Wood Burning Stoves
The basic wood fired unit consists of a heat chamber, ash pan, combustion air inlet, and an exhaust pipe that is referred to as a flue. The principle of operation is pretty simple.
A small fire is started manually and then fuel such as wood, coal, etc. is added. The unit draws in air from the room and uses it to burn the fuel.
The exhaust gases (smoke) are then drawn through the flue to the outside of the home. As the fuel is burned, ashes are produced and they fall through a grate where they are collected in the ash pan. These stoves were usually made of cast iron which was very good at absorbing heat and then radiating it out into the room.
One of the disadvantages of these basic units was that they required manually adjusting the combustion air to control the heat output. This problem was overcome by adding a component called a combustion or draft blower. This blower is used to control the amount of air that is supplied to the fire which in turn controls the amount of heat produced. The earliest forms were a simple single speed fan that was either on or off. Modern units incorporate variable speed motors into the combustion blower. When this technology is combined with digital sensors and controls, these motors allow for more accurate control of the heat output.
Another problem with the old basic units was that they were not very efficient. This lack of efficiency not only resulted in more wood used but also put a lot of pollutants into the air. It was found that the pollution could be reduced by burning the smoke from the exhaust but it required a relatively high temperature of at least 1000 degrees fahrenheit.
In the United States, the Environmental Protection Agency (EPA) has implemented a certification program for wood stoves to comply with the provisions of the Clean Air Act.
This certification process places limits on the emissions from these units. For non-catalytic units, the standard is 7.5 grams/hour while for catalytic units the standard for certification is 4.1 grams/hour.
**NOTE** Some cities and states have adopted more stringent emission limits.
This problem is overcome by adding a component called a catalytic combustor. This is a ceramic honeycomb shaped device that is made out of a special type of metal. The exhaust gases from the unit are forced through this combustor where the special metal combines with the smoke and lowers the temperature at which it burns to about 500 degrees. This device increases the efficiency of the stoves and decreases their pollution but they require periodic replacement.
Another way to solve these problems is to replace the catalytic combustor with a process called gasification or dual combustion. In these units, a second heating chamber is added and combustion air is injected into this chamber. The temperature is high enough to burn the flue gases.
The modern units also incorporate a fan system to transfer the heat from the unit to the home more efficiently. These blowers can be controlled through a temperature control or by a manual switch. This has allowed the units to effectively heat larger areas as long as they are not divided into separate rooms.
Some units also surround the firebox with a ceramic or soapstone exterior. This has an advantage in that the heat is readily absorbed and then slowly released into the room over time. Another advantage is these units look more like an elegant piece of furniture than a heating appliance.
Before purchasing a unit, you can check out our brand ratings page where we use our service experience to review the brands.