Solving Problems in Existing Chimneys

Existing chimneys can have several different faulty situations. Some affect safety, some affect performance (smoking), and some can affect both.

Some chimneys might even have more than one of these problems.

We should also mention that there are literally hundreds of variables that can affect how a chimney functions. It is SUCH a complex problem that analytical science really cannot explain much of why a chimney might work properly or why it might smoke. Over many centuries of people building chimneys and fireplaces, experience has established some things that generally seem to work and some things that don't.

Scientific analysis has taken this information and created formulas that relate chimney height to "mass flow rate" and other such relationships. Yet, many of the subjects we will address here are primarily based on history's past successes and failures regarding chimneys.

After we discuss the various problems, we will present various solutions that might improve or solve the problem(s).

Size

Probably the most common reason that we see for chimneys not "drawing" properly (sometimes spilling smoke into the room) has to do with the size of the chimney. In general, there is a guideline where you should measure the area of the fireplace opening to the room (say 32" wide by 24" high or 768 square inches) and take 1/10 of that (in this case 76.8 square inches). The SMALLEST area of the chimney smoke path should have at least this area. For example, a 10" round Class-A Chimney has an area of 78.5 square inches, so it should work fine. An 8" Class-A Chimney, though, would have an area of about 50 square inches, and all the smoke may not always go up the chimney, and it would be likely to smoke. An 8" by 12" tile-lined masonry chimney would have an area of 96 square inches and would be fine, while an 8" by 8" would only have 64 square inches and would probably smoke.

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Height

The height of a chimney is related to the amount of draft that it can develop. The taller the chimney (everything else being identical) the better a chimney can draw. But it's not a simple proportional relationship. For example, doubling the height of a chimney would not double the draft. Actually, it would require a chimney about four times as tall to double the draft.

Sometimes, when a chimney is SLIGHTLY to small in area, adding to the height of a chimney can solve an occasional smoking problem.

Location

Surprisingly, the location of a chimney can tremendously affect how well it draws. There are two separate subjects involved.

Interior versus Exterior Chimneys

A chimney in the center of a house is kept warm on all four sides by the warmth of the house. This situation generally enables very good and consistent draft in the chimney. An exterior chimney, one on an outside wall, has at least one surface that is exposed to the cold outside air. This has the effect of chilling the chimney and making it harder for the chimney structure to stay warm enough to provide consistent draft inside the chimney. Most outside chimneys are actually physically placed outside the wall of the house (to take up less floor space in the house) which cause three sides of the chimney to be exposed to the cold outside air.

Science isn't much good here, but it seems that many people think that an interior chimney draws about 20% better than exterior chimneys. In the case of a chimney that is close to the line of being under-sized (see above) that can make the difference between smoking or not. It also makes a major difference in first getting a fire started and getting the chimney to begin to draw.

If it is possible to get into the space around the body of the fireplace and chimney, it might be possible to install non-combustible fiberglass insulation in such a hollow chase. This should only be considered after making sure that the chimney manufacturer's clearances and installation requirements, and those of the fireplace manufacturer, are followed. A case might exist where adding such insulation might clog some necessary opening in a fireplace or chimney or otherwise cause it to dangerously overheat, so always check required clearances first.

For Exterior Chimneys, Which Wall?

Another surprising thing is that which side of a house a fireplace is installed on can greatly affect how it works! In much of the United States, winter winds generally come from the west (prevailing westerlies) and north. Say you have a fireplace on the west side of a building, and say the chimney is not quite as tall as the highest part of the building. Winds coming from the west had just easily flown by, before the building was there. With the building there, the winds run into it, and then must find some way of getting by it. "Running into it" really means that an area of high air pressure exists just outside that wall of the building. This increased pressure is the reason the air turns sideways to go around or over the building, to eventually get past the building. But what if you had a window slightly open on that wall and on the opposite wall of the house? The high pressure would push air in through that upwind window (actually even faster than the wind was blowing), through the house, and out the window on the downwind side of the house. Well, a chimney on an upwind wall of a building, particularly a chimney that was not at least several feet taller than the whole building, can act like that open window. The high pressure air actually gets forced down the chimney, to get into the house, then it finds leaks in the downwind wall to leak out of. Such a fireplace would often have a natural characteristic of having a negative draft! Even without any fire in the fireplace, the smell of the soot from the chimney would be smelled in the house.

Trying to get a fireplace to overcome this and cause smoke to actually go UP the chimney can be very inconsistent for this situation. A roaring fire will generally overcome it and the chimney will draw correctly, but a fire that is dying down might not be able to overcome the natural downdraft characteristic.

Interestingly, the same fireplace/chimney on the downwind side of the house would work excellently! The natural characteristic would be to leak house air outward, in other words up the chimney. Even without a fire, this chimney would virtually always have a positive draft!

Condition

It seems obvious, but still, bird's nests in chimneys are the cause of many complaints of chimney performance. Also, in older chimneys, bricks that have gotten loose and fallen into the inside of the chimney can block some of the necessary open area. Creosote accumulations on the inside walls of a chimney (generally occur only when an air-tight woodburner uses the chimney) can reduce the open dimensions of the chimney so the area is too small to allow all the smoke to go up (see the SIZE discussion above).

In addition, thick accumulations of creosote represent an INCREDIBLE danger to the house and occupants. During the 1980s, when airtight woodburners suddenly became popular, many thousands of chimney fires occurred and many thousands of houses burned down as a result. You should ALWAYS be aware of the condition inside your chimney, ESPECIALLY if you are using an air-tight woodstove. (Eventually, the Government mandated rules for manufacturers of air-tight products where they had to instruct owners of their products to physically CHECK the chimney every week and have it CLEANED every three weeks! Wow!)

Another condition of a chimney that requires attention generally only applies if the chimney is more than about 20 years old. Prior to that, most chimneys were UNLINED, which means they didn't have a clay tile liner and the bricks of the chimney were exposed directly to the smoke of the fire. This is dangerous for quite a few reasons. It turns out that the alternating heating and cooling of a chimney always causes the mortar between the bricks to eventually break loose. This makes the bricks come loose, which means that the smoke from inside the chimney now has ways of getting through the walls of the chimney. Only bad could come of that! If the result is excessive heat getting to nearby wood (which has become extremely dry over the years), the house could catch fire. If the woodburner was an air-tight, it naturally creates a certain amount of carbon monoxide. If that get out into the house, it's dangerous to the occupants. Again, for an air-tight wood-burner, if a lot of liquid creosote forms in the chimney (and it sometimes will), some could leak out to accumulate outside the chimney. That creosote WILL eventually catch fire, and nothing could then stop the house from burning down. If a very old un-lined chimney had ever been used for a gas burning furnace or stove, it turns out that the gases produced gradually disintegrate the bricks themselves! I first realized this when I needed to remove an old chimney to replace it with a metal one. I had expected to have a lot of strenuous work ahead of me in dismantling the existing brick unlined chimney, but the mortar joints were almost all loose and many of the bricks could be disintegrated by hand. I seldom needed even a hammer or crowbar! I was amazed that the chimney had never fallen over because nothing seemed to be holding it together. Imagine having a creosote chimney fire in such a chimney!

Get the point? If the chimney is not lined now, LINE it! We have a separate page that describes several methods of Re-lining an existing Chimney.

Other Causes of Problems

The subjects described above represent the primary reasons for problems in chimney and fireplace operation. However, there are dozens of lesser causes that could also apply. If one of the situations described above causes a borderline condition, an additional minor cause could make the difference between a fireplace sometimes drawing properly and sometimes smoking. So, for intermittent problems, the odds are that one of the conditions above is probably near the limit of acceptable condition, and that some lesser cause is then seeming to sometimes cause it to smoke. You might be able to solve the basic problem or the minor problem, either of which might eliminate the smoking.

Such minor conditions include: Very tall trees just upwind (to the west of) the house; A chimney on the west or north side of a building that does not extend at least 3 feet above the highest point of the building; A house that has a "lot" of leaks on the downwind side of the hosue and few on the upwind side, which would accentuate that downdraft characteristic described above; Those "leaks" could be bathroom powered ventilators, kitchen fans, clothes drier vents, etc. You should be able to figure out similar specific complications that might be affecting the performance of your chimney/fireplace. As we said, there are hundreds of things that can have such an effect.

Possible Solutions

Some suggestions were mentioned in the various preceding discussions. Here are several more general solutions, particularly useful if you are having trouble figuring out just what is causing the problem. We will start with the simplest and least expensive and proceed to the more involved and expensive solutions that might be necessary for more serious problems of smoking.