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PRINCIPLES OF WOOD BURNING

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.

principles of wood burningIn 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 pit outside fire, 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 three paragraphs up 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, demonstrating 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.

 

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