Principals Of Wood Burning
In An Open Fireplace
Combustion - The rapid oxidation of fuel accompanied by the production of
heat and light. The three elements required for combustion are fuel, heat, and
oxygen.
In almost any open fireplace, combustion can take place. The fuel is
firewood, and being an open fireplace, the fuel is surrounded by oxygen. Add
enough heat and you have combustion. What makes a good fireplace, is design
features that allow high enough temperatures to be reached to achieve near
complete combustion. Incomplete combustion produces unburned gasses that are
wasted and released into the atmosphere contributing to the pollution concerns
we already have.
The intention of good fireplace design is to attain as complete combustion as
possible and remove the remaining by-products of combustion while leaving as
much heat as possible for the comfort of the occupants of the home. Fireplaces
without good design properties are extremely wasteful in many ways. Here are
some examples of poor design:
Corbelled throats cause turbulence and allows cooler room air to mix with
hot air from
the fire resulting in a break down of the
combustion process.
Wide lintels cause turbulence.
Smoke shelves cause turbulence.
Standard throat dampers interfere with flow and cause turbulence.
Incomplete combustion causes pollution and dirties the atmosphere.
Incomplete combustion results in chimneys accumulating creosote faster.
Throats that are too large allow huge amounts of cooler room air to enter
the
chimney resulting in cooler flue gas
temperatures causing flues to collect creosote
deposits faster.
Throats that are too large allow huge losses of heated room air up the
chimney.
Squared off fire box side walls reflect little heat into the room.
Short firebox openings don’t allow much exposure of the flame to the
room.
Deep fireboxes allow too much room air into the throat.
Unfortunately most fireplaces we see today have all or most of the above
features.
To help understand good fireplace performance, the principles of density must
be understood. Density and the resulting position of elements can best be
described with the following examples. When water and oil are mixed, the oil
being less dense, rises to the top. Fluids and air, both when heated become less
dense and therefore lighter. As an example, if you were to take a bottle of hot
water that has been colored with food coloring and submerge it - open end up -
into a larger clear container of cool water, you would quickly see that the hot
water rises to the top. The same is true for air. When heated, air will rise—even
without draft present which is shown in the case of an open fire outside with no
chimney. The important point I’m making is, heated air will rise naturally. A
properly built fireplace will not contain obstructions or inconsistencies that
will prevent the heat and accompanying smoke from following it’s natural
tendency to flow up the chimney.
Understanding that heat rises naturally, when it is contained in a chimney,
the rising heat causes draft. The two functions have a compound effect on each
other. The more heat that is produced, the lighter the hot air becomes and the
faster the heat tends to rise which in effect causes more draft. More draft
pulls more oxygen into the heated fuel and the fire burns stronger.
Improper fireplace design, including the presence of any of the factors
listed on the preceding page can interrupt the process mentioned above and
result in poor functioning fireplaces.
The Hart-Rumford Fireplace Form system has taken all of these considerations
into account. The forms start the lintel at the top of the fireplace opening at
a single point and then gently curve the breast up into the throat. The finished
breast is perfectly smooth and leads directly into the flue which is located
exactly above the hearth and fire eliminating the unneeded smoke shelf. The
smooth breast and throat allows for a small curtain of air to enter under the
lintel and flow up into the flue without causing turbulence and the mixing of
hot and cold air. In some of my earlier fireplaces I would use white refractory
cement along the breast and even after years of burning, the breasts of these
fireplaces remain perfectly white proving the air curtain that is created. The
damper is placed where it belongs, at the top of the chimney. The walls of the
firebox are angled sharply to reflect more heat into the room and the firebox is
shallow to keep the fire closer to the room which cuts down on how much air
enters the firebox. The taller fireplace opening allows more of the flame
exposed to the room which not only contributes more heat to the living area but
allows the beautiful flame to be seen and enjoyed. All of these features help to
facilitate the ascent of smoke up the chimney.
Another factor to consider in the combustion process is the moisture content
of the wood being burned. For open fireplaces the drier the wood the less energy
will be needed to drive off the moisture in the wood. If the wood is dry, the
energy produced by the fire will be used to attain higher temperatures in the
flame area of the fire, resulting in more complete combustion. Wood will burn
best in an open fireplace when the moisture content is below 15%. The type of
wood burned is of little importance along as moisture content is low. Our intent
is to get as close as possible to complete combustion. Burning dry firewood in a
properly built Hart-Rumford fireplace will result in an almost smokeless fire
and very little ash volume left on the hearth when the fire burns out.
No grate is needed with the Hart– Rumford fireplace and in fact a grate is
not recommended. A grate allows cool air to enter under the fire which results
in a cooling effect and therefore reduces the amount of combustion that can take
place.
The best shaped flue for a fireplace is round. As heat rises, it does so in a
circular motion and a round flue has no corners to cause turbulence and slow the
flow of heat rising through the chimney. Square or rectangle flues cause
turbulence and slow the ascent of heat and smoke up the flue decreasing draft
and keeping smoke in the flue longer resulting in more creosote deposits.
Good heat management can be attained in fireplace performance by using the
Hart-Rumford Fireplace Forms and following these instructions.
For more information about Hart-Rumford Fireplaces, visit our Rumford website
at:
http://www.hart-pagosa.com/Hart-Rumford/
