Energy, fuels, and chemicals 3195 



Timber to be cut into firewood for personal use is usually available at low cost 

 from both State and National Forests. Owners of small woodlots also sell culled 

 hardwoods as domestic firewood. The pine-site hardwoods make excellent fire- 

 wood for home use. They are more dense and bum with a longer lasting fire than 

 resinous softwoods. Oak is probably best as it gives the most uniform flames 

 while producing hot and long lasting coals (USD A Forest Service 1974). 



Efficiency. — Fireplaces generally are not very efficient (10 percent or less) 

 because much heat is lost up the chimney (Shelton 1976). One device to make 

 fireplaces more efficient is a hollow grate through which air can be circulated. 

 Glass doors with controlled air intakes can be placed on a fireplace to reduce the 

 amount of air entering from the room. With reduced airflow the fire bums more 

 slowly and less heat is lost up the chimney. In new fireplace installations, 

 efficiency can be raised to about 40 percent by heating circulating fire boxes with 

 outside air intakes. 



Wood buming stoves are considerably more efficient than fireplaces; efficien- 

 cies can range from 25 to 75 percent;^ 50 to 55 percent efficiency is commonly 

 attained in airtight stoves of good design. Stove design is generally based on 

 controlling combustion air and enhancing heat transfer to the room. Combustion 

 air can be controlled with secondary air inlets, airtightness, and different airflow 

 pattems through the stove. Secondary air, which is often provided in a secondary 

 combustion chamber, is used to combust completely the volatile gases produced 

 from the buming wood. In non airtight stoves air leaks through cracks around the 

 door and at other places where parts are joined reduce efficiency because the 

 leaked air is not involved in combustion and takes heat out through the chimney. 

 Airtight stoves control combustion air more effectively because stove sections 

 are welded or cemented together. This control over airflow gives airtight stoves 

 longer buming times and causes them to use 25 to 50 percent less wood. 



While operation of wood-buming stoves in an airtight mode increases their 

 thermal efficiency, there are some dangers in such operation, i.e., combustion 

 can occur with less than stoichiometric oxygen resulting in generation of inter- 

 mediate combustion products which may condense in the chimney to form 

 flammable liquids. These liquids, sometimes called creosote, can cause chim- 

 ney fires if not periodically removed. 



Air-flow patterns in stoves. — Many different wood stoves are on the market 

 today. Most can be broadly categorized on the basis of the flow pattem of air 

 through the stove (Shelton 1976). The five basic pattems are: up, diagonal, 

 across, down, and "S" (fig. 26-22). 



In the updraft pattern the combustion air enters below the burning wood and 

 travels up through the grate. Secondary air may be necessary above the wood for 

 complete combustion of volatiles. Example of updraft stoves include the pot- 

 bellied stoves and the Shenandoah. 



The diagonal flow pattern is found in the simple box stove. Air enters the 

 stove at the bottom front of the combustion zone and travels diagonally through 

 the burning wood, exiting at the top back corner. Secondary air inlets again may 



