Cures For Backpuffing

Wood Stove Installation and Safety Thomas J. Karsky Wood as a Fuel No. 9 UI Extension Forestry Information Series House fires related to the use of wood heat are on the
increase in Idaho. Many of these fires would not have
occurred if stoves were installed properly. The hazards
of heating with a wood stove include fires started by
heat radiated or conducted by the stove, stove pipe or
chimney to walls, floors and other combustible mate-
rial; fires started by sparks and glowing coals falling
out of front loading stoves when opened, and fires
started by flames leaking out of faulty chimneys or
burning or glowing material coming out of the top of
the chimney. A chimney flow reversal is also possible,
leading to either flames or smoke coming out of the
stove’s air inlets. Before installing, seek advice from
your stove dealer, your local building
inspector, or fire department. And
check with your insurance agent. The
insurance company may have its own
specifications for installation and,
since you are changing the method of
heating your home, your agent must
be notified in order to maintain fire
insurance coverage on your home. The National Fire Protection Asso-
ciation (NFPA) has developed
standards for clearances from walls
and ceilings that are the basis for
many local building codes (Table 1).
All combustible materials, wood-
work, unprotected walls, furniture,
firewood, etc., should be no closer
than 36 inches to a wood stove. A
stove pipe should not be closer than
18 inches to an unprotected ceiling.
These distances are important be- cause wood that is continually reheated will ignite at
much lower temperatures than fresh wood. A new
wall will start to burn at between 500 and 700°F. If
this wall is continually heated over a period of time the
wood will dry and eventually may start to char be-
cause of radiant heat. The ignition temperature can
drop to 200 to 250°F. For this reason an improper
wood stove installation becomes a potential time
bomb. Fig. 1 shows a proper wood stove installation. Wall Protection. A simple test will tell if you have
enough clearance to an unprotected wall. Place your
hand on the closest surface. If you can keep your hand
there comfortably while the stove is operating, the Crimped end of
connector pipe
should point
downward. Creosote then
runs down
inside of pipe. Securely fasten
joint with sheet
metal screws. Joints lapped
and fastened
with screws Damper Joint Joint Proper Thimble Radiant type stove Floor protection pad Figure 1. A wood stove needs to be installed properly. See Table 1 for “A”,
the clearance from walls and ceilings. location passes the test. If not, you need additional
protection. Spacing asbestos millboard or 28 gauge steel 1-inch
away from the wall allows you to reduce the distance
a stove can be placed from the wall (Figure 2). These
materials absorb heat radiated from the stove and the
spacing lets air circulate behind the panel. The spacers
should be made of non-combustible material. A 1- to
1 ½- inch gap between the panel and floor and at the
top of the panel is necessary to provide proper air
flow. Asbestos millboard is different from asbestos cement
board or asbestos transite board. Cement board or
transite boards are both hard, slate-like panel materi-
als designed as a flame barrier. They provide little in
terms of heat resistance and will conduct heat to any
combustible surface to which they are attached.
Asbestos millboard is a soft, lightweight panel product
that can be easily cut with a saw or utility knife. WARNING: Inhaling asbestos fibers may be harmful. The
effect of long term exposure is not completely known.
However, you should wear a protective mask when cutting
asbestos products. Since brick and stone are good conductors of heat,
they offer little protection if placed against a combus-
tible wall or have wood studs behind them. To be
effective, bricks must be placed out at least 1-inch
from the wall with air gaps by using half bricks on the
top and bottom row. Stoves can be placed as close as
12 inches from the brick facing if you provide an air
space behind the brick. An inexpensive and temporary way to protect a wall if
you already have a stove installed closer than 36
inches to an unprotected wall is to provide a baffle.
This baffle could be sheet metal, hardware cloth or
cement board hung on metal brackets approximately 4
inches behind the stove. Floor Protection. All floors on which stoves are
installed, except concrete, must be protected from
both heat of the fire and hot coals falling out when fuel
is added. Metal with asbestos backing and asbestos
millboard are non-combustible materials used for floor
protection. Fireproof clay tile, slate, brick, colored
pebbles and marble chips can be used alone only if
they are mortared in place with no gaps. If they are Stove type Stove Type of protection Radiant Circulating pipe (inches) (inches) (inches) None 36 12 18 ¼-inch asbestos
millboard nailed to
wall 36 12 18 ¼-inch asbestos
millboard spaced out
1-inch 18 6 12 28 gauge sheet metal
on ¼-inch asbestos
millboard nailed to
wall 18 6 12 28 gauge sheet metal
spaced out 1-inch 12 4 9 28 gauge sheet metal
on 1/8-inch asbestos
millboard spaced out
1-inch 12 4 9 Table 1. NFPA recommended minimum* clearances from
combustible walls ** and ceilings. * Note these are minimum requirements. If you have an extremely
hot or a large stove with a considerable amount of surface area
facing the wall, you may want to increase these distances since
more heat will be radiated from the stove.
** Combustible walls are unprotected walls that contain wood,
which include dry wall (gypsum board) on wood studs or plaster on
wood backing. Heat is transmitted directly through these materials
to the wood.
NOTE - Use NFPA approved material when substituting for
asbestos millboard. Figure 2. By providing wall protection, you can place a
wood stove closer to the wall. Leave at least a 1-inch gap
between the protection material and the wall to provide
ventilation. Aluminum foil attached Wall shielding to wall for added protection Asbestos millboard, 24
gauge sheet metal, or
face brick Space out at
least 1-inch
from wall. 1-inch
spacers cut
from a
length of
1/4-inch pipe 1-inch off
of floor I</b>f face brick is used
leave gaps in mortar for
ventilation not mortared or have gaps, then metal or asbestos
millboard must be installed between them and a wood
floor. A 2-inch layer of ashes or sand or bricks laid in
the bottom of the stove helps to insulate the bottom of
the stove and protect the floor. In general, 18 inches is
enough clearance to protect the floor if it is covered by
non-flammable material, such as a sheet of 24 gauge
metal or brick or fireproof clay tile. If the stove legs
are from 6 to 18 inches long, 24 gauge sheet metal laid
over a ¼-inch sheet of asbestos millboard is needed.
Legs of 6 inches or less require 2 to 4 inches of hollow
masonry laid to provide air circulation and covered by
24 gauge sheet metal. If the stove has no legs, provide
a sturdy support to allow air circulation under the
stove. The floor protection should extend at least 12 inches
beyond the sides and rear of the stove, and at least 18
inches beyond the stove front, to protect against falling
embers and for loading wood or removing ashes. Before installing heavy protection materials such as
brick, check the floor to make sure it can handle the
increased weight. You may want to reinforce the joists
under the floor. Consult a carpenter if necessary. Stove Pipe. The stove pipe or chimney connector
runs from the stove to the chimney. Many fires associ-
ated with wood stoves are caused by unsafe stove
pipe installation. A safe installation requires proper
material, construction clearances and does provide
proper draft. A 24 gauge or thicker metal is recommended; lower
gauge numbers indicate thicker metal. This gauge will
provide better protection in the event of a chimney fire
and will also resist chemical corrosion longer. Most
stoves use either a 6- or 8-inch stove pipe. Using
stove pipe that is smaller in diameter than the fire box
outlet will reduce combustion efficiency and may cause
improper draft. Keep the connector pipe as short as possible. It
should not be longer than 75% of the vertical chimney
height above the flue inlet (where the connector pipe
enters the chimney). The maximum length is 10 feet. If
the pipe runs horizontally, it should have a rise of at
least ¼-inch per linear foot from the elbow or stove
outlet to the chimney inlet. Use 45° angles to create an
upward slope in the flue connector pipe. Try to have
no more than one right angle turn between the stove
and the chimney. Additional right angle bends can
cause soot and creosote to collect in the smoke pipe
or chimney, blocking flue gas flow and increasing the
danger of a fire. The connecter pipe diameter should be as large as the
flue collar (where the connector pipe joins the stove).
When joining sections of the pipe, overlap the joints at
least 2 inches, with the crimped (male) end pointing
down to prevent creosote drip or leak. Many house
fires have resulted from stove pipe joints vibrating
apart during a chimney fire. Secure each joint with at
least 3 sheet metal screws. A fireproof sealant may be
used in addition (Figure 1). Clearances from a connector pipe must be 3 times the
pipe diameter (a 6-inch pipe needs 18 inches clear-
ance) unless the wall is protected (Figure 3). You
should not pass a stove pipe through a combustible
wall but if a stove pipe must pass through an interior
combustible wall in order to hook up with a chimney
flue, there are 4 ways to do this safely: • Use an UL “all fuel” thimble extending through the wall, with a wall hole 4 inches larger than the
thimble diameter (Figure 4.1). This permits the
placement of an insulating material such as fiber-
glass or rock wool between the thimble and the
wooden framing of the wall. Figure 3. Stove pipe clearance with ceiling or wall
protection. “A”- see Table 1 for this distance; “B”-
minimum distance here is 3 times the diameter of smoke Asbestos
millboard or
sheet metal Smoke pipe Space of at least 1-inch Ceiling or wall Spacers • Use a ventilated thimble that is at least 3 times larger than the stove pipe (Figure 4.2). For a 6-
inch stovepipe, use a thimble that is 18 inches in
diameter. This type of thimble is not readily avail-
able but can be fabricated by a sheet metal shop.
Ventilation through this thimble is an essential
aspect of its design; the ventilating holes on either
side must not be blocked. • Use a fire clay thimble surrounded by 8 inches of brick work or non-combustible materials within 18
inches on all sides of the stove pipe (Figure 4.3).
Material for closing this opening must be non-
combustible, with insulating properties. • Use no thimble, but remove all combustable material within 18 inches of the stove pipe. Mate-
rial for closing this openingmust be noncombustible
with insulating properties. When the wall is cut between supporting studs for the
thimble, inspect the opening to make sure there are no
electrical wires or conduit in the space between
adjoining wall studs. Heat from the stove pipe may be
sufficient to melt the insulation on wire in this space,
causing an electrical fire. Stove pipe should not pass through ceilings, closets or
outside a building. Holes in the ceiling (including hot air
registers) permit fires through upper floors. A closet
fire could smolder and spread undiscovered. Running a stove pipe out a window and up the outside
wall of the house is a dangerous practice, because the
pipe cools faster than a prefabricated metal chimney
and allows a rapid creosote buildup. Wood burners
sometimes recommend long spans of single thickness
stove pipe as a heating device. This idea had some
merit when used with old fashioned inefficient stoves
where much of the heat went up the pipe. Today’s
airtight stoves are more efficient and this practice may
cause rapid creosote buildup. Some stove installations require a damper either built
into the stove or in the pipe near the stove to control Stove pipe Figure 4. 2. Ventilated metal thimble. Typical framing Figure 4.1. Prefabricated insulated chimney used as a
thimble. Thimble
supported
by
brackets Stove pipe Typical framing Must maintain 2-inch
clearance to
combustible surfaces Insulated chimney Outside diameter is 3
times diameter of the
stove pipe. Stove pipe Figure 4.3. Fire clay thimble Typical framing 8” Metal
pipe
sleeve 8 inches of non- combustible material - brick or concrete leave home with a roaring fire going in the stove. • Place ashes in a lidded metal container. Because they might be hot, clean up any ashes or cinders
that spill out on the floor. • Wear gloves when handling rough or splintery chunks of wood. If they are heavy, take care not to
strain yourself or drop them on your foot. • You can burn wood in a coal stove, but you shouldn’t burn coal in a wood stove unless it is
lined and designed for it. When you add coal to an
approved stove, keep the stove pipe damper open
until the fuel is burning well to avoid a potentially
explosive buildup of gases from the coal. Heavily
laden coal buckets can also cause strains and other
mishaps if they are not handled properly. • Take down the stove pipe at least once or twice during the heating season and clean out the soot.
Removing the accumulated soot saves fuel, in-
creases heat and minimizes the danger of fire. • If you have yet to equip your house with fire warning devices, be sure to do so when you install
a stove. Install a smoke detector in an adjacent
room to avoid false alarms when you recharge the
stove or from backpuffing due to wind. • Before opening the fire box to add fuel or just to look at the fire, always open the stove pipe
damper first. This allows gases to escape up the
chimney and eliminates the possibility of “flare up”
when air suddenly comes in through the door. • With today’s tightly-constructed houses, there may not be sufficient air leakage for efficient stove
operation. By providing an outside air inlet, you
prevent the possibility of a reverse draft which may
suck carbon monoxide fumes form combustion-
type (natural gas, etc.) appliances and discharge
them into the living area. About the Author: Thomas J. Karsky is an Extension Farm
Safety Specialist and Professor at the University of Idaho. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws. This information first appeared as CIS 485 and was part of the
Wood as a Fuel Series. draft and loss of volatile gases. Check the recommen-
dation of the stove manufacturer. When connecting the stove pipe to the chimney make
sure the fitting is snug at the flue inlet. Use the proper
thimble. The pipe must not project into the flue itself,
since it would hamper draft. Long stove pipes and those with restrictions should be
cleaned frequently to prevent creosote buildup and
possible chimney fires the entire length of the stove
pipe must be easily inspected, firmly fastened at the
joints and kept free of all combustible materials. Tap
your pipe to check its condition several times during
the heating season and before starting the stove each
year. Additional Precautions. • Chimney and chimney connectors require regular inspection and cleaning to remain reasonably safe.
Chimney fires are a common problem. There are
several factors that can cause a chimney fire. • Furniture, wood, newspapers, matches, etc., can ignite if placed or left too close to a stove. These
materials must be kept at least 36 inches away
from the stove. • Stove surfaces can become as hot as 800°F. At this temperature, combustible material can ignite
and plastic material will melt. Be careful when
drying clothing, making sure that nothing is dangling
too near. Also, remove any slipping or tripping
hazards near the stove to reduce the risk of falling
against it and perhaps suffering a severe burn.
Small children must be taught to stay away from
the stove. You should erect some kind of barricade
around the stove if you have crawling tots who are
too young to be verbally warned. • Never use kerosene or charcoal lighter fluids to start a fire. Also, do no t burn trash in your stove.
These materials lead to hot uncontrollable fires and
may cause a chimney fire. • Keep the fire controlled with the dampers. Do not let it get roaring hot. A fire properly controlled is
safer and more efficient. • If you want to keep your fire alive all night or when you are away from the house, band the fire with
ashes or damper it way down. Do not retire or