Heating Values - A More Detailed Discussion

The heating values provided in the Table 1 above are the "higher" or "gross" heating values of the fuels as estimated by the Energy Information Administration in the Annual Energy Review 2001 (November 2002; see: www.eia.doe.gov/aer ), and other sources. Higher (or gross) heating values are commonly used in energy calculations in the United States. "Net" or "lower" heating values may also be used. The difference between the two values is the amount of energy that is necessary to vaporize water that is contained in the fuel or created in the combustion process when hydrogen in the fuel is combined with oxygen to form water vapor. In general, this difference can range from as little as 2 percent to as much as 60 percent, depending on the hydrogen or moisture content of specific fuels. The heat energy contained in the water vapor is generally lost as the combustion gases leave the appliance vent or chimney. Some types of combustion appliances, however, such as high efficiency "condensing" forced-air furnaces, are able to capture much of the heat contained in the water vapor before it leaves the furnace vent (thus the term "condensing"). Since electricity is not burned in a heating appliance, the two values are equal.

Wood heating values can vary significantly. The most important factor affecting useful Btu content is the moisture content of the wood. Well-seasoned, air-dried wood will typically have a moisture content of around 20 percent (when compared to a "bone dry" sample of the wood). A very rough approximation of the effect of moisture content on the heating value is for every percent increase in moisture content (relative to a bone-dry sample) there is a one percent decrease in heating value. The other factor affecting heat content is the tree species of the wood. Higher heating values of wood can vary from 8,000 to 10,000 Btu per pound, bone dry. A cord of wood is a rough measuring unit; it is a stack of wood 4 feet high, 8 feet long, and 4 feet wide. A "good" cord of wood will be tightly packed. Pellet fuels are usually made from sawdust. The Btu content will therefore vary depending on the type of wood that the sawdust is from. Pellet fuels typically have a moisture content of around 10 percent.

Most of the appliance efficiencies given in the Table 2 above roughly account for the net heating value fuels used in a vented appliance (ie one that has a chimney). An unvented space heater, such as a kerosene heater or a natural gas fireplace insert, delivers nearly all of the heating value of a fuel to the space in which it is located. It also puts all the products of combustion including carbon dioxide, water vapor, and small amounts of carbon monoxide, sulfur dioxide, and nitrous oxides into the room. These types of heating units generally require that a window be opened (slightly) for safe operation, which reduces their overall heating capability.

The system used to distribute heat from a heating appliance, such as pipes in a hydronic heating system or ducts in a forced-air system, can greatly affect the amount of heat actually delivered to individual rooms in a house. This is especially true of forced-air heating systems, if ducts are improperly sized and installed, if they leak, and/or if they are uninsulated where they pass through attics or crawl spaces. Also, central heating systems use fans (in forced-air systems to distribute the heated air) and pumps (in hydronic systems to distribute hot water to radiators), which require electricity to operate. This electricity consumption is an additional cost for heating in these types of systems.