FNR-79
forestry & natural resources
MARKETING AND UTILIZATION
Wood for Home Heating
Daniel L. Cassens, Extension Forest Products Specialist
Although fossil fuels and electricity are currently in good supply compared to ten years ago, the relatively high prices continue to focus attention on the use of wood for home heating. Wood may be burned in stoves and even furnaces to reduce fossil fuel bills during cold weather or to help warm homes in emergencies. Also, an occasional “cheery fire on the hearth” can bring families together. Regardless of the reasons for burning wood, the benefits received will depend upon your knowledge of wood as a fuel. This publication discusses the potentials of using wood for heating and enjoyment, as well as the purchasing, cutting, and storage of firewood.
How Good is Wood as a Fuel?
The evaluation of wood as a fuel is difficult for several reasons. The fuel value per unit volume depends on the species’ weight and moisture content and is thus extremely variable. For the same volume, some types of wood may contain twice as much potential heat as others. The heating system in which wood is burned also affects the quantity of usable heat produced. Typical masonry fireplaces may recover only 10 percent of the available heat, while some of the better designed wood stoves are rated at an efficiency of 50 to 75 percent or higher.
To further complicate the evaluation of wood as a fuel, wood combustion occurs in consecutive overlapping stages. In the first stage, heat is absorbed by the fuel, and water in the wood is evaporated as steam. In the second stage, the volatile matter is liberated and burned. The volatiles ignite, burn, and give off heat at about 1,000°F. The third stage in combustion occurs when most of the volatile matter has been removed. The surface of the remaining residue (charcoal) reaches a glowing temperature and
burns when oxygen from the air is brought in contact with it. This combustion exposes additional surface area until the entire mass is consumed. Each stage, and thus the amount of heat derived, is affected by variables such as wood moisture, stove efficiency, size and temperature of the fire, and type and location of the chimney.
Comparative Heat Values
To evaluate wood as a potential fuel, the heat values and burning efficiencies of the fossil fuels must be known. The heating value of No. 2 oil is about 140,000 BTU*/gallon, bituminous coal 13,000 BTU/pound, and natural gas 1,000 BTU/cubic feet. A kilowatt hour (KWH) of electricity is equivalent to 3,400 BTU. The actual usable heat derived from these different fuels will depend upon equipment design, operating conditions, and method of installation. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) Systems Handbook estimates a 70 to 80 percent efficiency for fuel fired equipment. The conversion of electricity to heat is assumed to be 100 percent efficient.
A pound of oven dry wood of any species has a heating value of about 8,600 BTU. However, the available heat per pound of wood depends, in part, upon its moisture content when burned.
Freshly cut Indiana hardwoods commonly have a 75 percent moisture content on an oven-dry basis. That is, every pound of freshly cut wood contains .57 pounds of dry wood and .43 pounds of water. Therefore, the total available heat per pound of wood at 75 percent moisture content is 4,900 BTU (0.57 pounds x 8,600 BTU/pound). At a 50 percent burning efficiency, the usable heat is 2,450 BTU per pound. Air
*BTU or British Thermal Unit is the amount of heat required to raise the temperature of one pound of water 1 °F.
PURDUE UNIVERSITY • COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, INDIANA 47907