How to Clean a Chimney

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A professional sweep, demonstrating how to clean a chimney, empties creosote and soot from a stovepipe. The wire brush (foreground) is used to loosen waste material.
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Brittle, pyrolized creosote is the fuel for a potential chimney fire

Reprinted by permission from Harrowsmith Magazine,
copyright© 1980


Whether you’re learning how to clean a chimney or relying on the
services of a professional sweep, the burner of wood should be aware of
what is being removed and why it cannot be allowed to
accumulate in a flue.

Creosote is the bane of many contemporary wood heating
systems, and it is produced in quantity by a slow-burning,
smoldering fire … the very sort that makes for those 6-
to 16-hour burns of which many stove owners are so proud.

Creosote is the name of a specific chemical compound (C 8H
10
O 2 ), which, in its commercially available form, is
used to weatherproof railroad ties and other wood. It also
occurs in chimneys used to vent wood smoke, but creosote
from a sweep’s viewpoint is a complex mixture of wood tar,
soot, and other by-products produced by the burning of
wood.

It appears in three primary forms: a thin, watery fluid
that consists of creosote and soot mixed with water which
has condensed in the chimney; a dry, black, gray, or
brownish brittle crust found clinging to the inside of the
flue; and a tar-like, sticky layer which can seriously
clog a chimney. The sooty fluids and the tarlike layer are
practically impossible to remove before they become
pyrolized. These forms of creosote ignite only at extremely
high temperatures. It is the brittle, pyrolized creosote
which is the potential fuel for a flue fire.

Although relatively little research has been done on
creosote formation, it is believed that three main factors
determine the amount that is deposited at a given time or
in a given chimney.

[1] Smoke Density. Creosote builds up when thick smoke,
sometimes known as tar fog , condenses on the
walls of a cool flue. Dense smoke occurs when a fire is
oxygen-starved or when green wood is burned.

Contrary to the popular notion that seasoned hardwood will
not yield creosote, there is ample evidence to show that
even the driest, hardest of woods can cause problems if
improperly burned. An airtight stove kept constantly in a
tightly dampered state burns wood very slowly, producing
heavy smoke and large amounts of incompletely burned wastes no matter what fuel is used.

[2] Stack Temperature. A cold chimney serves as a
distillation tube for warm tar fog, and a slow-burning fire
is accompanied by lower flue temperatures and a
correspondingly high rate of creosote formation. Other
factors that come into play are the type of chimney and its
location. The newer, insulated metal flues are reported by
many sweeps to be much less prone to creosoting problems,
while masonry chimneys that run up an outside wall are
among the worst. The ideal chimney, in most opinions, is
one which is contained within the house and therefore is
much easier to keep warm.

[3] Residence Time. This term refers to the period
necessary for smoke to exit from the stove and chimney. In
a slow-burning fire, smoke rises sluggishly up the chimney,
providing increased exposure to the cooling effect of the
flue walls. Bends and elbows in the stovepipe and flue also
serve to increase the residence time by creating turbulence
and swirls of tar fog.

One environment in which creosote formation is kept to a
minimum is the typical open fireplace. With an unhampered
flow of air, the wood burns rapidly and completely, with
nearly complete combustion of flammable particles in the
smoke and wood gas. The chimney of such a fireplace should
be checked at least once each year, but cleaning may be
necessary only in alternating years.

At the other end of the spectrum we find the efficient
airtight stove, which, if kept in a perpetual slow burn,
may clog a flue with creosote in two weeks or less.

Sweeping Basics

Various folk methods of cleaning chimneys have evolved over
the years, and range from sending the Christmas goose up
the flue prior to beheading it, to pulling a small
evergreen through the pipes. Other techniques involve
drawing a burlap sack filled with sand up (or down) the
flue, or rattling lengths of chain in the chimney to
dislodge creosote and soot deposits (a tactic that may lead
to cracked masonry or flue tiles).

None of the above can compare with the
effectiveness–and ease–of using a modern steel
chimney brush. Several slow passes through the chimney with
one of these bristling devices is usually sufficient to
remove virtually all removable creosote.

Two types are commonly available, the first being a brush
with eyelets at the top and bottom, designed to be pulled
through the chimney on a rope or ropes. A one-man method
using this type of brush involves hanging a weight from the
lower eyelet and lowering the brush down through the
chimney. This method is occasionally used by professional
sweeps when faced with a restricted access chimney, in
which extension poles cannot be employed.

Most sweeps, however, prefer to work with a brush fitted to
the end of a rod. This system allows one to work either
from the rooftop or from inside the house, provided there
is adequate clearance for entry of the rod (the highly
flexible fiberglass lets the extension rod angle rather
sharply into and up a flue).

Assuming that the brush is of the correct size for the
chimney being cleaned (an eight-inch flue calls for an
eight-inch metal brush … but, if you’re using a
flexible plastic brush, it might be wise to use a size
larger than your flue), it is merely worked slowly through
the chimney either from the top down or the bottom
up. As the brush progresses, it should be worked in short,
up-and-down scrubbing strokes, with a pause at the end of
the downward stroke to allow time for loosened debris to
fall. Additional extension rods are screwed on as needed
until the brush emerges from the top of the chimney or
strikes the bottom.

The flex in the rods allows the brush to be worked around
most slight angles and jogs in the flue, but in especially
tortuous chimneys a plumber’s snake may have to be
employed. Most sweeps recommend steel brushes over the
cheaper plastic versions and, although they generally work
with 0.48-inch-diameter fiberglass rods, the homeowner
should be well served with the less expensive 0.35-inch
rods.

Brushes–round, square, and rectangular–can be
ordered to fit most chimneys, but if the exact size needed
is not available, the next size larger should be chosen
(one-quarter to one-half inch of excess bristle length will
cause no major problems).

Steps should be taken to assure that dislodged creosote,
soot, ash, and dust are kept from entering the house. If an
open fireplace is being cleaned from the top, its damper
should be shut tightly, and if this seems insufficient, a
sheet or a piece of paper can be taped over the mouth of
the fireplace. A closed airtight stove will contain any
clouds of dust, but if the stove has been disconnected for
cleaning, the wall exit should be stuffed with paper to
seal the flue from the interior of the house. (It’s also
advisable to wear a piece of cheesecloth–or other
material–over your nose and mouth while working to
reduce the chance of inhaling the carcinogenic dust.)

When one is cleaning from inside the house, it is important
that downdrafts be avoided, or loosened soot will be drawn
back into the house. Cleaning on a cool day usually assures
an upward flow of air, but it may be necessary to open a
first-floor window or two to create the proper flow of air.
(To determine the direction of flow, hold a smoking piece
of paper near the opening of the stove, fireplace, or
flue.)

Having cleaned out the chimney, one should next shovel up
the fallen creosote and soot, and bag it to go to the dump
(it serves no useful purpose around farm or garden). Using
a flashlight, inspect the chimney to assure that the walls
are now clean. They need not be shining bright or spotless,
but any obvious deposits of creosote should have been
removed. In an especially dirty chimney, the falling
creosote occasionally jams in the flue, creating a thick
block of fallen waste. If you are unable to visually check
the flue, insert a fire poker or other tool to assure
yourself that the chimney is open.

To clean the stovepipe connecting a stove to the chimney,
it is often best to take it down and move it outdoors to
avoid creating a mess inside. If the pipe is properly
screwed together at each joint, it may be possible to carry
it out in one piece and to clean it without totally
disassembling the sections. When taking apart any
stovepipe, it is wise to first make a small scratch at each
joint, so that the pieces can be rejoined easily (used
stovepipe sections can stubbornly refuse to be connected in
a new order or alignment). The pipes can be cleaned with a
sweep’s brush or with a simple long-handled wire brush
available from hardware stores.

For the occasional cleaning of the stove or fireplace
itself, a drop-cloth should be spread to protect
surrounding floor space and any hint of a downdraft
avoided. If the chimney is drawing air briskly, the
inevitable dust that arises will be carried up and out the
flue. Professional chimney sweeps make use of an industrial
vacuum to clean stoves and fireplaces, as well as to filter
dust from the air during the indoor cleaning process. The
average home vacuum should not, however, be used for
cleaning up heavy amounts of ash, as the fine particles can
damage the motor bearings. A wire brush, scraper,
flashlight, ash shovel, and whisk broom make it possible to
do a completely acceptable job of cleaning any stove.
Occasionally a fireplace will have an almost inaccessible
smoke ledge or other space where creosote, ash, and soot
have accumulated, and an industrial vacuum (available at
rental outlets) may have to be employed to complete the
job.

How Often?

The frequency of such cleanings, of course, depends on the
type of stove, the wood being burned, and the habits of
those who use the stove. It is generally recommended that
the chimney and stovepipes be checked after two weeks of
using a new stove. If no serious deposits are found, the
stove can be used for another two to four weeks, and
checked again until the owner has a feeling for how
often a sweeping is necessary. The standard guideline
states that any deposit of creosote more than a quarter inch
thick should be cleaned.

The prime place to check is the point where the stovepipe
enters the chimney flue. If the deposits are sticky and
tarlike, the chimney cannot be cleaned without fouling
one’s brush. A hot fire should be built to pyrolize the
creosote, turning it into dry flakes which are easily
swept. If a hard, slaglike deposit is found, it is usually
best left alone. It can be chiseled or hammered out, but
only at the risk of damaging the chimney.

In the wake of a chimney fire, the flue should be swept and
the entire system checked for the presence of leaks or
cracks. This can be done visually and with a smoke test, in
which a wet blanket or burlap sack is used to seal the top
of the chimney once a small but smoky fire has been built
(burning hay, green leaves, grass clippings, or wet
leaves). One person should be on the roof ready to apply
and remove the wet cloth, and another below to watch the
fire and check for smoke leaking out the chimney. While the
flue is well filled with smoke, someone should inspect its
entire length, looking for telltale wisps of escaping
smoke.

If all of this boggles the mind, calling a professional
sweep may be the answer (ignoring the chimney is not). A
good sweep will do more than clean the chimney. He should
be able to tell you whether a stove and chimney are safely
installed and should report on the condition of the
chimney. He should be willing to explain what he’s doing
and may provide the would-be chimney sweep with the
confidence needed to do the job himself.

To hire a sweep, ask for estimates from several
professionals in the area … if they exist. Ask how much
experience the sweep has had, whether he carries insurance
(a good sweep does), and whether he guarantees that the job
will be done without mess. A trail of soot across a rug is
the footprint of a careless and/or incompetent sweep.

Sweep Lore

Orrin C. Kerr–a professional part time sweep–is
bearded, long-haired, and as tall and wide as a lot of
chimneys. “Traditionally the sweep is considered a creature
of good luck,” said Kerr, as I watched him work inside a
very handsome Kingston home near Lake Ontario. “In Germany,
custom has it that if you touch the sweep’s brushes, good
luck will rub off on you. In England and the Scandinavian
countries, the luck is picked up by shaking the sweep’s
hand, or–in the case of ladies–by kissing him … this is the part of the job I like best. Another Olde
English custom is to invite a chimney sweep to the wedding
to kiss the bride and to shake the groom’s hand, insuring a
lucky marriage.”

Not being superstitious, yet at the same time not wishing
to take any unnecessary chances, I held out my hand. We
shook. As I left, Kerr turned his back to return to his
task and there was my opportunity: On the way to the door I sneaked a quick feel of the steel bristles …
just in case.