Copyright Goodheart-Willcox Co., Inc. Chapter 3 Boilers 45 but it is considered a renewable energy resource since trees and other organic materials can be replaced with new growth. The Btu content of wood varies according to the particular species of tree, with hardwood varieties containing the highest Btu content per cord. A cord of wood is the unit of measurement in the United States and Canada, equal to 128 cubic feet of volume, or approxi- mately 4′ × 4′ × 8′. In comparison, a cord of oak contains a heating content of about 36 million Btu while cottonwood possesses only about 16.8 million Btu/ cord. Biomass boilers may be placed in basements, but most are located out- doors, Figure 3-7. Outdoor wood boilers are discussed in Chapter 20, Outdoor Wood Boilers. 3.3 Construction Materials and Boiler Efficiency Boiler construction materials must hold up to high operating pressures and temperatures, and the evolution of boiler materials used over time reflects the technological advances seen throughout manufacturing, such as in the automo- tive industry. Today’s construction materials must adhere to local and national codes, and boilers must meet ASME standards. Several types of materials are used in the construction of modern-day hot water boilers, based on boiler efficiency. Boiler efficiencies and their corresponding types of construction materials can be grouped into the following categories: Lower-efficiency Boilers: ■ ■■ Natural draft that creates a flow of combustion gases ■■ Continuous pilot light ■■ Heavy-duty, cast-iron heat exchanger ■■ Metal flue pipe material ■■ Higher return water temperatures ■ 56 to 70% AFUE Mid-efficiency Boilers: ■ ■ Combustion exhaust fan that controls the flow of combustion air and gases more precisely ■ ■■ Electronic ignition (no pilot light) ■ Compact size and lighter weight to reduce cycling losses GREEN TIP The Natural Carbon Cycle Even though the burning of different types of biomass does generate greenhouse gases, burning wood releases about the same amount of carbon dioxide (CO2) into the atmosphere as does the decaying of that same amount of wood as part of the natural carbon cycle—only at a faster pace. The EPA has determined that wood burning is carbon neutral, yet environmentalists feel that this accelerated release of greenhouse gases contributes to climate change. One method of reducing wood-burning emissions is gasification technology, which is used in many of today’s wood-burning boilers. In the gasification pro- cess, exhaust gases from burning wood are forced through a secondary heat exchanger in the wood boiler and reburned at temperatures up to 2300°F. This re- sults in a cleaner wood-burning boiler that is virtually smoke free. These types of boilers have been independently verified to meet EPA phase 2 emissions output for reduced greenhouse gases. More information on EPA wood-burning standards can be found in the Burn Wise section of the EPA’s website. Goodheart-Willcox Publisher Figure 3-7. An example of an outdoor wood boiler, or biomass boiler.