Copyright Goodheart-Willcox Co., Inc. Chapter 12 Radiant Heating Systems 241 temperature through the individual radiant heating circuits, with each circuit acting independently of the others, Figure 12-6. Primary-secondary circuit pip- ing is discussed in greater detail in Chapter 7, Hydronic Piping Systems. 12.2.2 Condensing Boilers A condensing boiler is a high-efficiency boiler with output efficiency ratings usu- ally above 90%, Figure 12-7. These boilers achieve a higher efficiency rating by “squeezing” more heat out of the fuel. This simply means that more heat from combustion is transferred into the boiler water, and less heat passes through the chimney. This is accomplished by incorporating a secondary heat exchanger into the boiler. When flue gases from combustion pass through this secondary heat exchanger, water vapor condenses out of the gases. This secondary heat exchanger is made of a noncorrosive stainless steel, which resists the effects of flue gas condensation. Condensing boilers do not use conventional metal for their flue vents, but rather plastic piping such as PVC or PP that resists corro- sion caused by acidic condensate from the products of combustion. Condensing Primary loop Primary pump Secondary loop Secondary pumps Goodheart-Willcox Publisher Figure 12-6. An example of a boiler piped with a primary-secondary loop system.