Wood Heat: Good For You, Good For Our Environment.
A wood heating system is the right choice for the increasing number of people who are concerned about our environment. Unlike systems that rely on fossil fuels, wood is a totally renewable resource, which, when burned, results in no net carbon dioxide increase and helps reduce your carbon footprint. Carbon dioxide is part of the natural plant-growth cycle and occurs naturally when trees are allowed to rot on the forest floor. On the other hand, fossil fuels, when burned, release carbon dioxide which otherwise would stay trapped in the earth. The burning of fossil fuels causes a net increase in carbon dioxide, which is believed to be responsible for the heat-trapping greenhouse effect. In addition, harvesting firewood has a pruning effect on forests, which allows new growth to flourish. And the wood you burn most likely comes from your local area -- it's not imported, as is the case with oil.
Heating with Wood
You have a nice fire going, and it has burned down to the point where what you see is a collection of hot, glowing embers. The fire is still producing a lot of heat, but it is producing no smoke at all. You might have gotten to this point, either by starting with logs in a fireplace or by starting with charcoal.
If you now toss a piece of wood, or even a sheet of paper, onto this fire, what you will notice is that the new fuel produces a lot of smoke as it heats up. Then, all of a sudden (often with a small pop), it bursts into flame and the smoke disappears.
If you have a fireplace or wood stove, or if you have been around a lot of campfires, this little scene is very familiar to you. It tells you a lot about smoke. Let's look at what is happening.
A few things that you find in any Piece of Wood
Water – Freshly cut wood contains a lot of water (sometimes more than half of its weight is water). Seasoned wood (wood that has been allowed to sit for a year or two), or kiln-dried wood contains much less water, but it still contains some.
Volatile organic compounds – When the tree was alive, it contained sap and a wide variety of volatile hydrocarbons in its cells. Cellulose is a chief component of wood. This is a carbohydrate, meaning it is made of glucose. A compound is volatile if it evaporates when heated. These compounds are all combustible. Gasoline and alcohol are, after all, hydrocarbons. The volatile hydrocarbons in wood burn the same way.
Ash – Ash is the non-burnable minerals in the tree's cells, like calcium, potassium, and magnesium.
When you put the fresh piece of wood, or paper on a hot fire, the smoke you see is those volatile hydrocarbons evaporating from the wood. They start vaporizing at a temperature of about 300 degrees F.
If the temperature gets high enough, these compounds burst into flame. Once they start burning, there is no smoke because the hydrocarbons are turned into carbon dioxide and water (both invisible) when they burn.
Why and How Wood Burns
Stage One Combustion
The first stage of combustion is the heating and evaporating stage. Initially, heat is brought into contact with a piece of wood in the presence of air. Heat causes several reactions.
First, it raises the temperature of an area on the wood surface to some depth into the wood. As the wood’s surface temperature approaches 212 degrees Fahrenheit, the water in the wood begins to boil, then evaporates. As long as the water remains in the wood, its boiling and evaporation robbing heat energy from the source, thereby keeping the wood cells from gaining more heat. Moisture must be driven off before combustion can begin, so wood with a high moisture content is hard to ignite.
Unlike moisture, volatile gases are combustible. They burn and release heat. As the wood surface temperature rises beyond 212° F to about 450° F, major gases abundant in creosote are produced: carbon dioxide, carbon monoxide and acetic and formic acids. However, the gases generated in the first stage of combustion do not ignite until the moisture evaporates and the kindling temperature is hot enough.
Stage Two Combustion
After moisture is driven from the wood and the heat raises the temperature of the wood above 540° F, the second stage of combustion takes place. This is the heat-producing stage. It occurs at two different temperature levels: primary and secondary combustion.
The process by which gases are released from wood and burned is called primary combustion. Primary combustion begins at about 540° F, continues toward 900° F and results in the release of a large amount of energy. Primary combustion also releases large amounts of unburned combustible gases, including methane and methanol as well as more acid, water vapor and carbon dioxides.
These gases, called secondary gases, contain up to 60 percent of the potential heat in the wood. Their combustion is important to achieve high overall combustion efficiency. The secondary gases are not burned near the wood because of lack of oxygen (oxygen is being consumed by primary combustion) or insufficient temperature.
The conditions needed to burn secondary gases are sufficient oxygen and temperatures of at least 1100° F. The air supply is critical. Too little air will not support combustion and too much will cool the temperature to a point where combustion cannot occur.
Remember that air is about 80 percent inert gas and, when introduced into a wood stove, is well below the 1100° F needed to sustain secondary combustion. The more air that mixes with the secondary gases, the greater the quantity of heat absorbed by the nitrogen, and the lower the temperature of the secondary gas-air mixture.
Secondary combustion can and does occur in wood burning stoves that are designed to meet or exceed the EPA’s requirements for clean air, but only if the stove is used with seasoned wood, operated in a manner consistent with its design and is connected to a properly functioning chimney system.
Many people do not realize that the chimney is the engine that drives the stove (or fireplace) and that if the chimney is not up to par (sized correctly, have adequate height, or does not hold enough heat) then the draft will be inadequate and the best stove in the world will be a disappointment at best, and possibly even a danger at worst.
Stage Three Combustion
During wood burning, after the gases are driven from the wood, the carbon chains of cellulose and lignin molecules remain. Carbon, or charcoal, burns a long time with a low rate of heat output. Charcoal burning is important for two reasons. Additional energy is released, which is important to overall combustion efficiency. Also, charcoal burns at a low rate of combustion, which means that a good charcoal bed will burn a long time, allowing a fire to last the night. The fire can be rekindled by, adding wood and opening the draft to supply new oxygen.
Indoor or Outdoor Burning? Be Safe At Home
Safety is a major consideration when it comes to burning wood as a heat source. An estimated 73,800 home heating fires in 1994 killed 487 people and injured almost 2,000 people. But Central Boiler outdoor furnaces remove the fire hazard from the home or building being heated -- all burning takes place remotely, up to 500 feet away. There's no indoor burning, indoor smoke, and no threat of a devastating chimney fire. Also, indoor burning can affect air quality, and even create a dangerous buildup of carbon monoxide. With a Central Boiler outdoor furnace, these problems are eliminated. You get clean, comfortable warmth -- and peace of mind. All Central Boiler outdoor furnace models have been tested and listed to UL and CSA standards.
Energy Costs: Get A Handle On Your Heating Bills.
Energy costs are a major household or business expenditure, especially in harsher winter climates. As a fuel source, wood -- in addition to being readily available and totally renewable -- is also very inexpensive compared to other fuel sources. In fact, the energy savings realized by heating with a Central Boiler outdoor furnace can often pay back the purchase price in the first two years. By adding a domestic water-to-water heat exchanger, a Central Boiler outdoor furnace can also provide all of your hot water. You can then turn your water heater off and realize big savings on your electric or gas bill. Over a ten-year period, a homeowner or business may save $20,000 to $100,000 or more on heating costs.
Efficiency Means More Heat From Less Wood.
Central Boiler outdoor furnaces' energy efficient designs maximize the combustion and heat transfer process for better heat delivery. In other words, Central Boiler outdoor furnaces burn more completely and efficiently, and transfer more of the heat generated to your home. Current owners of Central Boiler outdoor furnaces report that they are using up to 25% to 70% less wood compared to traditional wood furnaces or other brands of outdoor wood furnaces. Indoor wood stoves can be less efficient because heated indoor air used for combustion is sent out the chimney, causing cold air to be drawn into the house to replace it.
It's Your Home. Why Not Enjoy It?
Your home is one of your biggest investments. It's where your family spends a considerable amount of time. And you spend a considerable amount of money making it comfortable and enjoyable. Firewood can be dirty, moldy and riddled with insects which can create a mess in your home. Burning wood indoors means smoke, ashes, odors, and eventual buildup of soot on walls and ceilings. With a Central Boiler outdoor furnace, you avoid all of these problems because you don't have to bring firewood inside, and all burning takes place remotely. Many Central Boiler outdoor furnace owners have also noticed that eliminating indoor burning has helped alleviate respiratory and allergy problems caused by exposure to moldy firewood, smoke, and ashes.
Burn Wood, Not Your Spare Time.
Continuous stoking of wood is a necessary part of fireplace and wood stove use. If you don't have the time or energy to continually babysit these heat sources, a Central Boiler outdoor furnace is your low-maintenance alternative. Since Central Boiler outdoor wood furnaces can burn larger pieces of wood they can dramatically reduce time spent preparing the wood. The large firebox door is designed for easy loading of wood and removal of ash.