Copyright Goodheart-Willcox Co., Inc. 46 Hydronic Heating: Systems and Applications ■ ■■ Steel and copper heat exchangers ■■ Metal flue pipe material ■■ Small-diameter flue pipe ■■ Higher return water temperatures ■ 80 to 83% AFUE High-efficiency Boilers: ■ ■ Combustion exhaust fan that controls the flow of combustion air and gases more precisely ■ ■ Condensing flue gases pass through a second heat exchanger for extra efficiency ■ ■■ Stainless steel heat exchangers ■■ Sealed combustion ■■ PVC flue pipe material ■■ Lower return water temperatures ■ 90 to 98.5% AFUE 3.3.1 Cast Iron One of the earliest boiler construction materials was cast iron, and it is still used today. Cast-iron boilers are often found in residential and light commercial applications. Heat exchangers in cast-iron boilers are built in vertical sections, similar to the slices in a loaf of bread. This design allows them to be expanded or enlarged for future use. Cast-iron boilers are very heavy and typically hold between 15 and 30 gallons of water. These attributes classify cast-iron boilers as high-mass boilers, which means they take longer to heat up than other types, such as low-mass boilers. However, they tend to hold their heat longer. Low-mass boilers typically have copper heat exchangers that allow them to heat up quickly, but they do not hold their heat as well over longer periods of time. Because of this, cast-iron boilers typically have longer run times and longer off cycles than other types of boilers. One drawback to cast-iron boilers is their susceptibility to corrosion if excessive air is trapped in the system over long periods. See Figure 3-8. Cast-iron boilers come in wet-base and dry-base designs. In a dry-base boiler, the area under the combustion cham- ber is dry—in other words, the boiler water is contained in a space above the combustion chamber. In a wet-base boiler, the water that is heated is located throughout the combus- tion chamber. See Figure 3-9. 3.3.2 Steel and Stainless Steel Steel and stainless steel boilers, Figure 3-10, include bundles of welded heat exchanger tubes submerged in water. These tubes are often electroplated, or clad with nickel or other corrosion-resistant materials, to protect against corrosion and increase the life of the heat exchangers. Another method of reducing corrosion is the inclusion of an anode rod made of aluminum or magnesium. This “sacrificial” rod is con- sumed by oxidation much faster than the treated steel heat exchanger, thus prolonging the life of the heat exchanger. Steel boiler heat exchangers are typically classified as either the fire-tube or water-tube type. Goodheart-Willcox Publisher Figure 3-8. Cast-iron boilers are considered high-mass boilers.