Heating Fuels

Only the principal characteristics of those heating fuels used for domestic heating purposes will be considered in any great detail in this chapter. Descriptions of firing methods for the more commonly used heating fuels are found in Chapter 1, “Oil Burners,” Chapter 2, “Gas Burners,” Chapter 3, “Burning Solid Fuels” in Volume 2, and Chapter 3, “Stoves, Fireplaces, and Chimneys” in Volume 3.

Heating fuels are measured in physical units, such as gallons (fuel

Oil, propane, and kerosene), cubic feet (natural gas), tons (coal and pellets), kilowatt hours (electricity), and cords or pounds (wood).

Another way to measure heating fuels is by heat content. The British thermal unit (Btu) is the value commonly used in the United States for expressing the energy value or heat content of a heating fuel. One Btu is the amount of energy required to raise the temper­ature of 1 pound of water 1 degree Fahrenheit (°F), when the temperature of the water is about 39°F. The average Btu contents of different heating fuels are listed in Table 5-1.

The heating fuel values listed in Table 5-1 are the gross (or higher) values commonly used in energy calculations in the United States. These values are estimated by the Energy Information Administration in the Annual Energy Review and other sources. Net (or lower) heat­ing values may also be used in energy calculations.

Table 5-1 Average Btu Content of Common Heating Fuels

Fuel Type Number of Btu/Unit

Fuel oil (No. 2)


Natural gas

1,025,000/thousand cubic feet







Wood (air dried)

20,000,000/cord or 8,000/pound





Courtesy Office of Energy Efficiency and Renewable Energy/U. S. Department of Energy

When a fuel is burned in an appliance (e. g., a furnace or boiler), the combustion process causes the water contained in the fuel to vaporize. The water vapor contains heat energy, which is lost when the combustion gases are vented to the exterior. The dif­ference between the higher and lower heating values is the amount of energy required to vaporize the water contained in a fuel or cre­ated in the combustion process. This difference can range from approximately 2 to 60 percent, depending on the fuel used.


The newer, high-efficiency, condensing forced-air furnaces are designed to capture much of the heat energy contained in the water vapor before it exits the furnace stack and enters the chimney. Because electricity is not burned in a combustion process, there is no difference between a gross (higher) and net (lower) value.

Heating fuels also can be classified as solid, liquid, or gaseous, depending upon their physical state. Examples of solid fuels are coal and coke. Fuel oils are classified as liquid fuels; gaseous fuels include natural gas, manufactured gas, liquefied petroleum gas, and related types.

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers