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).
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.
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.
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
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.
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.
- If the chimney has a physical flaw, FIX IT! Loose bricks, loose
mortar, extensive creosote accumulation, being unlined, are all very
unsafe and unacceptable. A local mason or chimner person should be
able to do what is needed.
- For occasional smoking, consider getting a set of fireplace glass
doors. We happen to have about 2,200
different appearances available!
- For fairly regular, minor smoking, a (decorative?) strip of metal
across the top of the opening of the fireplace might do the job. You're
actually reducing the effective area of the opening, so you're reducing
the necessary required chimney cross-sectional area as a result (see SIZE
discussion above). The width of the strip could be determined by trial
and error. Sometimes, just a one or two inch wide strip solves a minor
- For irregular smoking, that only seems to occur when wind comes
from one specific direction, consider experimenting at making the
top of the chimney "directional". If it is a masonry
chimney, with a flat stone covering it, I'd suggest temporarily
placing some loose bricks to block off the side of the opening
that faces upwind. The idea is that when wind would come from
that direction, a "venturi" effect is developed where
a low pressure is created just downwind of the obstruction you
made, in other words, in the top of the chimney. This can have the
effect of actually "sucking" smoke up the chimney!
For moderately serious smoking, particularly when it is windy or
gusting outside, a non-directional version of the effort described
above, is available. Whichever direction the wind would come from,
a venturi action would be developed, which increases the draw
of the chimney. These are available for any size metal (round)
or tile (rectangular) chimney. For normal-sized chimneys, they
generally run about $130-$150.
For truly terrible smoking problems, which cannot be solved by any of
the foregoing methods, there is a "brute-force" solution.
This is a motorized draft-inducer. Its paddle-wheel blades physically
push the smoke up the chimney. These are fairly expensive (around $300
to $600 for normal sized chimneys) and the side of the chimney must be
cut open to install them, so installation is far more involved than
any of the other approaches described here. But, since it is motorized,
it is certain to force smoke up the chimney.
There is only ONE situation where even this wouldn't work. Imagine
a brand new house that is absolutely drum-tight. No air whatever could
leak into or out of the house. So, you turn on the draft inducer, and it
starts to try to push smoke up the chimney. But, in order for it to
be able to do this, some air from somewhere would have to replace the
air that is being shot up out of the house! If no air source exists,
the draft inducer would have minimal effect at pushing smoke up and
out of the chimney!
A Different Problem, a Broken Damper
Sometimes, an existing fireplace works fine, but the lever damper up in
the throat of the chimney is broken or stuck open. When there is no
fire, house air is then lost up the chimney. Sometimes, it is possible to
repair or replace that damper, but sometimes such replacement would be
horrendously expensive due to labor costs. For these situations, there
is a type of damper that gets installed at the very top of the chimney
(called a chimney-top damper!). Most operate like a kitchen garbage can
lid, that flips up to open. In this case, a stainless steel cable
goes down through the inside of the chimney to allow a control handle
to be along the wall of the fireplace. You can open or close that
chimney top damper while tending the fire. These are generally compatible
with many chimney caps and they are available in sizes for most chimneys.
For normal-sized chimneys, they cost around $130-$150.
If you are going to need Class-A Chimney materials, our Comparison Shopper
page is at: /info/chim1.html
It calculates the parts needed and all of their individual costs for
ALL of the brands available in North America, and then applies the
discount that we can provide for you, giving you the grand total cost
for a whole Class-A Chimney system from each of the different manufacturers.
The JUCA Home Page is at: