Copyright Goodheart-Willcox Co., Inc. Chapter 3 Boilers 49 increasing heat transfer. Water inside the tubing is heated by combustion gases from the burner located inside the sur- rounding chamber. Because copper-tube boilers have less surface area than cast-iron boilers, they do not hold their heat as long. But because copper is a greater conductor of heat than cast iron, they are able to heat water more quickly and efficiently. When replacing an old cast-iron boiler in a residence or commercial building, a copper-tube boiler can operate more efficiently, saving the owner hundreds of dollars per year. Some copper-tube boilers are rated at and above 80% AFUE. AFUE stands for Annual Fuel Utilization Efficiency, which can result in a significant reduction in heating bills. Copper-tube boilers can be a good fit for snowmelt appli- cations. They generate a large volume of heat in a small pack- age. Furthermore, copper-tube boilers can run at a reduced rate with a constant load, conducive to the lower water temperature required in snowmelt applications. When choosing a copper-tube boiler, the water quality feeding the boiler is critical. Hard water tends to create a buildup of scale quickly on the tube-type heat exchanger, diminishing the boiler’s performance and reducing its lifespan. Always check with the boiler manufacturer regarding their water treatment specifications. Because copper-tube boilers need adequate water flow, ensure the circulating pump is sized correctly. Maintain the proper minimum water temperature through a copper-tube boiler it is critical that the return water temperature is above the dew point (130°F) to avoid condensation on the heat exchanger, as discussed in the next section. 3.4 Condensing Boilers A non-condensing, medium-efficiency boiler needs to main- tain a return water temperature above its condensing dew point. By definition, dew point is the temperature below which water vapor begins to condense. Figure 3-16 shows the relationship between boiler return water temperature, dew point, and boiler efficiency. Conventional lower-effi- ciency and medium-efficiency boilers tend to condense the products of combustion when the return water temperature approaches 130°F. When this occurs, it can create major operational, installation, and safety issues. Flue gas conden- sation is very acidic, with a pH between 4 and 5. This acidic condensate can reduce efficient heat transfer, damage the heat exchanger, and corrode the venting system, causing premature failure. This last condition can create a potentially dangerous spillage of flue gases. To remediate this situation, the minimum return water temperatures of lower- and medium-efficiency hot water boilers need to be maintained above 130–140°F. This can be accomplished by changing the supply water temperature set point on the boiler’s temperature controls. A 20°F tempera- ture difference is typically desired on lower- and medium- efficiency boilers. This would require that the boiler supply Goodheart-Willcox Publisher Figure 3-14. Copper-tube boilers are considered low mass because of their lightweight design. Mykhailo Motov/Shutterstock.com Figure 3-15. An example of a heat exchanger commonly used in a copper- tube boiler. Goodheart-Willcox Publisher Figure 3-16. This illustration shows the relationship between boiler return water temperature, dew point, and boiler efficiency. 80 82 84 86 88 90 92 94 96 98 100 40 60 80 100 120 140 160 180 200 Condensing mode operation Noncondensing mode operation 130° dew point Return Water Temperature (°F) Steady-State Boiler Efficiency %