since chimney caps are reasonably inexpensive. Even if the cap doesn't eliminate the smoking problem it gives you several other benefits. There are special caps which are designed to eliminate downdraft problems. These caps act to create a vacuum within the chimney as the wind blows across the cap. The harder the wind blows, the better the cap works. Note: Do not install attic turbines on a chimney as they are not intended for that purpose. Other solutions to wind induced downdrafts would be to raise the chimney above the region of turbulence if possible, or remove the obstruction causing the turbulence. (Example - Trimming or cutting back of tree branches.)

Fire starting practices - Smoking problems can be caused by the way a fire is started and maintained. Once, while investigating a smoking problem, we discovered that the fire only smoked when the husband built the fire and worked fine if the wife built the fire. A quick review of some basic fire techniques eliminated this problem. For more information see the section on "Starting Your Fire".

Flow reversals - Flow reversals can cause smoking problems that are either constant or erratic. A flow reversal occurs when the pressure surrounding the woodstove or fireplace opening is lower than the pressure in the chimney. This can be caused by several things. Leaks or cracks in the upper portions of the home can allow warm rising air to escape faster than it is replaced in the lower portions of the home. This can lower the pressure downstairs around the fireplace, which could cause smoke flow in the chimney to reverse or come out into the room. This same thing could happen if someone upstairs decided to open a window to let in fresh air. This type of problem can be solved by either closing or sealing cracks in the upper portions or installing an outside air supply for the fireplace or woodstove.

Another type of flow reversal can be created when two chimneys are located close together. Make up air for the chimney in use can be drawn down the unused flue, pulling smoke from the flue in use. The solution would be to close off the unused flue with a tight damper and install an outside air supply for the other.

As erratic smoking problems are the most difficult to diagnose, it will often be necessary to make observations during several different fires. Some things to make note of are:

	Wind direction and velocity
	Outdoor temperature
	Condition of the firewood
	Operation of any competing vents
	When the smoking occurred (beginning of fire, end of fire, all the time)
	Who built the fire
	Who was home at the time

	House stack effect - Stack effect is the tendency of warmer air to rise within a structure, creating lower pressure in the lower areas of the building. Leaky areas near the top of the house increase the stack effect. Stack effect works against chimney draft.
	Competing vents - Competing vents which exhaust air to the outdoors can cause negative pressure in the house. Common examples are the clothes dryer, central vacuum systems, kitchen and bath exhausts, and attic exhausts.
	Other appliances - Air being consumed by one fuel burning appliance may cause negative pressure in the area of another appliance. This is particularly a problem in homes with fireplaces on two different levels. Air exhausted by the upper fireplace may create negative pressure near the lower fireplace. If the chimneys exhaust close to each other the downstairs fireplace may draw in smoke from the upstairs fireplace as well as air.
	Wind loading - Wind creates pressure areas around a building. When wind strikes the building it creates high pressure on the windward side and low pressure on the downwind or leeward side. If the building has leaks on the downwind side it can draw air out of the building, creating negative pressure indoors.

Diagnosing smoking problems can be very challenging and frustrating; but if you succeed, you will be able to enjoy your fireplace or woodstove that much more. Be patient, be observant, and good luck. While we have attempted to outline the more common problems, it would be impossible to outline them all.

When all else fails, you may try installing a powered exhaust fan at the chimney top, but be careful here. If you are depending on a powered exhauster to prevent smoking, imagine what would happen if you had a nice fire going when the power went out. Additionally, a powered exhauster will probably draw all or most of the heat from the fire up the chimney. Powered exhausters should only be used as a last resort and with the advice of a professional sweep. Some building codes do not allow the use of powered exhausters for manually fired appliances.

It would be prudent to call your chimney sweep if you experience a problem. They should be able to professionally diagnose problems and recommend a solution.

We have mentioned the use of an outside air supply several times in the above paragraphs and would like to more fully explain this concept. Outside air supplies are used to eliminate negative pressure near the fuel burning appliance. Negative pressure in the area of the appliance or fireplace opening can also cause many problems for natural draft chimneys. Negative pressure can overcome the natural draft in the chimney. When this happens a flow reversal occurs. When a flow reversal occurs, air from outside is being drawn down the chimney to relieve the negative pressure at the base of the chimney. This negative pressure can be caused by many factors, several of the more common are:

Outside air supplies can be natural or mechanical. Natural draft outside air supplies are undependable. Because wind direction is always changing the air inlet terminal may be surrounded by high pressure one day and work very well while the following day it may be surrounded by low pressure and actually draw air from the house. For this reason we recommend mechanical introduction of make-up (outside) air when the use of outside air is required. The use of mechanical outside air insures that air moves from outdoors to indoors to create the desired effect. A mechanical outside air system can be as simple as a small electric fan connected to a short section of ductwork with a weatherproof hood on the outside wall.