Copyright Goodheart-Willcox Co., Inc. 50 Hydronic Heating: Systems and Applications water temperature be set between 160°F and 180°F. This topic is discussed further in Chapter 7, Hydronic Piping Systems and Chapter 11, Terminal Devices. How- ever, raising the return water temperature tends to reduce the boiler’s efficiency because not as much heat will have been extracted from the water. For these reasons, the industry has developed condensing boilers, which are considered high-efficiency boilers, Figure 3-17. High-efficiency boilers contain a secondary heat exchanger that “squeezes” additional heat from the flue gases before they exit the boiler. To understand how a condensing boiler operates, first review the by-products of perfect combustion: heat, carbon dioxide (CO2), and water. The boiler’s combustion process can be expressed as follows: [C + H (fuel)] + [O2 (Air)] → (Combustion Process) → [CO2 + H2O + (Heat)] where C = Carbon H = Hydrogen O = Oxygen The heat of combustion is either transferred into the boiler water or is passed through the boiler and up the chimney. The goal in increasing boiler efficiency is to reduce the amount of heat that passes through the chimney and increase the amount of heat transferred into the boiler water. However, in order to decrease the amount of heat passing through the chimney, we must “squeeze” more heat out of the fuel, which will create condensation, or water, in the flue gases. As A B Goodheart-Willcox Publisher Figure 3-17. The outside and inside view of a condensing boiler.