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CONTRAFLOW MASONRY HEATERS/OVENS

For thousands of years, fire has played an important role in
northern hemisphere cultures, providing heat, light, and cooking.
Much of the heat from an open fire is lost, either to deep sky
radiation, up a smokehole, chimney, or through air infiltration
in a drafty dwelling. Huge quantities of fuel wood and large
tracts of forest are wasted because of these heat losses.

The masonry heater, developed out of necessity to cope with
the frigid northern winters of Finland and other northern
countries, is a vast improvement over open fires and fireplaces.
For generations, masonry wood-fired heaters have been used by
northern cultures to provide an even and steady warmth. Masonry
cookstoves, bake ovens, space heaters, sleeping platforms and
heat-storage fireplaces all spring from this ancient tradition.
Masonry heaters combine a fast, clean, fuel-efficient burn with a
large passive thermal storage mass to offer an aesthetically
pleasing, and physically comfortable form of heating. A modern,
carefully built and centrally located masonry heater fueled once
or twice a day for 1-2 hours per burn can easily serve as the
primary heat source in a new, well-insulated single family-home.

The rapid combustion of wood fuel insures that maximum heat
is generated while minimizing the emission of particulates to the
atmosphere. This is of critical importance in places where air
quality suffers due to excessive particulates during inversion
conditions. Thankfully, sulphur dioxide and nitrogen oxide
emissions are practically negligible in wood combustion. Masonry
heating systems definitely point the way towards a renewable and
efficient energy future for wood burning, with virtually no
negative impacts on air quality.

1. Advantages

• The masonry heater is inexpensive if built by the owner
  (no sending money to a distant stove manufacturer). The cost
  depends materials used (e.g. scrap metal and recycled brick,
  local native stone, or Cinva-ram earth bricks) but it can be as
  low as $800 (mainly brick and mortar cost). Considerable savings
  are also realized in lower wood usage.
• The heater produces virtually no tar, soot or creosote
  build-up, which means a greatly reduced risk of chimney fires.
• Wood saved per year may amount to a cord or more per
  building heated, thus conserving forest resources as well as
  saving money for the homeowner.
• The high efficiency of the heater allows for burning of
  trash paper like gummed envelopes or soiled paper which cannot
  normally be recycled.
• The design of the stove can include a bake oven for all
  types of food: Biscuits, breads, cakes, casseroles, pizzas, etc.
• The design of the stove can also include a wood cook
  stove for meal preparation on the range top.
• The need for non-renewable fuels (natural gas, propane,or
  nuclear - or coal-fired electricity) can be eliminated.
• The low surface temperatures of the heater are much safer
  for children than the extremely hot surfaces of steel
  manufactured wood or coal burning stoves.
• The masonry heater exemplifies the positive and efficient
  use of locally available materials -- one of the key features of
  any bioregionally appropriate technology.
• The heater can serve as thermal storage for passive solar
  designs when located near south-facing windows, clerestories or
  greenhouse glazing.
• A hot water heating system can be included in the
  construction of the unit if desired.
• The heaters can be extraordinarily beautiful in any
  setting, especially if glass doors are included in the design to
  allow viewing the fire.

2. The Contraflow Principle

The secret to the efficiency and clean burning of the
masonry fireplaces is in the design. Firewood (split, dried
hardwood 4 inches in diameter is preferred) is burned in the
firebox. Directly above the firebox is a secondary combustion
chamber where gases, which contain up to two-thirds of the
potential heat energy in the wood, are burned. Combustion
temperatures of 1650-1800F ensure a complete burn without
formation of creosote, a major fire hazard associated with air-
tight stoves.

While masonry heaters are not common in the United States,
they have been used for centuries in Europe. The Finnish
contraflow design is distinctive in that it uses a secondary
combustion chamber along with two vertical baffles which ensure
that the heat from combustion is absorbed by the masonry of the
heater and is radiated to the living space. One damper is
located at the top of the secondary combustion chamber to
facilitate drawing smoke when the fire is initially lit, then is
closed once the wood has finished burning in order to keep any
heat from escaping up the chimney.

The contraflow principle is demonstrated by the downward
flow of heat in the unit from the secondary combustion chamber to
the flue located below the level of the firebox. This downward
heat flow is opposite the movement of cooler currents of air on
the floor in living space which, when reaching the masonry
heater, begin an upward convective flow. In this way, hot spots
immediately adjacent to the heater are eliminated, and a more
even and complete heating of the entire living space is achieved.

3. Variations in Heater Design

A bake oven for biscuits, breads, cakes, casseroles, pizzas,
etc. can be easily included into the design of the stove. The
bake oven is typically located just above the firebox, with the
base of the oven constructed out of firebrick for even heat
distribution. Channels surround the oven to allow heat and gases
to circulate up to the secondary combustion chamber where the
gases are consumed. The heat then flows downward, down the
baffles to the plenum beneath the firebox and out to the chimney
flue.

Another practical feature that can be included into the
design of the masonry heater is a wood cookstove for range top
meal preparation. This can be constructed alongside the masonry
heater with a separate firebox (and oven if desired) and stove
top for cooking. Using masonry construction around the firebox
and oven beneath the stove top helps to dissipate heat and
eliminate any danger of injury from contact with hot surfaces.
Refrigerators and freezers should be kept away from the cook
stove's warmth.

An integrated loop of pipe can be installed into the heater
during construction to provide for domestic hot water needs.
Used in combination with and as a back-up for a solar hot water
system, no other energy sources should be required for water
heating.

4. Masonry Heater Construction

• Build steel-reinforced concrete pad for foundation of
  heater. Foundation footings for the heater should be dug to
  below frostline on undisturbed earth. A twelve-inch thick
  (minimum) concrete pad should be poured with two grid sections of
  one-half inch diameter, reinforcing steel; one 12" x 12" grid in
  the bottom one-third of the foundation, and the other 12" X 12"
  grid in the top third of the pad. The foundation should extend
  at least six inches beyond the perimeter of the masonry heater.
• Draft design, including bake oven, stove, top and/or
  water heating apparatus, and assemble list of building materials.
• Gather building materials.
• Build the heater one brick course at a time, paying close
  attention to design details. A copy of the overall design with
  several details of variations with notes may be obtained from
  Paul Gallimore, Long Branch Environmental Education Center, P.O.
  Box 369, Leicester, NC 28748, Price $25.00. Special attention
  should be paid to details involving the use of refractory mortars
  used to bond firebricks.

5. Notes on Heater Replacement

If possible, select a central location for the heater.
Carefully consider the location of the heater in the proposed
building. In some cases, existing venting and chimney
arrangements limit site options. Try to select a location that
is well ventilated, as near as possible to the center of the
structure being heated, where there is enough space for the
massive heater, and within reasonable access to doors for
bringing in firewood. Be sure that a clear vertical path for the
chimney exists. The major problem in locating a heater on an
outside wall is that convective air flows are restricted in the
living space and heat distribution to more distant locations in
the house is uneven. The closer the heater is located to the
center of the structure, the more even heat transfer to the
entire space will be.

• Assemble metal parts for dampers, door, ash clean-out,
  grate, etc. Some welding may be required.
• After constructing the main body of heater with fire
  brick where necessary, apply veneer (rock, Cinva-ram earth
  bricks, bricks, soapstone, flagstones, etc.). The exterior of
  the heater will be the final and lasting tribute to the builder's
  care and sense of aesthetics.
• Construct a chimney for the unit using, at a minimum, an
  eight-by-eight inch clay chimney flue liner. A slide damper to
  shut off any escape of heat should be included in the
  construction of the chimney.
• Allow sufficient time for the masonry to cure by lighting
  only small "setting fires" with just ten pounds of kindling at a
  time. These small fires can be lit as many as three times per
  twenty-four hour period for a week or until the heater becomes
  quite dry. The purpose of this process is to ensure that no
  cracking or thermal stressing will occur. Firewood should be
  stacked log cabin fashion to allow the best circulation of air
  for combustion.
• An additional feature that should be considered is the
  use of outside air for combustion. A screened, six-inch diameter
  pipe from the outside should be brought in to a closable floor
  register in front of the heater. This will avoid the problem of
  consuming already warmed interior air for the combustion process
  and will also diminish air infiltration around doors, windows,
  etc. The register can be closed when the heater is not in use to
  prevent unwanted infiltration.

6. Possible Difficulties

• Insufficient knowledge of the design and
  construction procedure.
• Insufficient space to place the heater.
• Absence of good convection current and access to
  outside air.
• Insecure ground and lack of a deep foundation.
• Overly rapid curing
• Use of the wrong cement products.
• Failure to install proper expansion joints which
  results in cracking of the heater.
• Poor chimney construction.
• Burning green wood.
• Failure to remove dust which accumulates in the
  building of the heater.
• Difficulty in obtaining metal heater accessories.

The Maine Wood Heat Company can provide many of the
materials that are not available locally: Albert Barden, Maine
Wood Heat Company, Box 640 RFD 1, Norridgwock, Maine 04957
(207) 696-5442.

7. Notes on Wood

The moisture content of freshly cut wood is about 50%. If
stored for a year this can drop to 25%. In a dry and ventilated
storage area, moisture content can be reduced to 20% -- 15% or
even 10% if stored in a warm indoor space. About 20-30 pounds of
dried wood (20% or less moisture content) is recommended for each
firing cycle. The number of cycles depends on the severity of
cold weather and building size. Generally, only one or two
firings per day is needed to maintain a comfortable temperature.

Note: Under no conditions should Chromate-copper, arsenate
(CCR) green pressure-treated wood be burned. It is extremely
toxic when combusted in the home. Bronchitis, pneumonia, ear
infections, blackouts, gastrointestinal ills, nosebleeds, muscle
cramps, dermatitis over the arms, legs and soles of the feet, and
other ailments may result.

The user of the fireplace comes to understand that fire,
like the sun is a life-sustaining and renewing force, that the
real purpose of a fireplace is to renew the energy of those who
gather around it.

FINNISH FIREPLACES: The Hearth of
the Home, Albert Barden - Heikki Hyyti„nen

REFERENCES

Barden, Albert. Finnish Fireplaces. Norridgewock, ME: Main
Wood Heat, 1988. ($25 postpaid from: RFD 1 Box 640,
Norridgewock, ME 04957)

Lyle, David. Masonry Stoves. Brick House Publishing, 1984.