Copyright Goodheart-Willcox Co., Inc. 56 Hydronic Heating: Systems and Applications Chapter Review Summary■ ■ There are several different types of boilers classified by pressures, construction materials, and efficiencies. ■ ■ Boilers can be classified as low-pressure, medium-pressure, and high-pressure applications. ■ ■ Various types of fuels may be used as heating sources for modern-day hot water boilers, including fuel oil, natural gas, propane, electricity, and biomass. ■ ■ Boilers are constructed from cast iron, steel and stainless steel, and copper. The types of materials used in boiler construction can influence their efficiency. Cast-iron boilers are very heavy and take longer to heat up, but they tend to hold their heat longer. Steel fire-tube boilers have a simple, compact design, require less stringent water treatments, clean easily, and are well suited for space heating as well as for industrial process applications. However, they require extensive time to raise the water temperature and do not respond quickly to changes in load. Steel water-tube boilers provide a faster response to heating load, are available in a greater range of sizes, and can reach very high temperatures. The disadvantages of water-tube boilers are higher initial capital costs, difficulty of cleaning, and their often-challenging physical size. Copper-tube boilers do not hold their heat as long, but they are able to heat water more quickly and efficiently. ■ ■ In a dry-base boiler, 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—both above and below—surrounding the combustion chamber. The boiler’s condensing dew point can influence the boiler’s efficiency. ■ ■ Condensing boilers are classified as high-efficiency boilers and extract additional heat by adding a second heat exchanger to the system. ■ ■ A condensing boiler uses latent heat as a means to achieve higher efficiencies. When additional heat is removed by the secondary heat exchanger, part of the combustion gases condense into liquid, releasing latent heat into the boiler’s water. ■ ■ Modulating/condensing (mod/con) boilers can modulate their firing rate to achieve higher efficiencies and comfort. ■ ■■ Turndown ratio is the rate at which a modulating boiler can throttle its output. ■ Electric boilers are used where other potential fossil fuels are scarce. They are virtually 100% energy efficient, durable, and relatively small in size. However, there is a limit to how much water they can heat at one time, and most areas’ electricity costs outweigh the cost of other fossil fuels. ■ ■ Low NOx boilers are required in various areas of the United States and Canada in an effort to reduce toxic emissions that contribute to smog and acid rain.