There are many ways that firewood can be used for home heating. These can range from stoves to fireplaces to outdoor boilers. New designs, along with proper use, work well in limiting smoke, and for producing heat. Each type of wood heater works well in specific situations.
Today’s wood stoves are manufactured to limit the amount of smoke produced. Smoke reduction is accomplished through two methods. One is by passing combustion gases and smoke through a ceramic apparatus that ignites the gases. This is called a catalytic combustion system.
In a non-catalytic system, the stoves use large baffles and pre-heat the combustion air.
These designs can reduce smoke by significant amounts compared to older stoves.
Pellet stoves make use of sawdust or other types of biomass that has been pelletized for use. These stoves provide a more even combustion process, which makes for lower emissions, comparable to modern, well-made wood stoves.
Fireplace designs can range from the traditional open fireplace to controlled combustion units such as masonry heaters.
Traditional open fireplaces often have problems if placed in a newly built home, not because of the fireplace itself, but because of air flow. Large amounts of air, in the range of several hundred cubic feet per minute, are needed for proper combustion and exhaust flow. Most new home construction does not allow this much air flow into and out of the house. Air flow needs can be reduced by using glass doors. Fireplaces that are EPA certified, or designed to match the house, can also greatly reduce this potential problem.
Fireplace inserts are stoves that can be fitted into a masonry fireplace. These can also be designed to greatly reduce smoke. Typically a chimney liner is required for proper operation.
Controlled Combustion Fireplaces
In a controlled combustion system, air flow needs are reduced substantially. One example of this type of system is a masonry heater. Common in northern Europe, they are designed around a large masonry mass, which radiates heat. The flue system is designed to wind through the masonry, transferring heat to the mass. Heating cycles burn wood quickly and at a very high temperature. Since the heat radiates out through the masonry, burn cycles are only needed once or twice a day.
These units must be designed to fit specific building characteristics.
Outdoor boilers are insulated buildings where the firebox is located, surrounded by pipes containing water. As the water is heated, it is transferred to the house or other buildings. Excessive smoke may be an issue with older or less technologically advanced systems, particularly when there are long periods between heating cycles.
In terms of heating efficiency, traditional open fireplaces generally have efficiencies in the 10% range. Advanced low-emission models will increase this considerably, sometimes near new stove efficiencies. Older wood stoves have efficiencies up to 30%, with new non catalytic models in the 50 to 60% range. Catalytic combustion stoves may reach 70%.
Pellet stoves and masonry heaters can reach 80 to 90% heating efficiency.
Newer outdoor boilers may reach 80%.
All of these efficiencies are based on optimum performance, and will be reduced if the appliance is not operated or maintained properly.
As with any appliance that works with high heat, regular maintenance of both the appliance and chimney is necessary for proper operation and to make sure there are no potential fire hazards.
Screens should be used in front of open fireplaces to keep sparks from flying out onto combustible materials.
Fireplaces should be inspected each year to verify firebox integrity. Masonry fireplaces should not have cracks in the mortar or brick. With catalytic wood stoves, the combuster needs to be in good working order. In non catalytic stoves, baffles should be inspected.
If you are not sure what should be inspected, contact a professional. If you have someone clean your chimney, they are often trained to inspect the stove or fireplace, as well.
One of the biggest safety concerns with wood burning is chimney fires. This occurs when exhaust gases cool and condense on chimney walls, creating creosote. This material is highly flammable. If allowed to build up, it can start burning in the chimney when heated.
Chimneys should be inspected and cleaned annually. Creosote buildup can be decreased by making sure you use seasoned wood, having hot fires instead of long, slow-burning ones, or employing low smoke-producing systems.
Proper chimney caps should be in place to arrest sparks, and to keep your friendly neighborhood wildlife from visiting.
Ash disposal is another concern. Ashes should be kept in a metal container with a lid. The container should not be placed next to or on top of combustible materials, and should be outdoors.
Duane Friend is a University of Illinois Extension environmental stewardship and energy educator.